Python: Difference between != and "is not"

Question

I'm unclear about the difference between the syntax != and is not. They appear to do the same thing:

>>> s = 'a'
>>> s != 'a'
False
>>> s is not 'a'
False

But, when I use is not in a list comprehension, it produces a different result than if I use !=.

>>> s = "hello"
>>> [c for c in s if c is not 'o']
['h', 'e', 'l', 'l', 'o']
>>> [c for c in s if c != 'o']
['h', 'e', 'l', 'l']

Why did the o get included in the first list, but not the second list?

Solution 1

is tests for object identity, but == tests for object value equality:

In [1]: a = 3424
In [2]: b = 3424

In [3]: a is b
Out[3]: False

In [4]: a == b
Out[4]: True

Solution 2

is not compares references. == compares values

Solution 3

Depending on how you were confused, this might help.

These statements are the same:

[c for c in s if c != 'o']
[c for c in s if not c == 'o']

Solution 4

I'd like to add that they definitely do not do the same thing. I would use !=. For instance if you have a unicode string....

a = u'hello'
'hello' is not a
True
'hello' != a
False

With !=, Python basically performs an implicit conversion from str() to unicode() and compares them, whereas with is not, it matches if it is exactly the same instance.

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Comments

[user1066101]

When you looked at the Python language reference for these two operators, what did you see? This seems clear: docs.python.org/reference/expressions.html#notin. What part confused you?

[michaelsnowden]

This is really strange, I can't recreate this: Python 2.7.5 (default, Aug 25 2013, 00:04:04) [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> s = "hello" >>> [c for c in s if c is not 'o'] ['h', 'e', 'l', 'l'] >>> [c for c in s if c != 'o'] ['h', 'e', 'l', 'l']

[ajwood]

Why does a==123 return true after setting a=123?

[Wilduck]

@ajwood: because they have the same value. == compares values.

[Wilduck]

The reason is not worked for the strings in the op's example is an artifact of python's method of assigning strings to variables. Since strings are immutable, in the global namespace, two variables which contain the same string will be referencing the same object.

[Wooble]

A better question is why "a is 123" returns True in most implementations. (see stackoverflow.com/questions/306313/… )

[eyquem]

What sort of Python do you use ? With Python 2.7, I obtain True for the instruction a is b !

[tkerwin]

This is using Python 2.6.4. The implementation can make the choice to cache certain objects. On my system, 'a' passes the is test, but 'apple sauce' does not.

[ncoghlan]

For the curious, that happens because 'a' looks like an identifier (so the interpreter caches it), but 'apple sauce' does not (due to the space), so if you want it cached you must do so explicitly via intern().

[ajwood]

@Wooble That is indeed that question I meant to ask