Turning text into numbers and back, what converts silently, and what raises rather than guessing.
int(), float(), str(), list().int("3.9") raises; int(3.9) truncates toward zero.round(2.5) is 2, not 3 - it rounds to the nearest even on a tie.bool("0") is True. Any non-empty string is.Python does not convert types behind your back. You ask, using the type name as a function, and it either succeeds or raises — it will not guess.
int("42") float("3.14") str(42) list("abc") bool("")
Each type's name doubles as its conversion. That is why there is nothing to memorise beyond the types themselves.
Anything read from a user, a file or a network arrives as text. "10" + "10" is "1010", and no error is raised, because concatenating strings is a perfectly sensible thing to do. Converting is on you:
int(text) + int(text)
This is the single most common source of confusion for beginners, and it is not really about conversion — it is about noticing that input is always a string.
int("3.9") # ValueError
int(3.9) # 3
From a string, int refuses anything that is not a whole number, because guessing which way to go would be a decision it has no business making. From a float, it truncates toward zero — int(-3.9) is -3, not -4. If you want nearest, say round.
It does tolerate surrounding whitespace, which is convenient when parsing scruffy input.
round(2.5) # 2
round(3.5) # 4
On an exact tie, Python rounds to the nearest even number. This is deliberate: always rounding halves up biases a long run of numbers upward. It surprises people once, and it is correct.
0.1 + 0.2 == 0.3 # False
Binary floating point cannot represent 0.1 exactly, so the sum is a hair off. This is not a Python quirk; it is how floats work everywhere. Compare with a tolerance, round before comparing, or use decimal.Decimal for money.
Do not check first — try, and handle the failure:
try:
return int(text)
except (ValueError, TypeError):
return default
ValueError covers bad text and TypeError covers None. Testing text.isdigit() first looks tidier and gets negative numbers and whitespace wrong.
bool("0") is True, because the string is not empty. Falsy values are: 0, 0.0, "", [], {}, set(), None and False. Everything else is truthy, including "False".
Answer without scrolling back up.
What does `int("3.9")` do?
From a string, int refuses anything that is not a whole number. From a float, int(3.9) truncates to 3 - the two behave differently on purpose.
`round(2.5)` returns what?
Python rounds a tie to the nearest even number, which avoids biasing a long run of values upward. round(3.5) is 4.
`bool("0")` is:
Only an empty string is falsy. "0" has a character in it, so it is True - a classic bug when reading text input.
Python does not convert types behind your back. You ask, using the type name as a function, and it either succeeds or raises — it will not guess.