python return false,为什么... = = True在Python 3中返回False？

I am learning python, but I'm a bit confused by the following result.

In [41]: 1 == True

Out[41]: True

In [42]: if(1):

...: print('111')

...:

111

In [43]: ... == True

Out[43]: False <===== why this is False while '1 == True' is True in previous sample

In [44]: if (...): <==== here ... just behaves like True

...: print('...')

...:

...

According to the documentation, ... has a truth value of True.

But I still feel the above code a bit inconsistent.

...And something more interesting:

In [48]: 2==True

Out[48]: False <===== why 1==True returns True while 2==True returns False?

In [49]: if(2):

...: print('222')

...:

222

解决方案

You're mixing two concepts: equality testing and truth-value testing. They are not the same in Python.

I think what triggered the question is that Python does an implicit casting when you do if something (it casts the something to bool) but it does not do implicit casting when you do something1 == something2.

Pythons data model actually explains how these operations are done:

It starts by checking if the object implements the __bool__ method and if it does it uses the returned boolean.

If it doesn't define a __bool__ method it looks at the __len__ method. If it's implemented it will use the result of len(obj) != 0.

If it doesn't have either the object is considered True.

For integers the __bool__ method returns True except when the integer value is 0 (then it's False).

Equality testing relies on the __eq__ method of both arguments. It's more a chain of operations:

It checks if the first operand implements __eq__ when the second operand is passed as argument.

If it doesn't then it checks if the second operand implements __eq__ when the first operand is passed as argument.

If it doesn't then Python checks for object identity (if they are the same object - similar to pointer comparisons in C-like languages)

The order of these operations may vary.1

For built-in Python types these operations are explicitly implemented. For example integers implement __eq__ but the CHECK_BINOP makes sure that it returns NotImplemented if the other one isn't an integer.

The Ellipsis object doesn't implement __eq__ at all.

So when you compare integers and Ellipsis Python will always fallback to object identity and so it will always return False.

On the other hand booleans are a subclass of integers so they actually compare with int (they are another int after all). The booleans are implemented as 1 (True) and 0 (False). So they compare equal:

>>> 1 == True

True

>>> 0 == False

True

>>> 1 == False

False

>>> 0 == True

False

Even though the source code is probably hard to understand I hope I explained the concepts well enough (the source code is for the CPython implementation, the implementation in other Python implementations like PyPy, IronPython may differ!). The important take-away message should be that Python doesn't do implicit conversions in equality checks and equality testing is not related to truth value testing at all. The built-in types are implemented that they almost always give senseable results:

all number-types implement equality in some way (floats compare to integers, complex compare to integers and floats)

and everything not-zero and not-empty is truthy.

However if you create your own classes you can override equality and truth value testing as you like (and then you can spread a lot of confusion)!

1 In some cases the order is changed:

If the second operand is a subclass of the first operand the first two steps are reversed.

For some implicit equality checks the object identity is checked before any __eq__ methods are called. For example when checking if some item is in a list, i.e. 1 in [1,2,3].