Python

Python Variables

Variables are one of the most fundamental concepts in any programming language. In Python, variables are used to store data that can be referenced and manipulated in a program. This section will cover the various aspects of Python variables, including their types, naming conventions, scope, and best practices.

What is a Variable?

A variable in Python is a name that refers to a memory location where data is stored. The value of a variable can change throughout the execution of a program. Variables are essential for writing flexible and dynamic code.

Creating and Assigning Variables

In Python, variables are created when you assign a value to them using the assignment operator (=).

x = 10
y = "Hello"
z = 3.14

In the examples above:

  • x is assigned the integer value 10.
  • y is assigned the string value "Hello".
  • z is assigned the float value 3.14.

Variable Naming Conventions

Python has specific rules and conventions for naming variables:

  1. Must start with a letter or an underscore (_):
  • Valid: _my_var, myVar
  • Invalid: 1myVar, -myVar

2. Can only contain alphanumeric characters and underscores:

  • Valid: var1, my_var
  • Invalid: my-var, my var

3. Case-sensitive:

  • myVar and myvar are considered different variables.

4. Cannot use Python reserved keywords: Examples of reserved keywords are class, finally, return, is, try, except, etc.

Dynamic Typing

Python is a dynamically typed language, meaning you don’t have to declare the type of a variable explicitly. The type is inferred based on the value assigned.

a = 5        # a is an integer
a = "Hello"  # a is now a string
a = 3.14     # a is now a float

Multiple Assignments

You can assign values to multiple variables in a single statement.

a, b, c = 1, 2, "Hello"

You can also assign the same value to multiple variables simultaneously.

x = y = z = 0

Variable Scope

The scope of a variable determines where it can be accessed in the program. Python has three types of variable scopes:

  1. Local Scope: Variables declared inside a function are local to that function.
  2. Global Scope: Variables declared outside all functions are global and can be accessed anywhere in the program.
  3. Nonlocal Scope: Variables declared inside nested functions that are not in the local or global scope.
x = "global"

def outer_function():
    x = "outer local"

    def inner_function():
       # nonlocal x
        x = "inner local"
        print("Inner:", x)

    inner_function()
    print("Outer:", x)

outer_function()
print("Global:", x)

Global Keyword

If you need to modify a global variable inside a function, you can use the global keyword.

x = 10

def change_global():
    global x
    x = 20

change_global()
print(x)  # Output: 20

Deleting Variables

You can delete a variable using the del keyword.

a = 10
del a
# print(a) would raise a NameError since 'a' is deleted

Best Practices for Using Variables

  1. Meaningful Names: Use descriptive and meaningful variable names to make your code more readable.
   num_students = 50
   avg_score = 75.5
  1. Consistency: Stick to a naming convention throughout your code. Common conventions are:
  • snake_case: my_variable
  • camelCase: myVariable
  • PascalCase: MyVariable
  1. Avoid Single Letter Names: Except for in temporary or loop variables.
   for i in range(10):
       pass
  1. Use Comments: Add comments to explain the purpose of variables if it’s not immediately clear.
   interest_rate = 0.05  # Annual interest rate in percentage

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