Counterintuitive behaviour of int() in python

Question

It's clearly stated in the docs that int(number) is a flooring type conversion:

int(1.23) 1 

and int(string) returns an int if and only if the string is an integer literal.

int('1.23') ValueError  int('1') 1 

Is there any special reason for that? I find it counterintuitive that the function floors in one case, but not the other.

Answer 1

There is no special reason. Python is simply applying its general principle of not performing implicit conversions, which are well-known causes of problems, particularly for newcomers, in languages such as Perl and Javascript.

int(some_string) is an explicit request to convert a string to integer format; the rules for this conversion specify that the string must contain a valid integer literal representation. int(float) is an explicit request to convert a float to an integer; the rules for this conversion specify that the float's fractional portion will be truncated.

In order for int("3.1459") to return 3 the interpreter would have to implicitly convert the string to a float. Since Python doesn't support implicit conversions, it chooses to raise an exception instead.

Answer 2

This is almost certainly a case of applying three of the principles from the Zen of Python:

Explicit is better implicit.

[...] practicality beats purity

Errors should never pass silently

Some percentage of the time, someone doing int('1.23') is calling the wrong conversion for their use case, and wants something like float or decimal.Decimal instead. In these cases, it's clearly better for them to get an immediate error that they can fix, rather than silently giving the wrong value.

In the case that you do want to truncate that to an int, it is trivial to explicitly do so by passing it through float first, and then calling one of int, round, trunc, floor or ceil as appropriate. This also makes your code more self-documenting, guarding against a later modification "correcting" a hypothetical silently-truncating int call to float by making it clear that the rounded value is what you want.