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:

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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 <=.)