Truthiness, bool & min/max

Every object is either truthy or falsy

In Python you don't only write if x == 0: — you can write if x: and let the object decide. When an object appears in a boolean context (an if, a while, an and/or), Python asks it: "are you truthy or falsy?" The complete list of falsy values is short and worth memorising — everything not on it is truthy:

• False, None
• any zero number: 0, 0.0, 0j
• any empty container: "", [], {}, set(), ()

The traps live in the gaps. -3 is truthy (only zero is falsy, not "negative"). "0" is truthy (it's a non-empty string — the characters don't matter). [0] is truthy (a non-empty list, even though its one element is falsy). Click any chip to check it yourself:

True and False are numbers

Here's the fact that makes exam Q3 click: in Python bool is a subclass of int. True is literally 1 and False is literally 0 for every arithmetic and comparison purpose. So True + True == 2, False < 5 is True, and sum([True, False, True]) == 2 (a handy way to count matches). This is why min and max — which compare by value — will happily mix booleans and numbers.

But there's a twist that the exam leans on: min and max return the actual winning element, not a fresh number. So if False wins a comparison, you get the object False back — even though it compared as 0. Watch it resolve step by step:

The punchline: the result is False, not 0 — even though they're equal in value. max(False, -3, -4) compares 0, -3, -4, the biggest is 0, and the element that carried that value was False, so max hands back False. Then min(False, 2, 7) compares 0, 2, 7, the smallest is 0 → False again. print() shows False. If the answer key says "0" it's numerically right but the object Python returns is the boolean.

Short-circuit and / or — and what they actually return

One more piece that surprises Java developers: in Python and and or don't return True/False — they return one of the operands, and they short-circuit (stop as soon as the answer is known).

• a and b → returns a if a is falsy (and never looks at b); otherwise returns b.
• a or b → returns a if a is truthy (and never looks at b); otherwise returns b.

That's why the idiom name = user_input or "guest" works: if user_input is an empty string (falsy), you fall through to "guest". And why x and x.method() safely skips the call when x is None. Try a few:

Why this matters

Truthiness is everywhere in idiomatic Python and ML code: if results: (got any rows?), config.get("lr") or 0.01 (default), while queue: (drain it). The cost of the convenience is exactly the traps above — 0, "", and empty containers are falsy, which is great until a legitimate zero (a valid price of 0.0, an empty-but-real list) gets silently skipped. When zero is a meaningful value, be explicit: write if x is None: instead of if not x:.