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Re: None in string => TypeError?

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2014-06-09 23:34 GMT+08:00 Roy Smith <roy@panix.com>:

> 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.
>

It's a little bit inconsistent.  But it's clearly documented here:
https://docs.python.org/3/reference/expressions.html#in

Which, according to its own logic, the string is not  a *container* type.
It's just some chars, and that totally makes sense for to restrict the type
of x in "str" to be convertible to type str. On the other hand, containers
like list, and tuple, they are heterogeneous by default in Python, so a
item by item comparison is needed.



-- 

吾輩は猫である。ホームーページはhttp://introo.me。

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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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