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.
The __str__() Function
The __str__() function controls what should be returned when the class object is represented as a string.
If the __str__() function is not set, the string representation of the object is returned:
Example
The string representation of an object WITHOUT the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
p1 = Person("John", 36)
print(p1)
Try it Yourself »Example
The string representation of an object WITH the __str__() function:
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
def __str__(self):
return f"{self.name}({self.age})"
p1 = Person("John", 36)
print(p1)
Try it Yourself »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:
Delete Object Properties
You can delete properties on objects by using the del keyword:
Delete Objects
You can delete objects by using the del keyword:
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.
Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.
Lists are created using square brackets:
Example
Create a List:
thislist = ["apple", "banana", "cherry"]
print(thislist)
List Items
List items are ordered, changeable, and allow duplicate values.
List items are indexed, the first item has index [0], the second item has index [1] etc.
Ordered
When we say that lists are ordered, it means that the items have a defined order, and that order will not change.
If you add new items to a list, the new items will be placed at the end of the list.
Note: There are some list methods that will change the order, but in general: the order of the items will not change.
Changeable
The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.
Allow Duplicates
Since lists are indexed, lists can have items with the same value:
Example
Lists allow duplicate values:
thislist = ["apple", "banana", "cherry", "apple", "cherry"]
print(thislist)
List Length
To determine how many items a list has, use the len() function:
Example
Print the number of items in the list:
thislist = ["apple", "banana", "cherry"]
print(len(thislist))
List Items - Data Types
List items can be of any data type:
Example
String, int and boolean data types:
list1 = ["apple", "banana", "cherry"]
list2 = [1, 5, 7, 9, 3]
list3 = [True, False, False]
A list can contain different data types:
Example
A list with strings, integers and boolean values:
list1 = ["abc", 34, True, 40, "male"]
Try it Yourself »type()
From Python's perspective, lists are defined as objects with the data type 'list':
Example
What is the data type of a list?
mylist = ["apple", "banana", "cherry"]
print(type(mylist))
The list() Constructor
It is also possible to use the list() constructor when creating a new list.
Example
Using the list() constructor to make a List:
thislist = list(("apple", "banana", "cherry")) # note the double round-brackets
print(thislist)
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, unchangeable*, and unindexed. No duplicate members.
- Dictionary is a collection which is ordered** and changeable. No duplicate members.
*Set items are unchangeable, but you can remove and/or add items whenever you like.
**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.
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.