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

[Dick Moores]

http://www.python.org/doc/lib/string-methods.html has
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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