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Date: Sun, 19 Jan 2014 01:24:56 +0000
From: John Allsup <pydev@allsup.co>
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To: python-list@python.org
Subject: Re: How to write this as a list comprehension?
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Hi,

I'd agree with the advice that it's not the best idea: readability sucks 
here, but consider the following:


import time

def somefunc(a,b,c,d): # dummy function
     return "{} - {} - {} : {}".format(a,b,c,d)
l = [(time.time(),"name {}".format(n)) for n in range(100)] # dummy data

# the line in question
labels = [somefunc(*(lambda t,n: 
(t.tm_mon,t.tm_mday,t.tm_wday,n))(time.localtime(x[0]),x[1])) for x in l]


print(labels) # just to see the result


If you find that hard to decipher, the consider the maintainability of 
code you write that uses such comprehensions.  You need to include 
comments that explain what this does, and it is easier to write a 
longhand version using .append() and variable assignments.  I presume
performance won't be an issue determining the right approach, since then 
you'd be using C or C++.

John

On 17/01/2014 23:49, Dan Stromberg wrote:
> On Fri, Jan 17, 2014 at 3:19 PM, Piet van Oostrum <piet@vanoostrum.org> wrote:
>> Hi,
>>
>> I am looking for an elegant way to write the following code as a list
>> comprehension:
>>
>> labels = []
>> for then, name in mylist:
>>      _, mn, dy, _, _, _, wd, _, _ = localtime(then)
>>      labels.append(somefunc(mn, day, wd, name))
>
> My recomendation: Don't use a list comprehension.  List comprehensions
> and generator expressions are great for quick little things, but
> become less readable when you have to string them over multiple
> physical lines.
>
>> labels = [somefunc(mn, day, wd, name)
>>              for then, name in mylist
>>              for _, mn, dy, _, _, _, wd, _, _ in [localtime(then)]]