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Variables in Python

Variables store data values. Python has no command for declaring a variable - it is created when you assign a value to it.

Naming Rules

# Valid variable names
my_name = "Alice"
_private = "hidden"
camelCase = True
myVar2 = 42

# Invalid variable names (causes error)
# 2myvar = 42      # cannot start with number
# my-var = 42      # no hyphens
# my var = 42      # no spaces

Data Types

# Numeric types
x = 42              # int
y = 3.14            # float
z = 2 + 3j          # complex

# Text type
name = "Alice"      # str

# Boolean
is_valid = True     # bool

# Sequence types
fruits = ["apple", "banana"]   # list
coords = (10, 20)              # tuple
letters = range(5)             # range

# Mapping type
person = {"name": "Alice", "age": 25}  # dict

# Set types
unique = {1, 2, 3}             # set
frozen = frozenset([1, 2, 3])  # frozenset

# None
result = None                  # NoneType

Type Conversion

# Implicit conversion
x = 10      # int
y = 3.14    # float
z = x + y   # float (13.14)

# Explicit conversion
s = "42"
n = int(s)          # string to int
f = float(s)        # string to float
l = list("hello")   # string to list: ["h","e","l","l","o"]

Multiple Assignment

# Assign multiple variables
a, b, c = 1, 2, 3

# Same value
x = y = z = 0

# Unpack a list
colors = ["red", "green", "blue"]
r, g, b = colors

Scope

# Global vs Local
x = 10  # global

def my_func():
    y = 20  # local
    print(x)  # can read global
    print(y)

my_func()
print(x)  # OK
# print(y)  # Error: y is not defined

Summary

• Python has several built-in data types: int, float, str, bool, list, tuple, dict, set
• Type conversion is done with int(), float(), str(), list(), etc.
• Multiple variables can be assigned in one line
• Variables inside functions are local by default