python variables

Python Variables

Last Updated | Jan, 04, 2021 By Kingsley Ijomah
In this python tutorial post, you will learn about Python variables and assignments and how they are used to store different data types in memory, we will also explore what is a legal variable name and an illegal variable.

We will also look at a simple command to show you all reserved keywords in Python, by the end of this post you will be very well versed in Python variables as well as understand how Python variables act as a pointer to objects in memory and at which point Python's garbage collector is required to reclaim allocated memory.

Variables are containers used to hold values, this makes it possible to reference the contained value multiple times throughout the execution life cycle of the program.

Without variables, it will be a real challenge to write computer programmes.

Topics covered in this post:

  1. Variable Assignment
  2. Variable Names
  3. Reserved Keywords
  4. Get Variable Type
  5. Object Identity
  6. Object Reference
  7. Final Thoughts

Let's explore how to use variables...

python variable explained

Variable Assignment

To assign a value to a variable you use the = operator, the variable name would be on the left side of the = operator and the value to the right. Below we are going to explore different ways of assigning a value to a variable.

code
# add "Kingsley" to the variable `name`
name = "Kingsley"

# output => Kingsley
print(name)

# assign same value to multiple variables on the same line
a = b = c = 'cat'

print(a) # output => cat
print(b) # output => cat
print(c) # output => cat

# assign a different value to multiple variables on the same line
a, b, c = 'cat', 'dog', 'horse'

print(a) # output => cat
print(b) # output => dog
print(c) # output => horse

# reuse variable name, last assignment is printed
colour = 'Red'
colour = 'Blue'

print(colour) # output => Blue

Variable Names

Above you learnt how to assign values to variables, now we are going to learn the rules around variables names, not all names are legal, let's have a look at some examples below.

code
# legal variable names
firstname = "Kingsley"
first_name = "Kingsley"
_first_name = "Kingsley"
firstName = "Kingsley"
firstname2 = "Kingsley"
FIRSTNAME = "Kingsley"

# illegal variable names
2firstname = "Kingsley"
first-name = "Kingsley"
first name = "Kingsley"

Underscores are allowed in variable names, numbers are allowed as long as it is not placed as the first character in the variable name, you cannot use dashes or have spaces in the variable names.

Using an invalid variable name will product an invalid syntax error: SyntaxError: invalid syntax

Reserved Keywords

There are additional invalid names when it comes to creating variables, and these are the reserved keywords. Python 3.8 has 35 reserved keywords that you cannot use as variable names.

Below is a list produced by typing into python console: help("keywords")

code
'''
False               class               from                or
None                continue            global              pass
True                def                 if                  raise
and                 del                 import              return
as                  elif                in                  try
assert              else                is                  while
async               except              lambda              with
await               finally             nonlocal            yield
break               for                 not                 
'''

False = 'hello' # output => SyntaxError: invalid syntax
class = 30 # output => SyntaxError: invalid syntax
from = 'hi' # output => SyntaxError: invalid syntax

Get Variable Type

In some other languages, variables a statically typed, which means that once you create them you also declare what type of data they accept and therefore they cannot accept any other data types.

In python things are done differently, a variable can change its type at any point during execution, let's have a look at how to override a variable value type and how to print out the type of value held by any variable using the type function.

code
# first assign a string to var
var = "Hello World!"

print(var) # output => Hello World!
type(var) # output => <class 'str'> 

# assign an int to var
var = 23

print(var) # output => 23
type(var) # output => <class 'int'>

Above shows that we can create a var assign a value of type string into it and then override that and assign a value to type int to the same variable and it is all perfectly legal.

Object Identity

In python every object that is created is given a unique number for identifying that object, so when we create a variable and assign value into it, that variable will also have a unique id associated to it.

code
# print the id for int stored in a variable
score = 400

id(score) # example output => 4341576592

As you can see from the above example, calling id() method will give you a unique identifier. But what happens as int he example below?

code
# score variable is saved into pb by reference
score = 400
pb = score

id(score) # output => 4342358576
id(pb) # output => 4342358576

As you can see from the output above the id printed for both score and pb are exactly the same, your id will be different values compared to mine since they are unique for each computer, but they will be identical to each other.

Python simply points to the same reference, so both score and pb are pointing to the same object in this case, below we will look at object reference in more detail.

Object Reference

When you assign a value to a variable ( score = 100 ) the following happens:

  • Int object containing 100 is created
  • The variable score points to it.

If you then assign the variable score to another variable pb, then below is what happens:

  • score variable point to int object containing 100
  • pb variable also point to the same int object containing 100
code
'''
both score and pb point to same int object

score ---------> int 100 <--------- pb
'''
score = 100
pb = score

type(score) # output => <class 'int'>
type(pb) # output => <class 'int'>
score # output => 100
pb # output => 100


'''
point pb to new location (int 20), score remains pointing to int 100

pb ---------> int 20
score ---------> int 100
'''
pb = 20

type(score) # output => <class 'int'>
type(pb) # output => <class 'int'>
score # output => 100
pb # output => 20


'''
point score to str 'Completed' this means int 100 still exists but not being
referenced by any variable and therefore at some point will be garbage collected by Python.

pb ---------> int 20
score ---------> str 'Completed'
---------> int 100
'''
score = 'Completed'

type(score) # output => <class 'str'>
type(pb) # output => <class 'int'>
score # output => 'Completed'
pb # output => 20

You will come across object lifecycle from time to time, as you saw from above, when we create int or str objects we reference them by pointing a variable to it, we can increase the number of references to each object as seen above with score and pb pointing to the same object int 100.

When the reference goes down to zero, e.g no variable is pointing to that object, then Python will reclaim that allocated memory using a process known as garbage collection.

Final Thoughts

In this blog post you learnt the basics of Python variables, explored how to assign values to them, multiple values in one line etc, you also learnt about object reference and object identification.

Read this again, book mark this page if you need to, know it by heart because Python variables will play a big role as you continue on your journey into learning to code Python.

Please share this post with your audience.

Happy coding...

MY STORY

My name is Kingsley Ijomah, I am the founder of CODEHANCE, an online education platform built with you in mind, a place where I express my gratitude to a skill ( coding ) which has changed my life completely.

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