Python Variable

Variable and Value

• A variable is a named memory location where data can be stored. For example: roll_no, amount, name.
• A value is the data stored in a variable, which can be a string, numeric value, etc. For example: "Sara", 120, 25.36.

Key Points About Python Variables:

• Variables are created when they are first assigned a value.
• They must be assigned before being referenced.
• The value stored in a variable can be accessed or updated later.
• No declaration is required before using a variable.
• The type of the variable (e.g., string, int, float) is determined automatically by Python based on the value assigned.
• Python manages memory allocation based on the data type of the variable.

Python Variable Name Rules

• Must begin with a letter (a-z, A-Z) or an underscore (_).
• Subsequent characters can be letters, numbers, or underscores.
• Case-sensitive: age, Age, and AGE are considered different variables.
• Can be of any reasonable length.
• Reserved words (like if, for, while, etc.) cannot be used as variable names.

Good Variable Naming Practices

• Choose meaningful names, such as roll_no, which is clearer than rn.
• Avoid unnecessarily long variable names (e.g., Roll_no_of_a_student is too long).
• Be consistent in your naming convention: roll_no or RollNo, but not both.
• Start variable names with an underscore (_) when you need a special case or private variables.

Python Assignment Statements

Assignment statements create variables and assign values to them. The basic syntax for an assignment is:

Syntax:

<variable> = <expr>

Where the equal sign (=) is used to assign value (right side) to a variable name (left side). See the following statements :

>>> Item_name = "Computer" #A String
>>> Item_qty = 10 #An Integer
>>> Item_value = 1000.23 #A floating point
>>> print(Item_name)
Computer
>>> print(Item_qty)
10
>>> print(Item_value)
1000.23
>>> 

Note: Assignment reads right to left. This is why a = 12 works, but 12 = a results in a syntax error.

Example:

a = 12 is correct, but 12 = a does not make sense to Python, which creates a syntax error. Check it in Python Shell.

>>> a = 12
>>> 12 = a
SyntaxError: can't assign to literal
>>> 

Multiple Assignment

The basic assignment statement works for a single variable and a single expression. You can also assign a single value to more than one variables simultaneously.

Syntax:

var1=var2=var3...varn= = <expr>

Example:

x = y = z = 1 

Now check the individual value in Python Shell.

>>> x = y = z = 1
>>> print(x)
1
>>> print(y)
1
>>> print(z)
1
>>> 

Alternatively, you can assign multiple values to multiple variables in a single line.

Syntax:

<var>, <var>, ..., <var> = <expr>, <expr>, ..., <expr>

Example:

x, y, z = 1, 2, "abcd"

In the above example x, y and z simultaneously get the new values 1, 2 and "abcd".

>>> x,y,z = 1,2,"abcd"
>>> print(x)
1
>>> print(y)
2
>>> print(z)
abcd 

You can reuse variable names by simply assigning a new value to them :

>>> x = 100
>>> print(x)
100
>>> x = "Python"
>>> print(x)
Python
>>>  

Other ways to define value

Python allows formatting of large numbers and decimal values for better readability.

>>> five_millions = 5_000_000
>>> five_millions

Output:

5000000

>>> small_int = .35
>>> small_int

Output:

0.35

>>> c_thousand = 10e3
>>> c_thousand

Output:

10000.0

Swap variables

In Python, you can swap the values of two variables in a single line.

Syntax:

var1, var2 = var2, var1

Example:

>>> x = 10
>>> y = 20
>>> print(x)
10
>>> print(y)
20
>>> x, y = y, x
>>> print(x)
20
>>> print(y)
10
>>>

Local and global variables in Python

• Global variables are accessible throughout the entire program, even within functions.
• Local variables are defined within a function and cannot be accessed outside it.
• A variable is assumed to be local unless explicitly declared as global using the global keyword.

Example:

var1 = "Python"
def func1():
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  Python

You can use a global variable in other functions by declaring it as global keyword :

Example:

def func1():
    global var1
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  PHP

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