比较int和float时，Python何时执行类型转换? [英] When does Python perform type conversion when comparing int and float?

问题描述

当我比较具有相同值的int和float对象时，为什么Python返回True?

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

例如:

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

推荐答案

它不像类型转换那样简单.

It's not as simple as a type conversion.

10 == 10.0委托给参数的__eq__方法，首先尝试(10).__eq__(10.0)，然后在第一次调用返回NotImplemented的情况下尝试(10.0).__eq__(10).它不尝试转换类型. (从技术上讲，方法查找使用一个特殊的例程，该例程绕过实例__dict__条目和__getattribute__/__getattr__覆盖，因此，这并不等同于自己调用方法.)

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__不知道如何处理浮点数:

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

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

但是float.__eq__知道如何处理整数:

but float.__eq__ knows how to handle ints:

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

float.__eq__也不只是在内部执行强制转换.它具有超过100行的代码处理浮点/整数比较，而不会产生未经检查的强制转换会导致的舍入错误. (如果C级比较例程不必同时处理>，>=，<和<=，则其中一些可以简化.)

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 <=.)

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