Truthiness

Python · Reference cheat sheet

📋 Overview

In boolean contexts (if, while, and/or, bool(x)), objects are truthy or falsy. Prefer explicit checks (is None, len, ==) when zero/empty must be distinguished from missing.

🔧 Core concepts

Everything else is truthy by default — including "0", [0], and custom objects.

💡 Examples

Typical checks:

name = ""
items: list[int] = []

if not name:
    print("missing name")
if items:
    print("has items")

None vs empty:

def title(value: str | None) -> str:
    if value is None:
        return "default"
    if not value:
        return "empty"
    return value

print(title(None), title(""), title("Hi"))

and / or return operands:

print("" or "fallback")     # "fallback"
print("hi" or "fallback")   # "hi"
print("hi" and "there")     # "there"
user = None
print((user or {}).get("id"))

Custom bool:

class Gate:
    def __init__(self, open_: bool) -> None:
        self.open = open_

    def __bool__(self) -> bool:
        return self.open

g = Gate(False)
print(bool(g), "yes" if g else "no")

⚠️ Pitfalls

• if xs is False for empty lists — OK for "any items?", wrong for "was provided?".
• if count: fails for 0 even when zero is valid.
• NumPy arrays: truth value is ambiguous — use .any() / .all().
• Don't use == True / == False; use is True only for actual bools, else rely on truthiness carefully.
• __bool__ and __len__ must be consistent if both exist.

🔗 Related

• Conditionals
• Operators
• Lists
• Dictionaries
• Magic methods
• Types