identity = lambda x: x -- a Pythonic idiom?

[Marco Antoniotti]

George Demmy <gdemmy at layton-graphics.com> writes:

> I do a bunch of list-oriented programming with a bunch of one-off
> programs. For a variety of reasons these lists tend to have a bit
> of cruft associated with them (I don't write it -- just process
> it!). So stuff like this shows up quite a bit:
> 
> data = filter(lambda x: x, crufty_list)
> 
> I did it so often, that stuff like this shows up:
> 
> identity = lambda x: x # like this in practice (lazy)
> 
> def identity(x): # Pythonic -- shows up in profiler, etc
>     return x
> 
> data = filter(identity, crufty_list)
> 
> There are, of course, other nifty things to do with identity, and I
> find it used all the time -- far more often than many Python
> built-ins. How prevalent is identity usage in the Python community? Is
> there anyone that would rather suffer hellfire and damnation that do
> something like this?
> 
> Curiously,

The function `identity' is part of the Common Lisp standard. It is
defined extacly as you suspected.

Within CL, it turns out that `identity' is useful in a number of
cases, especially when dealing with sequence operators.  E.g. the
`sort' function is really defined as

	sort <sequence> <predicate> &key (key #'identity)

So that you can write things like

	* (sort '(2 3 4 1) #'<)
        '(1 2 3 4)
        * (sort '((1 3) (3 0) (33 9) (19 5)) #'< :key #'second)
	((3 0) (1 3) (19 5) (33 9))

All in all a good thing.

Cheers

-- 
Marco Antoniotti ========================================================
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