[Python-Dev] sorted() (original) (raw)

Fran�ois Pinard pinard@iro.umontreal.ca
Wed, 25 Sep 2002 17:07:13 -0400

Hi, Guido, and people.

It recurrently happens that newcomers on the Python mailing list are surprised that list.sort() does not return the sorted list as value. I quite understand and agree that this is a good thing, because sorting is done in place, and Python programmers should stay aware and alert of this fact.

Yet, I often see myself writing things like:

keys = messages.keys()
keys.sort()
for key in keys:
    DO_SOMETHING

This is not difficult to write, only slightly annoying. Writing:

def sorted(list):
    list = list[:]
    list.sort()
    return list

with the goal of simplifying the first excerpt into:

for key in sorted(message.keys()):
    DO_SOMETHING

it is not really worth for small programs. But in larger programs, where one often loops over the sorted element of a list, it might become reasonable to write this extra definition. My feeling is that the idiom is common enough to be worth a list method, so the above could be written instead:

for key in message.keys().sorted():
    DO_SOMETHING

I immediately see an advantage and an inconvenient. The inconvenient is that users might confuse .sort()' with .sorted()', however we decide to spell sorted', so the existence of both may be some kind of trap. The advantage is that the .sorted()' method fits well within how Python has evolved recently, offering more concise and legible writings for frequent idioms.

Tim invested a lot of courageous efforts so Python sort' becomes speedier. A .sorted()' method requires separate space to hold the result, using the same size as the original, and that guaranteed extra-space may eventually be put to good use for speeding up the sorting even more. The constraint of a sort being in-place has indeed a cost, and deep down, we agree that this constraint is artificial in contexts where `.sorted()' is really what the user needs.

-- Fran�ois Pinard http://www.iro.umontreal.ca/~pinard