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Groups > comp.lang.python > #73045

Re: None in string => TypeError?

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On 2014-06-09 16:34, Roy Smith 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.
>
When working with strings, it's not entirely the same. For example:

 >>> 'oo' in 'foo'
True

If you iterated over the string, it would return False.

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None in string => TypeError? Roy Smith <roy@panix.com> - 2014-06-09 08:34 -0700
  Re: None in string => TypeError? Ryan Hiebert <ryan@ryanhiebert.com> - 2014-06-09 10:42 -0500
  Re: None in string => TypeError? Ian Kelly <ian.g.kelly@gmail.com> - 2014-06-09 09:50 -0600
  Re: None in string => TypeError? Paul Sokolovsky <pmiscml@gmail.com> - 2014-06-09 18:57 +0300
    Re: None in string => TypeError? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-06-09 16:14 +0000
      Re: None in string => TypeError? Chris Angelico <rosuav@gmail.com> - 2014-06-10 02:31 +1000
  Re: None in string => TypeError? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2014-06-09 16:07 +0000
  Re: None in string => TypeError? MRAB <python@mrabarnett.plus.com> - 2014-06-09 17:06 +0100
  Re: None in string => TypeError? Shiyao Ma <i@introo.me> - 2014-06-10 00:13 +0800
  Re: None in string => TypeError? Roy Smith <roy@panix.com> - 2014-06-09 12:53 -0400
  Re: None in string => TypeError? Chris Angelico <rosuav@gmail.com> - 2014-06-10 02:59 +1000
  Re: None in string => TypeError? Ian Kelly <ian.g.kelly@gmail.com> - 2014-06-09 11:22 -0600
  Re: None in string => TypeError? Chris Angelico <rosuav@gmail.com> - 2014-06-10 03:40 +1000
  Re: None in string => TypeError? Ian Kelly <ian.g.kelly@gmail.com> - 2014-06-09 11:58 -0600
  Re: None in string => TypeError? Chris Angelico <rosuav@gmail.com> - 2014-06-10 04:02 +1000

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