Variables, Types, and Operators¶

Python is dynamically typed - variables don't need type declarations, and their type is determined at runtime. Everything in Python is an object, and variables are references (pointers) to objects.

Data Types Overview¶

Check type with type(x). Prefer isinstance(x, int) over type(x) == int - it respects inheritance.

Key Facts¶

• Python integers have arbitrary precision - no overflow
• 10 / 2 returns 5.0 (float), not 5. Use // for integer division
• round(2.5) returns 2 (banker's rounding to nearest even)
• 0.1 + 0.2 != 0.3 due to IEEE 754 floating-point representation
• id(x) shows memory address; is checks identity (same object), == checks equality
• Falsy values: 0, 0.0, "", [], (), {}, set(), None, False

Patterns¶

Number Arithmetic¶

10 + 3    # 13  addition
10 - 3    # 7   subtraction
10 * 3    # 30  multiplication
10 / 3    # 3.333...  true division (always float)
10 // 3   # 3   floor division
10 % 3    # 1   modulo
10 ** 3   # 1000  exponentiation

Floating-Point Precision¶

0.1 + 0.2 == 0.3               # False!
round(0.1 + 0.2, 1) == 0.3     # True
abs((0.1 + 0.2) - 0.3) < 1e-9  # True (epsilon comparison)

from decimal import Decimal
Decimal('0.1') + Decimal('0.2')  # Decimal('0.3') exactly

Variables and Assignment¶

x = 10              # assignment
x = "hello"         # rebinding (dynamic typing)
x, y = 1, 2         # multiple assignment
x, y = y, x         # swap values
a = b = c = 0       # chained assignment

Type Conversion¶

int(3.7)      # 3 (truncates, does NOT round)
float(3)      # 3.0
int("42")     # 42
str(42)       # "42"
bool(0)       # False
bool(1)       # True

Reference Semantics¶

a = [1, 2, 3]
b = a              # b points to SAME list
b.append(4)
print(a)           # [1, 2, 3, 4] - a changed too!

c = a.copy()       # c is a NEW list
c.append(5)        # only c changed

Comparison and Logical Operators¶

# Chaining
1 < x < 10  # equivalent to: 1 < x and x < 10

# Short-circuit evaluation - returns actual values
0 or "default"      # "default"
"value" and 42      # 42
None or "fallback"  # "fallback"

Special Float Values¶

float('inf')    # positive infinity
float('-inf')   # negative infinity
float('nan')    # not a number

Gotchas¶

• Never name variables same as built-in functions (list, str, dict, print) - shadows them
• b = a for mutable types creates a shared reference, not a copy
• int("3.14") raises ValueError - must use float("3.14") first, then int()
• math.floor(-2.3) is -3, int(-2.3) is -2 (truncates toward zero)

See Also¶

• strings and text - string operations and formatting
• data structures - lists, tuples, sets, dictionaries
• type hints - static type annotations