Python is vs == [duplicate]

This question already has answers here: Closed 11 years ago. 开发者_高级运维

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 == ?

---

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

---

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.