On Mon, Jun 9, 2014 at 9:34 AM, Roy Smith <r...@panix.com> wrote: > We noticed recently that: > >>>> None in 'foo' > > raises (at least in Python 2.7) > > TypeError: 'in <string>' requires string as left operand, not NoneType > > This is surprising. The description of the 'in' operatator is, 'True if an > item of s is equal to x, else False '. From that, I would assume it behaves > as if it were written: > > for item in iterable: > if item == x: > return True > else: > return False > > why the extra type check for str.__contains__()? That seems very unpythonic. > Duck typing, and all that.
I guess for the same reason that you get a TypeError if you test whether the number 4 is in a string: it can't ever be, so it's a nonsensical comparison. It could return False, but the comparison is more likely to be symptomatic of a bug in the code than intentional, so it makes some noise instead. -- https://mail.python.org/mailman/listinfo/python-list
