Python Cookbook 2Nd Edition Jun 1002005 [Electronic resources]

David Ascher, Alex Martelli, Anna Ravenscroft

نسخه متنی -صفحه : 394/ 36

Recipe 1.19. Checking a String for Any of Multiple Endings

Credit: Michele Simionato

Problem

For a certain string s, you must check whether s has any of several endings; in other words, you need a handy, elegant equivalent of s.endswith(end1) or s.endswith(end2) or s.endswith(end3) and so on.

Solution

The itertools.imap function is just as handy for this task as for many of a similar nature:

import itertools
def anyTrue(predicate, sequence):
return True in itertools.imap(predicate, sequence)
def endsWith(s, *endings):
return anyTrue(s.endswith, endings)

Discussion

A typical use for endsWith might be to print all names of image files in the current directory:

import os
for filename in os.listdir('.'):
if endsWith(filename, '.jpg', '.jpeg', '.gif'):
print filename

The same general idea shown in this recipe's Solution is easily applied to other tasks related to checking a string for any of several possibilities. The auxiliary function anyTrue is general and fast, and you can pass it as its first argument (the predicate) other bound methods, such as s.startswith or s._ _contains_ _. Indeed, perhaps it would be better to do without the helper function endsWithafter all, directly coding

    if anyTrue(filename.endswith, (".jpg", ".gif", ".png")):

seems to be already readable enough.

Bound Methods

Whenever a Python object supplies a method, you can get the method, already bound to the object, by just accessing the method on the object. (For example, you can assign it, pass it as an argument, return it as a function's result, etc.) For example:

L = ['fee', 'fie', 'foo']
x = L.append

Now, name x refers to a bound method of list object L. Calling, say, x('fum') is the same as calling L.append('fum'): either call mutates object L into ['fee', 'fie', 'foo', 'fum'].

If you access a method on a type or class, rather than an instance of the type or class, you get an unbound method, not "attached" to any particular instance of the type or class: when you call it, you need to pass as its first argument an instance of that type or class. For example, if you set y = list.append, you cannot just call y('I')Python couldn't possibly guess which list you want to append I to! You can, however, call y(L, 'I'), and that is just the same as calling L.append('I') (as long as isinstance(L, list)).

This recipe originates from a discussion on