How to learn Python Newbies...: 2021

Python Dictionaries

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

Dictionary

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered*, changeable and does not allow duplicates.

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

Dictionaries are written with curly brackets, and have keys and values:

Example

Create and print a dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(thisdict)

Dictionary Items

Dictionary items are ordered, changeable, and does not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

Example

Print the "brand" value of the dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(thisdict["brand"])

Ordered or Unordered?

As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.

Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

Example

Duplicate values will overwrite existing values:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964,

"year": 2020

}

print(thisdict)

Dictionary Length

To determine how many items a dictionary has, use the len() function:

Example

Print the number of items in the dictionary:

print(len(thisdict))

Dictionary Items - Data Types

The values in dictionary items can be of any data type:

Example

String, int, boolean, and list data types:

thisdict = {

"brand": "Ford",

"electric": False,

"year": 1964,

"colors": ["red", "white", "blue"]

}

type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

Example

Print the data type of a dictionary:

thisdict = {

"brand": "Ford",

"model": "Mustang",

"year": 1964

}

print(type(thisdict))

Python Collections (Arrays)

There are four collection data types in the Python programming language:

List is a collection which is ordered and changeable. Allows duplicate members.

Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

Set is a collection which is unordered and unindexed. No duplicate members.

Dictionary is a collection which is ordered and changeable. No duplicate members.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

Python If ... Else

Python Conditions and If statements

Python supports the usual logical conditions from mathematics:

Equals: a == b

Not Equals: a != b

Less than: a < b

Less than or equal to: a <= b

Greater than: a > b

Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in "if statements" and loops.

An "if statement" is written by using the if keyword.

Example

If statement:

a = 33

b = 200

if b > a:

print("b is greater than a")

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that "b is greater than a".

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

If statement, without indentation (will raise an error):

a = 33

b = 200

if b > a:

print("b is greater than a") # you will get an error

Elif

The elif keyword is pythons way of saying "if the previous conditions were not true, then try this condition".

Example

a = 33

b = 33

if b > a:

print("b is greater than a")

elif a == b:

print("a and b are equal")

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that "a and b are equal".

Else

The else keyword catches anything which isn't caught by the preceding conditions.

Example

a = 200

b = 33

if b > a:

print("b is greater than a")

elif a == b:

print("a and b are equal")

else:

print("a is greater than b")

In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that "a is greater than b".

You can also have an else without the elif:

Example

a = 200

b = 33

if b > a:

print("b is greater than a")

else:

print("b is not greater than a")

Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

One line if statement:

if a > b: print("a is greater than b")

Short Hand If ... Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

One line if else statement:

a = 2

b = 330

print("A") if a > b else print("B")

This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

One line if else statement, with 3 conditions:

a = 330

b = 330

print("A") if a > b else print("=") if a == b else print("B")

And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, AND if c is greater than a:

a = 200

b = 33

c = 500

if a > b and c > a:

print("Both conditions are True")

The or keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, OR if a is greater than c:

a = 200

b = 33

c = 500

if a > b or a > c:

print("At least one of the conditions is True")

Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

x = 41

if x > 10:

print("Above ten,")

if x > 20:

print("and also above 20!")

else:

print("but not above 20.")

The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

Example

a = 33

b = 200

if b > a:

pass

Python While Loops

Python Loops

Python has two primitive loop commands:

while loops

for loops

The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

Example

Print i as long as i is less than 6:

i = 1

while i < 6:

print(i)

i += 1

Note: remember to increment i, or else the loop will continue forever.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.

The break Statement

With the break statement we can stop the loop even if the while condition is true:

Example

Exit the loop when i is 3:

i = 1

while i < 6:

print(i)

if i == 3:

break

i += 1

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

Example

Continue to the next iteration if i is 3:

i = 0

while i < 6:

i += 1

if i == 3:

continue

print(i)

The else Statement

With the else statement we can run a block of code once when the condition no longer is true:

Example

Print a message once the condition is false:

i = 1

while i < 6:

print(i)

i += 1

else:

print("i is no longer less than 6")

Python For Loops

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Example

Print each fruit in a fruit list:

fruits = ["apple", "banana", "cherry"]

for x in fruits:

print(x)

The for loop does not require an indexing variable to set beforehand.

Looping Through a String

Even strings are iterable objects, they contain a sequence of characters:

Example

Loop through the letters in the word "banana":

for x in "banana":

print(x)

The break Statement

With the break statement we can stop the loop before it has looped through all the items:

Example

Exit the loop when x is "banana":

fruits = ["apple", "banana", "cherry"]

for x in fruits:

print(x)

if x == "banana":

break

Example

Exit the loop when x is "banana", but this time the break comes before the print:

fruits = ["apple", "banana", "cherry"]

for x in fruits:

if x == "banana":

break

print(x)

The continue Statement

With the continue statement we can stop the current iteration of the loop, and continue with the next:

Example

Do not print banana:

fruits = ["apple", "banana", "cherry"]

for x in fruits:

if x == "banana":

continue

print(x)

The range() Function

To loop through a set of code a specified number of times, we can use the range() function,

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

Example

Using the range() function:

for x in range(6):

print(x)

Note that range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):

Example

Using the start parameter:

for x in range(2, 6):

print(x)

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

Example

Increment the sequence with 3 (default is 1):

for x in range(2, 30, 3):

print(x)

Else in For Loop

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

Example

Print all numbers from 0 to 5, and print a message when the loop has ended:

for x in range(6):

print(x)

else:

print("Finally finished!")

Note: The else block will NOT be executed if the loop is stopped by a break statement.

Example

Break the loop when x is 3, and see what happens with the else block:

for x in range(6):

if x == 3: break

print(x)

else:

print("Finally finished!")

Nested Loops

A nested loop is a loop inside a loop.

The "inner loop" will be executed one time for each iteration of the "outer loop":

Example

Print each adjective for every fruit:

adj = ["red", "big", "tasty"]

fruits = ["apple", "banana", "cherry"]

for x in adj:

for y in fruits:

print(x, y)

The pass Statement

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

Example

for x in [0, 1, 2]:

pass

Python - Add Set Items

Add Items

Once a set is created, you cannot change its items, but you can add new items.

To add one item to a set use the add() method.

Example

Add an item to a set, using the add() method:

thisset = {"apple", "banana", "cherry"}

thisset.add("orange")

print(thisset)

Add Sets

To add items from another set into the current set, use the update() method.

Example

Add elements from tropical into thisset:

thisset = {"apple", "banana", "cherry"}

tropical = {"pineapple", "mango", "papaya"}

thisset.update(tropical)

print(thisset)

Add Any Iterable

The object in the update() method does not have to be a set, it can be any iterable object (tuples, lists, dictionaries etc.).

Example

Add elements of a list to at set:

thisset = {"apple", "banana", "cherry"}

mylist = ["kiwi", "orange"]

thisset.update(mylist)

print(thisset)

Python - Access Set Items

Access Items

You cannot access items in a set by referring to an index or a key.

But you can loop through the set items using a for loop, or ask if a specified value is present in a set, by using the in keyword.

Example

Loop through the set, and print the values:

thisset = {"apple", "banana", "cherry"}

for x in thisset:

print(x)

Example

Check if "banana" is present in the set:

thisset = {"apple", "banana", "cherry"}

print("banana" in thisset)

Change Items

Once a set is created, you cannot change its items, but you can add new items.

Python Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.

Creating a Function

In Python a function is defined using the def keyword:

Example

def my_function():

print("Hello from a function")

Calling a Function

To call a function, use the function name followed by parenthesis:

Example

def my_function():

print("Hello from a function")

my_function()

Arguments

Information can be passed into functions as arguments.

Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.

The following example has a function with one argument (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

def my_function(fname):

print(fname + " Refsnes")

my_function("Emil")

my_function("Tobias")

my_function("Linus")

Arguments are often shortened to args in Python documentations.

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that are passed into a function.

From a function's perspective:

A parameter is the variable listed inside the parentheses in the function definition.

An argument is the value that is sent to the function when it is called.

Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):

print(fname + " " + lname)

my_function("Emil", "Refsnes")

If you try to call the function with 1 or 3 arguments, you will get an error:

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):

print(fname + " " + lname)

my_function("Emil")

Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly:

Example

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):

print("The youngest child is " + kids[2])

my_function("Emil", "Tobias", "Linus")

Arbitrary Arguments are often shortened to *args in Python documentations.

Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):

print("The youngest child is " + child3)

my_function(child1 = "Emil", child2 = "Tobias", child3 = "Linus")

The phrase Keyword Arguments are often shortened to kwargs in Python documentations.

Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

Example

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):

print("His last name is " + kid["lname"])

my_function(fname = "Tobias", lname = "Refsnes")

Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.

Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

Example

def my_function(country = "Norway"):

print("I am from " + country)

my_function("Sweden")

my_function("India")

my_function()

my_function("Brazil")

Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

Example

def my_function(food):

for x in food:

print(x)

fruits = ["apple", "banana", "cherry"]

my_function(fruits)

Return Values

To let a function return a value, use the return statement:

Example

def my_function(x):

return 5 * x

print(my_function(3))

print(my_function(5))

print(my_function(9))

The pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

Example

def myfunction():

pass

Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself ("recurse"). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

Example

Recursion Example

def tri_recursion(k):

if(k > 0):

result = k + tri_recursion(k - 1)

print(result)

else:

result = 0

return result

print("\n\nRecursion Example Results")

tri_recursion(6)

Python Sets

myset = {"apple", "banana", "cherry"}

Set

Sets are used to store multiple items in a single variable.

Set is one of 4 built-in data types in Python used to store collections of data, the other 3 are List, Tuple, and Dictionary, all with different qualities and usage.

A set is a collection which is both unordered and unindexed.

Sets are written with curly brackets.

Example

Create a Set:

thisset = {"apple", "banana", "cherry"}

print(thisset)

Note: Sets are unordered, so you cannot be sure in which order the items will appear.

Set Items

Set items are unordered, unchangeable, and do not allow duplicate values.

Unordered

Unordered means that the items in a set do not have a defined order.

Set items can appear in a different order every time you use them, and cannot be referred to by index or key.

Unchangeable

Sets are unchangeable, meaning that we cannot change the items after the set has been created.

Once a set is created, you cannot change its items, but you can add new items.

Duplicates Not Allowed

Sets cannot have two items with the same value.

Example

Duplicate values will be ignored:

thisset = {"apple", "banana", "cherry", "apple"}

print(thisset)

Get the Length of a Set

To determine how many items a set has, use the len() method.

Example

Get the number of items in a set:

thisset = {"apple", "banana", "cherry"}

print(len(thisset))

Set Items - Data Types

Set items can be of any data type:

Example

String, int and boolean data types:

set1 = {"apple", "banana", "cherry"}

set2 = {1, 5, 7, 9, 3}

set3 = {True, False, False}

A set can contain different data types:

Example

A set with strings, integers and boolean values:

set1 = {"abc", 34, True, 40, "male"}

type()

From Python's perspective, sets are defined as objects with the data type 'set':

Example

What is the data type of a set?

myset = {"apple", "banana", "cherry"}

print(type(myset))

The set() Constructor

It is also possible to use the set() constructor to make a set.

Example

Using the set() constructor to make a set:

thisset = set(("apple", "banana", "cherry")) # note the double round-brackets

print(thisset)

Python Collections (Arrays)

There are four collection data types in the Python programming language:

List is a collection which is ordered and changeable. Allows duplicate members.

Tuple is a collection which is ordered and unchangeable. Allows duplicate members.

Set is a collection which is unordered and unindexed. No duplicate members.

Dictionary is a collection which is ordered* and changeable. No duplicate members.

*As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

Python Lambda

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.

Syntax

lambda arguments : expression

The expression is executed and the result is returned:

Example

Add 10 to argument a, and return the result:

x = lambda a : a + 10

print(x(5))

Lambda functions can take any number of arguments:

Example

Multiply argument a with argument b and return the result:

x = lambda a, b : a * b

print(x(5, 6))

Example

Summarize argument a, b, and c and return the result:

x = lambda a, b, c : a + b + c

print(x(5, 6, 2))

Why Use Lambda Functions?

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

def myfunc(n):

return lambda a : a * n

Use that function definition to make a function that always doubles the number you send in:

Example

def myfunc(n):

return lambda a : a * n

mydoubler = myfunc(2)

print(mydoubler(11))

Or, use the same function definition to make a function that always triples the number you send in:

Example

def myfunc(n):

return lambda a : a * n

mytripler = myfunc(3)

print(mytripler(11))

Or, use the same function definition to make both functions, in the same program:

Example

def myfunc(n):

return lambda a : a * n

mydoubler = myfunc(2)

mytripler = myfunc(3)

print(mydoubler(11))

print(mytripler(11))

Use lambda functions when an anonymous function is required for a short period of time.

Python Arrays

Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.

Arrays

Note: This page shows you how to use LISTS as ARRAYS, however, to work with arrays in Python you will have to import a library, like the NumPy library.

Arrays are used to store multiple values in one single variable:

Example

Create an array containing car names:

cars = ["Ford", "Volvo", "BMW"]

What is an Array?

An array is a special variable, which can hold more than one value at a time.

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

car1 = "Ford"

car2 = "Volvo"

car3 = "BMW"

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.

Access the Elements of an Array

You refer to an array element by referring to the index number.

Example

Get the value of the first array item:

x = cars[0]

Example

Modify the value of the first array item:

cars[0] = "Toyota"

The Length of an Array

Use the len() method to return the length of an array (the number of elements in an array).

Example

Return the number of elements in the cars array:

x = len(cars)

Note: The length of an array is always one more than the highest array index.Looping Array Elements

You can use the for in loop to loop through all the elements of an array.

Example

Print each item in the cars array:

for x in cars:

print(x)

Adding Array Elements

You can use the append() method to add an element to an array.

Example

Add one more element to the cars array:

cars.append("Honda")

Removing Array Elements

You can use the pop() method to remove an element from the array.

Example

Delete the second element of the cars array:

cars.pop(1)

You can also use the remove() method to remove an element from the array.

Example

Delete the element that has the value "Volvo":

cars.remove("Volvo")

Note: The list's remove() method only removes the first occurrence of the specified value.

Array Methods

Python has a set of built-in methods that you can use on lists/arrays.

Method Description

append() Adds an element at the end of the list

clear() Removes all the elements from the list

copy() Returns a copy of the list

count() Returns the number of elements with the specified value

extend() Add the elements of a list (or any iterable), to the end of the current list

index() Returns the index of the first element with the specified value

insert() Adds an element at the specified position

pop() Removes the element at the specified position

remove() Removes the first item with the specified value

reverse() Reverses the order of the list

sort() Sorts the list

Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.

Python Classes and Objects

Python Classes/Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a "blueprint" for creating objects.

Create a Class

To create a class, use the keyword class:

Example

Create a class named MyClass, with a property named x:

class MyClass:

x = 5Create Object

Now we can use the class named MyClass to create objects:

Example

Create an object named p1, and print the value of x:

p1 = MyClass()

print(p1.x)

The __init__() Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:

Example

Create a class named Person, use the __init__() function to assign values for name and age:

class Person:

def __init__(self, name, age):

self.name = name

self.age = age

p1 = Person("John", 36)

print(p1.name)

print(p1.age)

Note: The __init__() function is called automatically every time the class is being used to create a new object.

Object Methods

Objects can also contain methods. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

Example

Insert a function that prints a greeting, and execute it on the p1 object:

class Person:

def __init__(self, name, age):

self.name = name

self.age = age

def myfunc(self):

print("Hello my name is " + self.name)

p1 = Person("John", 36)

p1.myfunc()

Note: The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.

The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.

It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class:

Example

Use the words mysillyobject and abc instead of self:

class Person:

def __init__(mysillyobject, name, age):

mysillyobject.name = name

mysillyobject.age = age

def myfunc(abc):

print("Hello my name is " + abc.name)

p1 = Person("John", 36)

p1.myfunc()

Modify Object Properties

You can modify properties on objects like this:

Example

Set the age of p1 to 40:

p1.age = 40

Delete Object Properties

You can delete properties on objects by using the del keyword:

Example

Delete the age property from the p1 object:

del p1.age

Delete Objects

You can delete objects by using the del keyword:

Example

Delete the p1 object:

del p1

The pass Statement

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

Example

class Person:

pass

Python Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Parent class is the class being inherited from, also called base class.

Child class is the class that inherits from another class, also called derived class.

Create a Parent Class

Any class can be a parent class, so the syntax is the same as creating any other class:

Example

Create a class named Person, with firstname and lastname properties, and a printname method:

class Person:

def __init__(self, fname, lname):

self.firstname = fname

self.lastname = lname

def printname(self):

print(self.firstname, self.lastname)

#Use the Person class to create an object, and then execute the printname method:

x = Person("John", "Doe")

x.printname()

Create a Child Class

To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:

Example

Create a class named Student, which will inherit the properties and methods from the Person class:

class Student(Person):

pass

Note: Use the pass keyword when you do not want to add any other properties or methods to the class.

Now the Student class has the same properties and methods as the Person class.

Example

Use the Student class to create an object, and then execute the printname method:

x = Student("Mike", "Olsen")

x.printname()

Add the __init__() Function

So far we have created a child class that inherits the properties and methods from its parent.

We want to add the __init__() function to the child class (instead of the pass keyword).

Note: The __init__() function is called automatically every time the class is being used to create a new object.

Example

Add the __init__() function to the Student class:

class Student(Person):

def __init__(self, fname, lname):

#add properties etc.

When you add the __init__() function, the child class will no longer inherit the parent's __init__() function.

Note: The child's __init__() function overrides the inheritance of the parent's __init__() function.

To keep the inheritance of the parent's __init__() function, add a call to the parent's __init__() function:

Example

class Student(Person):

def __init__(self, fname, lname):

Person.__init__(self, fname, lname)

Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__() function.

Use the super() Function

Python also has a super() function that will make the child class inherit all the methods and properties from its parent:

Example

class Student(Person):

def __init__(self, fname, lname):

super().__init__(fname, lname)

By using the super() function, you do not have to use the name of the parent element, it will automatically inherit the methods and properties from its parent.

Add Properties

Example

Add a property called graduationyear to the Student class:

class Student(Person):

def __init__(self, fname, lname):

super().__init__(fname, lname)

self.graduationyear = 2019

In the example below, the year 2019 should be a variable, and passed into the Student class when creating student objects. To do so, add another parameter in the __init__() function:

Example

Add a year parameter, and pass the correct year when creating objects:

class Student(Person):

def __init__(self, fname, lname, year):

super().__init__(fname, lname)

self.graduationyear = year

x = Student("Mike", "Olsen", 2019)

Add Methods

Example

Add a method called welcome to the Student class:

class Student(Person):

def __init__(self, fname, lname, year):

super().__init__(fname, lname)

self.graduationyear = year

def welcome(self):

print("Welcome", self.firstname, self.lastname, "to the class of", self.graduationyear)

If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.

Python Iterators

An iterator is an object that contains a countable number of values.

An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.

Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().

Iterator vs Iterable

Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.

All these objects have a iter() method which is used to get an iterator:

Example

Return an iterator from a tuple, and print each value:

mytuple = ("apple", "banana", "cherry")

myit = iter(mytuple)

print(next(myit))

Even strings are iterable objects, and can return an iterator:

Example

Strings are also iterable objects, containing a sequence of characters:

mystr = "banana"

myit = iter(mystr)

print(next(myit))

Looping Through an Iterator

We can also use a for loop to iterate through an iterable object:

Example

Iterate the values of a tuple:

mytuple = ("apple", "banana", "cherry")

for x in mytuple:

print(x)

Example

Iterate the characters of a string:

mystr = "banana"

for x in mystr:

print(x)

The for loop actually creates an iterator object and executes the next() method for each loop.

Create an Iterator

To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.

As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__(), which allows you to do some initializing when the object is being created.

The __iter__() method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.

The __next__() method also allows you to do operations, and must return the next item in the sequence.

Example

Create an iterator that returns numbers, starting with 1, and each sequence will increase by one (returning 1,2,3,4,5 etc.):

class MyNumbers:

def __iter__(self):

self.a = 1

return self

def __next__(self):

x = self.a

self.a += 1

return x

myclass = MyNumbers()

myiter = iter(myclass)

print(next(myiter))

StopIteration

The example above would continue forever if you had enough next() statements, or if it was used in a for loop.

To prevent the iteration to go on forever, we can use the StopIteration statement.

In the __next__() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:

Example

Stop after 20 iterations:

class MyNumbers:

def __iter__(self):

self.a = 1

return self

def __next__(self):

if self.a <= 20:

x = self.a

self.a += 1

return x

else:

raise StopIteration

myclass = MyNumbers()

myiter = iter(myclass)

for x in myiter:

print(x)

Python Scope

A variable is only available from inside the region it is created. This is called scope.

Local Scope

A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.

Example

A variable created inside a function is available inside that function:

def myfunc():

x = 300

print(x)

myfunc()

Function Inside Function

As explained in the example above, the variable x is not available outside the function, but it is available for any function inside the function:

Example

The local variable can be accessed from a function within the function:

def myfunc():

x = 300

def myinnerfunc():

print(x)

myinnerfunc()

myfunc()

Global Scope

A variable created in the main body of the Python code is a global variable and belongs to the global scope.

Global variables are available from within any scope, global and local.

Example

A variable created outside of a function is global and can be used by anyone:

x = 300

def myfunc():

print(x)

myfunc()

print(x)

Naming Variables

If you operate with the same variable name inside and outside of a function, Python will treat them as two separate variables, one available in the global scope (outside the function) and one available in the local scope (inside the function):

Example

The function will print the local x, and then the code will print the global x:

x = 300

def myfunc():

x = 200

print(x)

myfunc()

print(x)

Global Keyword

If you need to create a global variable, but are stuck in the local scope, you can use the global keyword.

The global keyword makes the variable global.

Example

If you use the global keyword, the variable belongs to the global scope:

def myfunc():

global x

x = 300

myfunc()

print(x)

Also, use the global keyword if you want to make a change to a global variable inside a function.

Example

To change the value of a global variable inside a function, refer to the variable by using the global keyword:

x = 300

def myfunc():

global x

x = 200

myfunc()

print(x)

Python Modules

What is a Module?

Consider a module to be the same as a code library.

A file containing a set of functions you want to include in your application.

Create a Module

To create a module just save the code you want in a file with the file extension .py:

Example

Save this code in a file named mymodule.py

def greeting(name):

print("Hello, " + name)

Use a Module

Now we can use the module we just created, by using the import statement:

Example

Import the module named mymodule, and call the greeting function:

import mymodule

mymodule.greeting("Jonathan")

Note: When using a function from a module, use the syntax: module_name.function_name.

Variables in Module

The module can contain functions, as already described, but also variables of all types (arrays, dictionaries, objects etc):

Example

Save this code in the file mymodule.py

person1 = {

"name": "John",

"age": 36,

"country": "Norway"

}

Example

Import the module named mymodule, and access the person1 dictionary:

import mymodule

a = mymodule.person1["age"]

print(a)

Naming a Module

You can name the module file whatever you like, but it must have the file extension .py

Re-naming a Module

You can create an alias when you import a module, by using the as keyword:

Example

Create an alias for mymodule called mx:

import mymodule as mx

a = mx.person1["age"]

print(a)

Built-in Modules

There are several built-in modules in Python, which you can import whenever you like.

Example

Import and use the platform module:

import platform

x = platform.system()

print(x)

Using the dir() Function

There is a built-in function to list all the function names (or variable names) in a module. The dir() function:

Example

List all the defined names belonging to the platform module:

import platform

x = dir(platform)

print(x)

Note: The dir() function can be used on all modules, also the ones you create yourself.

Import From Module

You can choose to import only parts from a module, by using the from keyword.

Example

The module named mymodule has one function and one dictionary:

def greeting(name):

print("Hello, " + name)

person1 = {

"name": "John",

"age": 36,

"country": "Norway"

}

Example

Import only the person1 dictionary from the module:

from mymodule import person1

print (person1["age"])

Note: When importing using the from keyword, do not use the module name when referring to elements in the module. Example: person1["age"], not mymodule.person1["age"]

Python Datetime

Python Dates
A date in Python is not a data type of its own, but we can import a module named datetime to work with dates as date objects.
Example
Import the datetime module and display the current date:
import datetime
x = datetime.datetime.now()
print(x)
Date Output
When we execute the code from the example above the result will be:
2021-09-24 02:04:28.441207
The date contains year, month, day, hour, minute, second, and microsecond.
The datetime module has many methods to return information about the date object.
Here are a few examples, you will learn more about them later in this chapter:
Example
Return the year and name of weekday:
import datetime
x = datetime.datetime.now()
print(x.year)
print(x.strftime("%A"))
Creating Date Objects
To create a date, we can use the datetime() class (constructor) of the datetime module.
The datetime() class requires three parameters to create a date: year, month, day.
Example
Create a date object:
import datetime
x = datetime.datetime(2020, 5, 17)
print(x)

The datetime() class also takes parameters for time and timezone (hour, minute, second, microsecond, tzone), but they are optional, and has a default value of 0, (None for timezone).

The strftime() Method

The datetime object has a method for formatting date objects into readable strings.

The method is called strftime(), and takes one parameter, format, to specify the format of the returned string:

Example

Display the name of the month:

import datetime

x = datetime.datetime(2018, 6, 1)

print(x.strftime("%B"))

A reference of all the legal format codes:

Directive Description Example

%a Weekday, short version Wed

%A Weekday, full version Wednesday

%w Weekday as a number 0-6, 0 is Sunday 3

%d Day of month 01-31 31

%b Month name, short version Dec

%B Month name, full version December

%m Month as a number 01-12 12

%y Year, short version, without century 18

%Y Year, full version 2018

%H Hour 00-23 17

%I Hour 00-12 05

%p AM/PM PM

%M Minute 00-59 41

%S Second 00-59 08

%f Microsecond 000000-999999 548513

%z UTC offset +0100

%Z Timezone CST

%j Day number of year 001-366 365

%U Week number of year, Sunday as the first day of week, 00-53 52

%W Week number of year, Monday as the first day of week, 00-53 52

%c Local version of date and time Mon Dec 31 17:41:00 2018

%C Century 20

%x Local version of date 12/31/18

%X Local version of time 17:41:00

%% A % character %

%G ISO 8601 year 2018

%u ISO 8601 weekday (1-7) 1

%V ISO 8601 weeknumber (01-53) 01

Python Math

Python has a set of built-in math functions, including an extensive math module, that allows you to perform mathematical tasks on numbers.

Built-in Math Functions

The min() and max() functions can be used to find the lowest or highest value in an iterable:

Example

x = min(5, 10, 25)

y = max(5, 10, 25)

print(x)

print(y)

The abs() function returns the absolute (positive) value of the specified number:

Example

x = abs(-7.25)

print(x)

The pow(x, y) function returns the value of x to the power of y (xy).

Example

Return the value of 4 to the power of 3 (same as 4 * 4 * 4):

x = pow(4, 3)

print(x)

The Math Module

Python has also a built-in module called math, which extends the list of mathematical functions.

To use it, you must import the math module:

import math

When you have imported the math module, you can start using methods and constants of the module.

The math.sqrt() method for example, returns the square root of a number:

Example

import math

x = math.sqrt(64)

print(x)

The math.ceil() method rounds a number upwards to its nearest integer, and the math.floor() method rounds a number downwards to its nearest integer, and returns the result:

Example

import math

x = math.ceil(1.4)

y = math.floor(1.4)

print(x) # returns 2

print(y) # returns 1

The math.pi constant, returns the value of PI (3.14...):

Example

import math

x = math.pi

print(x)

Complete Math Module Reference

In our Math Module Reference you will find a complete reference of all methods and constants that belongs to the Math module.

Python JSON

JSON is a syntax for storing and exchanging data.

JSON is text, written with JavaScript object notation.

JSON in Python

Python has a built-in package called json, which can be used to work with JSON data.

Example

Import the json module:

import json

Parse JSON - Convert from JSON to Python

If you have a JSON string, you can parse it by using the json.loads() method.

The result will be a Python dictionary.

Example

Convert from JSON to Python:

import json

# some JSON:

x = '{ "name":"John", "age":30, "city":"New York"}'

# parse x:

y = json.loads(x)

# the result is a Python dictionary:

print(y["age"])

Convert from Python to JSON

If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.

Example

Convert from Python to JSON:

import json

# a Python object (dict):

x = {

"name": "John",

"age": 30,

"city": "New York"

}

# convert into JSON:

y = json.dumps(x)

# the result is a JSON string:

print(y)

You can convert Python objects of the following types, into JSON strings:

dict

list

tuple

string

int

float

True

False

None

Example

Convert Python objects into JSON strings, and print the values:

import json

print(json.dumps({"name": "John", "age": 30}))

print(json.dumps(["apple", "bananas"]))

print(json.dumps(("apple", "bananas")))

print(json.dumps("hello"))

print(json.dumps(42))

print(json.dumps(31.76))

print(json.dumps(True))

print(json.dumps(False))

print(json.dumps(None))

When you convert from Python to JSON, Python objects are converted into the JSON (JavaScript) equivalent:

Python JSON

dict Object

list Array

tuple Array

str String

int Number

float Number

True true

False false

None null

Example

Convert a Python object containing all the legal data types:

import json

x = {

"name": "John",

"age": 30,

"married": True,

"divorced": False,

"children": ("Ann","Billy"),

"pets": None,

"cars": [

{"model": "BMW 230", "mpg": 27.5},

{"model": "Ford Edge", "mpg": 24.1}

]

}

print(json.dumps(x))

Format the Result

The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.

The json.dumps() method has parameters to make it easier to read the result:

Example

Use the indent parameter to define the numbers of indents:

json.dumps(x, indent=4)

You can also define the separators, default value is (", ", ": "), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:

Example

Use the separators parameter to change the default separator:

json.dumps(x, indent=4, separators=(". ", " = "))

Order the Result

The json.dumps() method has parameters to order the keys in the result:

Example

Use the sort_keys parameter to specify if the result should be sorted or not:

json.dumps(x, indent=4, sort_keys=True)

Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

Import the re module:

import re

RegEx in Python

When you have imported the re module, you can start using regular expressions:

Example

Search the string to see if it starts with "The" and ends with "Spain":

import re

txt = "The rain in Spain"

x = re.search("^The.*Spain$", txt)

RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

Function Description

findall Returns a list containing all matches

search Returns a Match object if there is a match anywhere in the string

split Returns a list where the string has been split at each match

sub Replaces one or many matches with a string

Metacharacters

Metacharacters are characters with a special meaning:

Character Description Example

[] A set of characters "[a-m]"

\ Signals a special sequence (can also be used to escape special characters) "\d"

. Any character (except newline character) "he..o"

^ Starts with "^hello"

$ Ends with "world$"

* Zero or more occurrences "aix*"

+ One or more occurrences "aix+"

{} Exactly the specified number of occurrences "al{2}"

| Either or "falls|stays"

() Capture and group

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character Description Example

\A Returns a match if the specified characters are at the beginning of the string "\AThe"

\b Returns a match where the specified characters are at the beginning or at the end of a word

(the "r" in the beginning is making sure that the string is being treated as a "raw string") r"\bain"

r"ain\b"

\B Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word

(the "r" in the beginning is making sure that the string is being treated as a "raw string") r"\Bain"

r"ain\B"

\d Returns a match where the string contains digits (numbers from 0-9) "\d"

\D Returns a match where the string DOES NOT contain digits "\D"

\s Returns a match where the string contains a white space character "\s"

\S Returns a match where the string DOES NOT contain a white space character "\S"

\w Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character) "\w"

\W Returns a match where the string DOES NOT contain any word characters "\W"

\Z Returns a match if the specified characters are at the end of the string "Spain\Z"

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set Description

[arn] Returns a match where one of the specified characters (a, r, or n) are present

[a-n] Returns a match for any lower case character, alphabetically between a and n

[^arn] Returns a match for any character EXCEPT a, r, and n

[0123] Returns a match where any of the specified digits (0, 1, 2, or 3) are present

[0-9] Returns a match for any digit between 0 and 9

[0-5][0-9] Returns a match for any two-digit numbers from 00 and 59

[a-zA-Z] Returns a match for any character alphabetically between a and z, lower case OR upper case

[+] In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string

The findall() Function

The findall() function returns a list containing all matches.

Example

Print a list of all matches:

import re

txt = "The rain in Spain"

x = re.findall("ai", txt)

print(x)

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

Example

Return an empty list if no match was found:

import re

txt = "The rain in Spain"

x = re.findall("Portugal", txt)

print(x)

The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Example

Search for the first white-space character in the string:

import re

txt = "The rain in Spain"

x = re.search("\s", txt)

print("The first white-space character is located in position:", x.start())

If no matches are found, the value None is returned:

Example

Make a search that returns no match:

import re

txt = "The rain in Spain"

x = re.search("Portugal", txt)

print(x)

The split() Function

The split() function returns a list where the string has been split at each match:

Example

Split at each white-space character:

import re

txt = "The rain in Spain"

x = re.split("\s", txt)

print(x)

You can control the number of occurrences by specifying the maxsplit parameter:

Example

Split the string only at the first occurrence:

import re

txt = "The rain in Spain"

x = re.split("\s", txt, 1)

print(x)

The sub() Function

The sub() function replaces the matches with the text of your choice:

Example

Replace every white-space character with the number 9:

import re

txt = "The rain in Spain"

x = re.sub("\s", "9", txt)

print(x)

You can control the number of replacements by specifying the count parameter:

Example

Replace the first 2 occurrences:

import re

txt = "The rain in Spain"

x = re.sub("\s", "9", txt, 2)

print(x)

Match Object

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned, instead of the Match Object.

Example

Do a search that will return a Match Object:

import re

txt = "The rain in Spain"

x = re.search("ai", txt)

print(x) #this will print an object

The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match.

.string returns the string passed into the function

.group() returns the part of the string where there was a match

Example

Print the position (start- and end-position) of the first match occurrence.

The regular expression looks for any words that starts with an upper case "S":

import re

txt = "The rain in Spain"

x = re.search(r"\bS\w+", txt)

print(x.span())

Example

Print the string passed into the function:

import re

txt = "The rain in Spain"

x = re.search(r"\bS\w+", txt)

print(x.string)

Example

Print the part of the string where there was a match.

The regular expression looks for any words that starts with an upper case "S":

import re

txt = "The rain in Spain"

x = re.search(r"\bS\w+", txt)

print(x.group())

Note: If there is no match, the value None will be returned, instead of the Match Object.

Python PIP

What is PIP?

PIP is a package manager for Python packages, or modules if you like.

Note: If you have Python version 3.4 or later, PIP is included by default.

What is a Package?

A package contains all the files you need for a module.

Modules are Python code libraries you can include in your project.

Check if PIP is Installed

Navigate your command line to the location of Python's script directory, and type the following:

Example

Check PIP version:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip --version

Install PIP

If you do not have PIP installed, you can download and install it from this page: https://pypi.org/project/pip/

Download a Package

Downloading a package is very easy.

Open the command line interface and tell PIP to download the package you want.

Navigate your command line to the location of Python's script directory, and type the following:

Example

Download a package named "camelcase":

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip install camelcase

Now you have downloaded and installed your first package!

Using a Package

Once the package is installed, it is ready to use.

Import the "camelcase" package into your project.

Example

Import and use "camelcase":

import camelcase

c = camelcase.CamelCase()

txt = "hello world"

print(c.hump(txt))

Find Packages

Find more packages at https://pypi.org/.

Remove a Package

Use the uninstall command to remove a package:

Example

Uninstall the package named "camelcase":

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip uninstall camelcase

The PIP Package Manager will ask you to confirm that you want to remove the camelcase package:

Uninstalling camelcase-02.1:

Would remove:

c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camecase-0.2-py3.6.egg-info

c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camecase\*

Proceed (y/n)?

Press y and the package will be removed.

List Packages

Use the list command to list all the packages installed on your system:

Example

List installed packages:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip list

Result:

Package Version

-----------------------

camelcase 0.2

mysql-connector 2.1.6

pip 18.1

pymongo 3.6.1

setuptools 39.0.1

Python Try Except

The try block lets you test a block of code for errors.

The except block lets you handle the error.

The finally block lets you execute code, regardless of the result of the try- and except blocks.

Exception Handling

When an error occurs, or exception as we call it, Python will normally stop and generate an error message.

These exceptions can be handled using the try statement:

Example

The try block will generate an exception, because x is not defined:

try:

print(x)

except:

print("An exception occurred")

Since the try block raises an error, the except block will be executed.

Without the try block, the program will crash and raise an error:

Example

This statement will raise an error, because x is not defined:

print(x)

Many Exceptions

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:

Example

Print one message if the try block raises a NameError and another for other errors:

try:

print(x)

except NameError:

print("Variable x is not defined")

except:

print("Something else went wrong")

Else

You can use the else keyword to define a block of code to be executed if no errors were raised:

Example

In this example, the try block does not generate any error:

try:

print("Hello")

except:

print("Something went wrong")

else:

print("Nothing went wrong")

Finally

The finally block, if specified, will be executed regardless if the try block raises an error or not.

Example

try:

print(x)

except:

print("Something went wrong")

finally:

print("The 'try except' is finished")

This can be useful to close objects and clean up resources:

Example

Try to open and write to a file that is not writable:

try:

f = open("demofile.txt")

try:

f.write("Lorum Ipsum")

except:

print("Something went wrong when writing to the file")

finally:

f.close()

except:

print("Something went wrong when opening the file")

The program can continue, without leaving the file object open.

Raise an exception

As a Python developer you can choose to throw an exception if a condition occurs.

To throw (or raise) an exception, use the raise keyword.

Example

Raise an error and stop the program if x is lower than 0:

x = -1

if x < 0:

raise Exception("Sorry, no numbers below zero")

The raise keyword is used to raise an exception.

You can define what kind of error to raise, and the text to print to the user.

Example

Raise a TypeError if x is not an integer:

x = "hello"

if not type(x) is int:

raise TypeError("Only integers are allowed")