Python Truthy and Falsy Values

3Python Truthy & Falsy: explained step by step

When you write if something:, Python doesn't insist that something is literally True or False. It takes whatever value you give it and asks one question: does this value count as true? Every value can answer that question, so every value can be used as a condition.

The falsy values

Only a short list of values count as falsy (they act like False in a condition):

• False and None
• zero in any number type: 0 and 0.0
• empty text: ""
• empty collections: [], (), {} and set()

Everything else is truthy. The idea behind the list is "is there anything here?" Zero, nothing and empty containers hold nothing, so they count as false. Any other value holds something, so it counts as true.

Looking inside with bool()

The built-in bool() converts any value to True or False using exactly the rule that if uses, which makes it the quickest way to check a value you're unsure about.

print("0      ->", bool(0))
print("0.0    ->", bool(0.0))
print('""     ->', bool(""))
print("[]     ->", bool([]))
print("()     ->", bool(()))
print("{}     ->", bool({}))
print("set()  ->", bool(set()))
print("None   ->", bool(None))
print("False  ->", bool(False))

Now the other side. Some values that look empty or false are actually truthy:

print('"hi"  ->', bool("hi"))
print('" "   ->', bool(" "))
print('"0"   ->', bool("0"))
print("-5    ->", bool(-5))
print("[0]   ->", bool([0]))
print("[[]]  ->", bool([[]]))

A single space is text, so it is truthy. The list [0] has one item, so it is truthy even though that item is falsy: Python only asks about the container itself, never about what is inside it.

Shorter conditions

Because an empty collection is falsy, if items: means "if there is anything in items". It does the same job as if len(items) > 0: with less to read, and it works for text, tuples, dictionaries and sets too.

cart = []
if cart:
    print("Items in cart:", len(cart))
else:
    print("Your cart is empty")

cart.append("headphones")
if cart:
    print("Items in cart:", len(cart))

Keep the len() version for when you really need the number ("at least 3 items"). For a plain "is there anything?" check, the short form is what most Python programmers write.

and / or give back one of their operands

Here is the part that surprises people: or and and don't always produce True or False. They hand back one of the two values you wrote.

• a or b gives a if a is truthy, otherwise b.
• a and b gives a if a is falsy, otherwise b.

Both stop as soon as the answer is known, so the right-hand side may never even run. Inside an if you only care whether the result is truthy, so you rarely notice. But when you store the result, you get the real value. That is what makes the popular default value trick work: name = entered or "guest".

entered = ""
name = entered or "guest"
print(name)

entered = "Asha"
name = entered or "guest"
print(name)

print("Asha" and "Ben")
print(repr("" and "Ben"))

Pitfall 1: 0 is falsy, even when 0 is a real answer

or can't tell the difference between "no value" and a value that merely happens to be falsy. If 0, 0.0 or "" are legitimate values in your program, or will quietly replace them. When you really mean "nothing was provided", test for None with is None:

score = 0          # the player really scored zero
shown = score or "not played yet"
print(shown)

if score is None:
    shown = "not played yet"
else:
    shown = score
print(shown)

The first version prints the wrong message, because a score of 0 is a real result. The second version only reacts to None, which is Python's way of saying "no value here".

Pitfall 2: the text "False" is truthy

Text is only falsy when it is completely empty. The words "False", "0" and "no" are ordinary non-empty text, so they are all truthy. This catches people out when values come from files, forms or input(), because those always arrive as text. Compare the text itself instead, for example answer.strip().lower() == "yes".

answer = "False"
print(bool(answer))
print(answer == "True")
print(bool(""))