Variables

In Python, variables are created when you assign a value. No type declaration is needed.

name = "Ana"        # str
age = 25            # int
height = 1.75       # float
is_student = True   # bool

Note

Python uses dynamic typing: the type is determined at runtime and can change with reassignment.

Naming conventions

• Use snake_case for variables and functions.
• Start with a letter or _, never with a number.
• Names are case-sensitive (name ≠ Name).

my_variable = 10      # valid
_private = 5          # valid (convention for internal use)
2name = "error"       # invalid

Warning

Avoid using Python keywords as variable names (if, for, class, import, etc.).

Assignment

x = 10                # simple assignment
x, y, z = 1, 2, 3    # multiple assignment
x = y = 0             # same value to multiple variables
x, *rest = [1, 2, 3, 4]  # x = 1, rest = [2, 3, 4]

Swapping variables

Python allows swapping without a temporary variable:

a = 5
b = 10
a, b = b, a   # a = 10, b = 5

Scope

Variables have scope depending on where they are defined:

global_var = "global"
 
def my_function():
    local_var = "local"        # only exists inside the function
    print(global_var)          # can read global variables
 
my_function()
print(global_var)              # works
# print(local_var)             # error: local_var is not defined

Tip

Use global to modify a global variable inside a function, but it is generally discouraged. Prefer passing values as parameters.

Constants

Python has no real constants. By convention, use UPPER_CASE names:

MAX_RETRIES = 3
PI = 3.14159

Warning

This is only a convention. Python does not prevent reassignment of UPPER_CASE variables.

Next: Data Types