Python — Booleans & Conditionals

◆Python◆Beginner

Introduction

Booleans represent truth values — True and False. In Python, bool is a subclass of int, and every object has an inherent truth value used in conditions.

Boolean Literals

booleans.py

Python

1	# Only two boolean values
2	is_ready = True
3	is_done = False
4
5	# bool is a subclass of int
6	print(True == 1) # → True
7	print(False == 0) # → True
8	print(True + True) # → 2
9	print(True * 10) # → 10
10
11	# bool() constructor
12	print(bool(1)) # → True
13	print(bool(0)) # → False
14	print(bool("hi")) # → True
15	print(bool("")) # → False

Truthiness

Every Python object is truthy or falsy. Falsy values evaluate to False in boolean contexts like if statements.

Falsy Values	Reason
False	Boolean false
None	Null value
0, 0.0, 0j	Zero numeric values
""	Empty string
[], (),	Empty collections
set(), range(0)	Empty set/range

truthiness.py

Python

1	# Truthiness in conditions
2	if "hello": # truthy → runs
3	if 0: # falsy → skipped
4	if []: # falsy → skipped
5	if [1, 2]: # truthy → runs
6	if None: # falsy → skipped
7
8	# Custom objects default to truthy
9	class AlwaysTrue:
10	pass
11
12	obj = AlwaysTrue()
13	if obj: # True by default
14	print("always truthy")
15
16	# Override with __bool__ or __len__
17	class MyList:
18	def __init__(self, items):
19	self.items = items
20	def __bool__(self):
21	return len(self.items) > 0
22	def __len__(self):
23	return len(self.items)
24
25	print(bool(MyList([]))) # → False
26	print(bool(MyList([1]))) # → True

ℹ

info

Use if x: instead of if x is True: or if x == True:. It's more Pythonic and works for all truthy/falsy values.

Logical Operators

logical.py

Python

1	# and, or, not
2	print(True and False) # → False
3	print(True or False) # → True
4	print(not True) # → False
5
6	# Short-circuit evaluation
7	# and: stops at first falsy
8	def a():
9	print("called a"); return False
10	def b():
11	print("called b"); return True
12	result = a() and b() # b() never called (short-circuits)
13
14	# or: stops at first truthy
15	result = b() or a() # a() never called
16
17	# and/or return the last evaluated value, not bool
18	print(0 and 42) # → 0 (falsy, short-circuits)
19	print(3 and 42) # → 42 (both truthy, returns last)
20	print(0 or 42) # → 42 (falsy, evaluates second)
21	print(3 or 42) # → 3 (truthy, short-circuits)
22
23	# Common pattern: default value
24	name = input() or "Guest"
25	# If input is empty string (falsy), defaults to "Guest"

Comparison Operators

comparisons.py

Python

1	# Equality
2	print(5 == 5) # → True
3	print(5 == "5") # → False (different types)
4	print(5 != 3) # → True
5
6	# Ordering
7	print(5 < 10) # → True
8	print(5 <= 5) # → True
9	print(5 > 10) # → False
10
11	# Chained comparisons (unique to Python!)
12	x = 5
13	print(1 < x < 10) # → True
14	print(1 < x < 4) # → False
15	print(1 < x <= 5) # → True
16	# Equivalent to: 1 < x and x < 10
17
18	# Identity: is vs ==
19	a = [1, 2, 3]
20	b = [1, 2, 3]
21	c = a
22	print(a == b) # → True (same value)
23	print(a is b) # → False (different objects)
24	print(a is c) # → True (same object)
25
26	# None checks — ALWAYS use 'is'
27	x = None
28	if x is None: # correct
29	if x is not None: # correct
30	if x == None: # technically works but avoid
31
32	# Membership
33	print(2 in [1, 2, 3]) # → True
34	print("x" in "hello") # → False

Conditional Expressions

ternary.py

Python

1	# Ternary (conditional expression)
2	age = 20
3	status = "adult" if age >= 18 else "minor"
4
5	# Chained ternary (avoid for complex cases)
6	score = 85
7	grade = "A" if score >= 90 else "B" if score >= 80 else "C"
8
9	# Nested ternary — less readable, use if/elif
10	grade = (
11	"A" if score >= 90 else
12	"B" if score >= 80 else
13	"C" if score >= 70 else
14	"F"
15	)
16
17	# In comprehensions: ternary in expression (not filter)
18	values = ["even" if x % 2 == 0 else "odd" for x in range(5)]
19	# → ["even", "odd", "even", "odd", "even"]

all() and any()

all_any.py

Python

1	# all() — True if ALL items are truthy
2	print(all([True, True, True])) # → True
3	print(all([True, False, True])) # → False
4	print(all([])) # → True (vacuous truth)
5
6	# any() — True if ANY item is truthy
7	print(any([False, False, True])) # → True
8	print(any([False, False, False])) # → False
9	print(any([])) # → False
10
11	# Practical examples
12	nums = [1, 2, 3, 4, 5]
13	print(all(x > 0 for x in nums)) # → True (all positive)
14	print(any(x > 3 for x in nums)) # → True (4 and 5)
15
16	# Validation
17	users = [
18	{"name": "Alice", "active": True},
19	{"name": "Bob", "active": False},
20	]
21	print(all(u["active"] for u in users)) # → False

Common Pitfalls

pitfalls.py

Python

1	# 1. Chaining vs 'and' chaining
2	# Python's chained comparisons are NOT like 'and'
3	def check(x):
4	return 0 < x < 10
5
6	def check_and(x):
7	return 0 < x and x < 10
8
9	# These seem equivalent but chained evaluates x only once
10	# Useful when x is an expression
11
12	# 2. 'is' vs '==' with integers (small integer caching)
13	a = 256
14	b = 256
15	print(a is b) # → True (CPython caches -5 to 256)
16
17	a = 257
18	b = 257
19	print(a is b) # → False (different objects!)
20	# Always use == for value comparison
21
22	# 3. Comparing different types
23	print(1 < "2") # TypeError in Python 3
24
25	# 4. == vs is for None/True/False
26	x = True
27	if x is True: # fine for singletons
28	if x == True: # works but less idiomatic
29
30	# 5. Floating point equality
31	print(0.1 + 0.2 == 0.3) # → False!
32	# Use math.isclose instead
33	import math
34	print(math.isclose(0.1 + 0.2, 0.3)) # → True

$Blueprint — Engineering Documentation·Section ID: PYTHON-BOOL·Revision: 1.0

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