2.4. Python Variables#

Defination: A variable is a container for a value, which can be of various types.

A variable is created the moment you first assign a value to it.

Variable names are case-sensitive.

Variable names can be short (like x and y) or more descriptive (age, carname, total_volume).

2.4.1. Rules for Python variables:#

  1. A variable name must start with a letter or the underscore character

  2. A variable name cannot start with a number

  3. A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )

  4. Variable names are case-sensitive (age, Age and AGE are three different variables)

2.4.2. Creating Varibales#

Python has no command for declaring a variable.

A variable is created the moment you first assign a value to it.

# Example
x = 4 # x is of type int
print(x)

4

Python allows you to assign values to multiple variables in one line:

# Example
x, y, z = "Orange", "Banana", "Cherry"
print(x)
print(y)
print(z)

Orange
Banana
Cherry

And you can assign the same value to multiple variables in one line:

# Example
x = y = z = "Orange"
print(x)
print(y)
print(z)

Orange
Orange
Orange

To combine both text and a variable, Python uses the + character:

# Example
x = "awesome"
print("Python is " + x)

Python is awesome

You can also use the + character to add a variable to another variable:

# Example
x = "Python is "
y = "awesome"
z =  x + y
print(z)

Python is awesome

For numbers, the + character works as a mathematical operator:

# Example
x = 5
y = 10
print(x + y)

15

If you try to combine a string and a number, Python will give you an error:

# Example
x = 5
y = "John"
print(x + y)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In [7], line 4
      2 x = 5
      3 y = "John"
----> 4 print(x + y)

TypeError: unsupported operand type(s) for +: 'int' and 'str'

To add a space between them, add a ” “:

# Example
x = 5
y = "John"
print(x + y)

2.4.3. Casting#

If you want to specify the data type of a variable, this can be done with casting.

# Example
x = str(3) # x will be '3'
y = int(3) # y will be 3
z = float(3) # z will be 3.0

2.4.4. Get the Type#

You can get the data type of a variable with the type() function.

# Example
x = 5
y = "John"
print(type(x))
print(type(y))

<class 'int'>
<class 'str'>

2.4.5. Single or Double Quotes?#

String variables can be declared either by using single or double quotes:

# Example
x = "John"
# is the same as
x = 'John'

2.4.6. Case-Sensitive#

Variable names are case-sensitive.

# Example
a = 4
A = "Sally"
#A will not overwrite a
print(a)
print(A)

4
Sally

2.4.8. Illegal variable names#

  1. 2myvar

  2. my-var

  3. my var

# Illegal variable names with examples
2myvar = "Apple"
print(2myvar)

  Input In [18]
    2myvar = "Apple"
     ^
SyntaxError: invalid syntax

my-var = "Apple"
print(my-var)

  Input In [33]
    my-var = "Apple"
    ^
SyntaxError: cannot assign to operator

my var = "Apple"
print(my var)

  Input In [41]
    my var = "Apple"
       ^
SyntaxError: invalid syntax

2.4.9. Unpacking a Collection#

Unpacking a collection of values into variables is called unpacking.

# Example
animals = ["Dog", "Cat", "Bird"]
(x, y, z) = animals
print(x)
print(y)
print(z)

Dog
Cat
Bird

2.4.10. Python Output Variables#

Defination: The Python print statement is often used to output variables.

Python uses the print statement to output data to the standard output device (screen).

# Example:
x = "awesome"
print("Python is " + x)

Python is awesome

We can also output multiple variables at the same time

# Example:
x = "Python is "
y = "awesome"
z =  x + y
print(x + y)

Python is awesome

2.4.11. Python Global Variables#

Defination: Variables that are created outside of a function (as in all of the examples above) are known as global variables.

x = "awesome"

def myfunc():
    print("Python is " + x)

myfunc()

Python is awesome

Global variables can be used by everyone, both inside of functions and outside.

# Example:
x = "awesome"

def myfunc():
    x = "fantastic"
    print("Python is " + x)

myfunc()

print("Python is " + x)

Python is fantastic
Python is awesome

Defining a global variable inside a function is done by using the global keyword.

# Example:
x = "awesome"

def myfunc():
    global x
    x = "fantastic"

myfunc()

print("Python is " + x)

Python is fantastic