I'd like to make the python's list to ['a','a','b',b','c','c'] from ['a','b','c']

Question

I'd like to make the python's list to ['a','a','b',b','c','c'] from ['a','b','c'].

Anyboday know to do this? Thank you !

Answer 1

For something that more or less says “I want to repeat each element twice”, there’s the nested list comprehension with range:

>>> l = ['a', 'b', 'c']
>>> [x for x in l for _ in range(2)]
['a', 'a', 'b', 'b', 'c', 'c']

You can make it a tiny bit shorter with a list multiplication if you find that more readable and won’t need to extend 2 to a large number and convert the list comprehension to a generator expression:

>>> l = ['a', 'b', 'c']
>>> [y for x in l for y in [x, x]]

If you’re a fan of Haskell, where l >>= replicate 2 would work, you can imitate that:

import itertools
from functools import partial
from operator import mul


def flat_map(iterable, f):
    return itertools.chain.from_iterable(map(f, iterable))


l = ['a', 'b', 'c']
flat_map(l, partial(mul, 2))

Answer 2

You could always create a new list:

for x in oldList: 
  newList.append(x)
  newList.append(x)

Note that this will create a new list rather than modifying the old one!

Answer 3

source = ['a','b','c']
result = [el for el in source for _ in (1, 2)]
print(result)

gives you

['a', 'a', 'b', 'b', 'c', 'c']