[docs] unittest documentation

[Arthur Goldberg]

Hello

The first example in the unittest documentation 
(https://docs.python.org/3.4/library/unittest.html and 
https://docs.python.org/2/library/unittest.html) contains the comment:

    # check that s.split fails when the separator is not a string

However, this comment does not correctly describe the code, and reflects 
a common misconception about testing. More accurately, the comment 
should say:

    # check that s.split fails when the separator is an int

Test code to "check that s.split fails when the separator is not a 
string" would need to test every type other than string, which is 
impossible because the set of possible types is infinite.

Thanks

Arthur


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