Python is vs == [duplicate]

问题

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Closed 9 years ago.

Possible Duplicate:
String comparison in Python: is vs. ==
When is the == operator not equivalent to the is operator? (Python)

I'm pretty new to Python still. I heard someone say use is, not == because "this isn't C". But I had some code x is 5 and it was not working as expected.

So, following proper Python/PEP style, when is the time to use is and when is the time to use == ?

回答1:

You should use == to compare two values. You should use is to see if two names are bound to the same object.

You should almost never use x is 5 because depending on the implementation small integers might be interned. This can lead to surprising results:

>>> x = 256
>>> x is 256
True
>>> x = 257
>>> x is 257
False

回答2:

The two operators have different meaning.

• is tests object identity. Do the two operands refer to the same object?
• == tests equality of value. Do the two operands have the same value?

When it comes to comparing x and 5 you invariably are interested in the value rather than the object holding the value.

来源：https://stackoverflow.com/questions/7718790/python-is-vs