Variables & Data Types

1# ============================================================
2# Variables are references (labels), not boxes
3# ============================================================
4a = [1, 2, 3]
5b = a           # b points to the SAME list object
6b.append(4)
7print(a)        # [1, 2, 3, 4] — a is also modified!
8print(a is b)   # True — same object in memory
9print(id(a), id(b))  # Same memory address
10
11# To make a copy:
12c = a.copy()    # Shallow copy — new list, same elements
13c.append(5)
14print(a)        # [1, 2, 3, 4] — a is NOT affected
15print(a is c)   # False — different objects
16
17# ============================================================
18# Numeric types
19# ============================================================
20# Integers have unlimited precision in Python!
21big_num = 10 ** 100  # No overflow!
22print(big_num)       # A googol
23
24# Readable large numbers with underscores
25population = 7_900_000_000
26budget = 1_500_000.50
27
28# Different bases
29hex_val = 0xFF      # 255 in hexadecimal
30oct_val = 0o77      # 63 in octal
31bin_val = 0b1010    # 10 in binary
32
33# Float precision gotcha
34print(0.1 + 0.2)           # 0.30000000000000004
35print(0.1 + 0.2 == 0.3)    # False!
36
37# Use decimal for financial calculations
38from decimal import Decimal
39print(Decimal('0.1') + Decimal('0.2'))  # 0.3 (exact)
40print(Decimal('0.1') + Decimal('0.2') == Decimal('0.3'))  # True
41
42# ============================================================
43# Boolean values and truthiness
44# ============================================================
45# False values (falsy):
46print(bool(0))          # False
47print(bool(0.0))        # False
48print(bool(""))         # False
49print(bool([]))         # False
50print(bool({}))         # False
51print(bool(None))       # False
52
53# Everything else is truthy:
54print(bool(1))          # True
55print(bool(-1))         # True — any non-zero number
56print(bool("hello"))    # True — any non-empty string
57print(bool([0]))        # True — non-empty list (even if contains falsy)
58
59# ============================================================
60# String types
61# ============================================================
62single = 'hello'
63double = "hello"
64triple = """Multi-line
65string that preserves
66line breaks"""
67
68# f-strings (Python 3.6+) — the preferred way to format strings
69name = "Alice"
70age = 30
71print(f"Name: {name}, Age: {age}")
72print(f"Next year: {age + 1}")          # Expressions in f-strings
73print(f"Name uppercase: {name.upper()}")  # Method calls
74print(f"{42:08b}")                        # Format spec: binary with padding
75
76# Raw strings — no escape processing
77path = r"C:\Users\alice\docs"  # Backslashes are literal
78print(path)  # C:\Users\alice\docs
79
80# ============================================================
81# None — Python's null
82# ============================================================
83result = None
84print(result is None)       # True — always use 'is' for None checks
85print(result is not None)   # False
86print(type(result))         # <class 'NoneType'>
87
88# BAD: if result == None  (works but not Pythonic)
89# GOOD: if result is None (identity check, faster and correct)
90
91# ============================================================
92# Type checking and conversion
93# ============================================================
94x = "42"
95print(type(x))          # <class 'str'>
96print(isinstance(x, str))  # True
97
98# Type conversion
99num = int("42")          # str -> int
100flt = float("3.14")     # str -> float
101s = str(42)              # int -> str
102b = bool(1)              # int -> bool
103
104# Multiple assignment
105a, b, c = 1, 2, 3
106x = y = z = 0            # All point to same object
107
108# Swap variables (Pythonic way)
109a, b = b, a              # No temp variable needed!