[BangPypers] which one is the best pythonic way .

Anand Balachandran Pillai abpillai at gmail.com
Tue Feb 9 07:48:14 CET 2010

On Tue, Feb 9, 2010 at 10:52 AM, Srinivas Reddy Thatiparthy <
srinivas_thatiparthy at akebonosoft.com> wrote:

> Thanks for the replies and I avoid using lambdas..
> Btw,Shall I avoid using filter and map ?
> Because what ever filter and map do,I could seem to do the same with
> Listcomprehensions..
> Is there any situation in which they fare better than list
> comprehensions?

   map, filter, reduce at al are orders slower when compared to
   list comprehensions, cuz each invocation of these cost one
   function call, whereas list comprehensions are optimized in
   the language.

  For example,

 >>> def f1(): [i for i in range(1000) if i % 3 == 0]
 >>> def f2(): filter(lambda x: x%3==0, range(1000))

 And using timeit,

>>> mytimeit.Timeit(f1, number=10000)
'211.47 usec/pass'
>>> mytimeit.Timeit(f2, number=10000)
'319.08 usec/pass'

The only advantage of map, filter etc over list comprehensions is
that they are sometimes concise over the equivalent list
comprehensions, when combined with already defined functions.

For example,

>>> [ x for x in range(1000) if (x%3==0) or (x%5==0)]
>>> def f(x): (x%3==) or (y %5==0)
>>> map(f, range(1000))

But again, keep in mind that these are much slower when compared
to the list comprehension or generator equivalent.

There are a few cases when a map function looks more "elegant"
when compared to the equivalent list comp.

For example, let us say you want to add together two lists.

>>> l1=range(5, 10)
>>> l2=range(10, 15)

>>> map(lambda x,y: x+y, l1, l2)
[15, 17, 19, 21, 23]

There is no direct way to do this in list comp except taking
the help of "zip".

>>> [a+b for a,b in zip(l1,l2)]
[15, 17, 19, 21, 23]

But practicality beats purity. Given a choice, I will always use the
latter instead of the former, irrespective of the "elegance", which
is questionable and subjective anyway :)

> Regards,
> ~ Srini T
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