[Tutor] Question about startswith() and endswith() in 2.5

[Carroll, Barry]

> -----Original Message-----
> Date: Mon, 25 Sep 2006 02:59:45 -0700
> From: Dick Moores <rdm at rcblue.com>
> Subject: [Tutor] Question about startswith() and endswith() in 2.5
> To: tutor at python.org
> Message-ID: <7.0.1.0.2.20060925013857.03a65758 at rcblue.com>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
> 
> http://www.python.org/doc/lib/string-methods.html has
> =============================================
> startswith( prefix[, start[, end]])
> Return True if string starts with the prefix, otherwise return False.
> prefix can also be a tuple of suffixes to look for. With optional
> start, test string beginning at that position. With optional end,
> stop comparing string at that position.
> 
> Changed in version 2.5: Accept tuples as prefix.
> ==============================================
> 
> and
> 
> ================================================
> endswith( suffix[, start[, end]])
> Return True if the string ends with the specified suffix, otherwise
> return False. suffix can also be a tuple of suffixes to look for.
> With optional start, test beginning at that position. With optional
> end, stop comparing at that position.
> 
> Changed in version 2.5: Accept tuples as suffix.
> ==================================================
> 
> Through experimentation I now see a use for a tuple in which start
> and end are indexes (as with the startswith() and endswith() of
2.4.3):
> 
>  >>> s = "qwerty"
>  >>>
>  >>> s.startswith("er",2,3)
> False
>  >>>
>  >>> s.startswith("er",2,4)
> True
>  >>>
> 
> but
>  >>> s.startswith("er","q","ty")
> 
> Traceback (most recent call last):
>    File "<pyshell#55>", line 1, in <module>
>      s.startswith("er","q","ty")
> TypeError: slice indices must be integers or None or have an __index__
> method
> 
> On http://docs.python.org/whatsnew/other-lang.html I found
> 
> ==================================================
> The startswith() and endswith() methods of string types now accept
> tuples of strings to check for.
> 
> 
> def is_image_file (filename):
>      return filename.endswith(('.gif', '.jpg', '.tiff'))
> 
> ====================================================
> 
> This is the only example I've been able to find in the documentation
> that uses the new tuple of strings, and I don't understand it. The
> function is_image_file() will return filenames ending in '.gif', but
> what do '.jpg' (as start) and '.tiff' (as end) do? What kind of
> data(?) would this function be applied to? A Python list of filenames?
> 
> Thanks,
> 
> Dick Moores
> 

Hello, Dick.

Let's compare your final startswith method and the endswith method in
is_image_file:

>>>>>>>
s.startswith("er","q","ty")
filename.endswith(('.gif', '.jpg', '.tiff'))
>>>>>>>

Notice that, while startswith has THREE parameters, endswith has only
ONE.  ('.gif', '.jpg', '.tiff') is a tuple, and the interpreter sees it
as a single parameter.  In other words your method is passing the
following parameters: 

	prefix = "er"
	start = "q"
	end = "ty

while the example method is passing:

	suffix = ('.gif', '.jpg', '.tiff')
	start = None
	end = None

Does that make sense?

Good luck.  

Regards,
 
Barry
barry.carroll at psc.com
541-302-1107
________________________
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