None and Truthiness

Python · Reference cheat sheet

📋 Overview

None means “no value” (Python’s null). Truthiness means how values behave in if / while without an explicit comparison: some values count as false, everything else as true.

🔧 Core concepts

Functions that “return nothing” actually return None.

💡 Examples

None as a missing value:

user = None

if user is None:
    print("Please log in")
else:
    print(user)

Default then assign:

def find_user(user_id):
    # pretend lookup failed
    return None


result = find_user(42)
name = result if result is not None else "anonymous"
print(name)

Truthiness in conditions:

items = []
text = ""
count = 0

if not items:
    print("list is empty")
if not text:
    print("no text")
if count:
    print("never runs for 0")

Truthy gotchas with numbers and containers:

print(bool(0), bool(1), bool(-1))      # False True True
print(bool(""), bool("0"))             # False True
print(bool([]), bool([0]))             # False True
print(bool(None))                      # False

⚠️ Pitfalls

• if x == None works but style guides prefer if x is None.
• Empty containers are falsy — that is often useful, sometimes surprising.
• The string "False" and the number 0 inside a list [0] are truthy as containers/strings.
• Returning None accidentally from a function that should return data is a common bug.

🔗 Related

• boolean_logic.md
• variables.md
• truthiness.md
• input_output_basics.md