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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: bug in str.startswith() and str.endswith()
Date: Thu, 26 May 2011 23:00:32 -0400
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6363

On 5/26/2011 7:27 PM, Ethan Furman wrote:
> I've tried this in 2.5 - 3.2:
>
> --> 'this is a test'.startswith('this')
> True
> --> 'this is a test'.startswith('this', None, None)
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> TypeError: slice indices must be integers or None or have an __index__
> method
>
> The 3.2 docs say this:
>
> str.startswith(prefix[, start[, end]])
> Return True if string starts with the prefix, otherwise return False.
> prefix can also be a tuple of prefixes to look for. With optional start,
> test string beginning at that position. With optional end, stop
> comparing string at that position

To me, that says pretty clearly that start and end have to be 
'positions', ie, ints or other index types. So I would say that the 
error message is a bug. I see so reason why one would want to use None 
rather that 0 for start or None rather than nothing for end.

-- 
Terry Jan Reedy