The documentation for str.expandtabs([tabsize]) is wrong:
 
"Return a copy of the string where all tab characters are replaced by one or more spaces, depending on the current column and the given tab size. [...]"

This should read "zero or more spaces":

>>> 'a\tb'.expandtabs(0)
'ab'
>>> 'a\tb'.expandtabs(-1)
'ab'

The description in Objects/unicodeobject.c does not include this error.