CODEWITHSUNDEEP
pythonlist-comprehension
Kenny Ostrom
Kenny Ostrom

Reputation: 5871

Can list comprehension have a prefix, suffix?

I need to create a list comprehension and add one extra item at the start and end. Is there a better way than just using a helper function to do it?

# convert tuple from permutation into a list with predecessor and successor
def perm_to_list(predecessor, perm, successor):
    result = [predecessor]
    result.extend(list(perm))
    result.append(successor)
    return result

candidates = [perm_to_list(prefix, x, suffix) 
              for x in permutations(something)]

Upvotes: 3

Views: 433

Answers (3)

VPfB
VPfB

Reputation: 17247

There is a new feature available only in Python 3.5:

Unpacking of iterables anywhere in a list:

[prefix, *iterable, suffix]

(I wish they added it sooner)

Upvotes: 2

ospahiu
ospahiu

Reputation: 3525

As an alternative if speed is critical, thought I'd offer this approach. You can use a deque which has native support for appending to either the head or tail of the structure.

It's a double-ended queue data structure, and as such offers O(1) prepending as opposed to O(n) for lists.

from collections import deque
dq = deque(range(1,10))  # Test list passed to deque -> deque([1, 2, 3, 4, 5, 6, 7, 8, 9])
dq.appendleft(0)
dq.append(10)
print dq
deque([0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

Upvotes: 1

J Earls
J Earls

Reputation: 1812

To match the sample code (where you add prefix and suffix to each element of a list comprehension)

candidates = [[prefix] + list(x) + [suffix] for x in permutations(something)]

To answer the question you actually asked (to add a prefix and suffix to the list comprehension itself)

candidates = [prefix] + [somefunction(x) for x in permutations(something)] + [suffix]

Upvotes: 4

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