Michael Puckett Michael Puckett - 1 month ago 5
Python Question

Creating sublists

The opposite of list flattening.

Given a list and a length n return a list of sub lists of length n.

def sublist(lst, n):
sub=[] ; result=[]
for i in lst:
sub+=[i]
if len(sub)==n: result+=[sub] ; sub=[]
if sub: result+=[sub]
return result


An example:

If the list is:

[1,2,3,4,5,6,7,8]


And n is:

3


Return:

[[1, 2, 3], [4, 5, 6], [7, 8]]


Is there a more eloquent / concise way?

An aside, what is preferred when appending lists to lists (in the context above):

list1+=[list2]


Or:

list1.append(list2)


Given that (according to Summerfeild's 'Programming in Python 3') they are the same?

Thanks.

Answer

Such a list of lists could be constructed using a list comprehension:

In [17]: seq=[1,2,3,4,5,6,7,8]
In [18]: [seq[i:i+3] for i in range(0,len(seq),3)]
Out[18]: [[1, 2, 3], [4, 5, 6], [7, 8]]

There is also the grouper idiom:

In [19]: import itertools
In [20]: list(itertools.izip_longest(*[iter(seq)]*3))
Out[20]: [(1, 2, 3), (4, 5, 6), (7, 8, None)]

but note that missing elements are filled with the value None. izip_longest can take a fillvalue parameter as well if something other than None is desired.


list1+=[list2] -- noting the brackets this time -- is equivalent to list1.append(list2). My highest priority when writing code is readability, not speed. For this reason, I would go with list1.append(list2). Readability is subjective, however, and probably is influenced greatly by what idioms you're familiar with.

Happily, in this case, readability and speed seem to coincide:

In [41]: %timeit list1=[1,2,3]; list1.append(list2)
1000000 loops, best of 3: 612 ns per loop

In [42]: %timeit list1=[1,2,3]; list1+=[list2]
1000000 loops, best of 3: 847 ns per loop
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