When does Python perform type conversion when comparing int and float?

Question

Why does Python return True when I compare int and float objects which have the same value?

For example:

>>> 5*2 == 5.0*2.0
True

Answer 1

It's not as simple as a type conversion.

10 == 10.0 delegates to the arguments' __eq__ methods, trying (10).__eq__(10.0) first, and then (10.0).__eq__(10) if the first call returns NotImplemented. It makes no attempt to convert types. (Technically, the method lookup uses a special routine that bypasses instance __dict__ entries and __getattribute__/__getattr__ overrides, so it's not quite equivalent to calling the methods yourself.)

int.__eq__ has no idea how to handle a float:

>>> (10).__eq__(10.0)
NotImplemented

but float.__eq__ knows how to handle ints:

>>> (10.0).__eq__(10)
True

float.__eq__ isn't just performing a cast internally, either. It has over 100 lines of code to handle float/int comparison without the rounding error an unchecked cast could introduce. (Some of that could be simplified if the C-level comparison routine didn't also have to handle >, >=, <, and <=.)

Answer 2

Objects of different types, except different numeric types, never compare equal.

And:

Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the “narrower” type is widened to that of the other, where integer is narrower than floating point, which is narrower than complex. Comparisons between numbers of mixed type use the same rule.

_{https://docs.python.org/3/library/stdtypes.html#numeric-types-int-float-complex}

The comparison logic is implemented by each type's __eq__ method. And the standard numeric types are implemented in a way that they support comparisons (and arithmetic operations) among each other. Python as a language never does implicit type conversion (like Javascript's == operator would do implicit type juggling).