Can be done more pythonic?

Question

Here is my code:

for x in range(len(patterns)):
 for y in range(len(patterns[x])):
 patterns[x][y] = list(patterns[x][y])

It takes that as patterns:

[['oxooo', 'oxoxo'], ['oxooo']]

And gives that as patterns:

[[['o', 'x', 'o', 'o', 'o'], ['o', 'x', 'o', 'x', 'o']], [['o', 'x', 'o', 'o', 'o']]]

For me, looks not pythonic. Can it be done better? Output should be the same.

• ivan_pozdeev: possible duplicate of Transforming nested Python loops into list comprehensions

Answer 1

A list comprehension is what you're looking for.

[[list(z) for z in y] for y in x]

Answer 2

Here is a general recursive solution that will achieve this for an arbitrary nesting of strings in lists.

def listify(x):
 if isinstance(x, str):
 return list(x)
 else:
 return [listify(i) for i in x]
>>> l = [['oxooo', 'oxoxo'], ['oxooo']]
>>> listify(l)
[[['o', 'x', 'o', 'o', 'o'], ['o', 'x', 'o', 'x', 'o']], [['o', 'x', 'o', 'o', 'o']]]

Answer 3

In your example you have a nested list in two levels. If that's always the case, you can use a list comprehension:

[[list(p) for p in pp] for pp in patterns]

If you don't know how many levels you have, you need to use a recursive function, for instance:

def expand_str(v):
 if isinstance(v, str): # basestring for python2
 return list(v)
 else: # assume that otherwise v is iterable
 return [expand_str(vv) for vv in v]

• Cory Kramer: You basically just copied the two answers above you... and it took you 5+ minutes to do so. And in fact your second version doesn't even work.
• Francis Colas: Thanks for nicely pointing out the typo. When I wrote my message the two answers weren't above me; I actually wondered which approach would be correct for OP (because they're not equivalent) and decided in providing them both. I'll try to improve my typing skills, thanks for the advice.

Answer 4

If you want to modify the original list like your own code does use enumerate:

for ind, ele in enumerate(l):
 l[ind] = [list(s) for s in ele]
print(l)
[[['o', 'x', 'o', 'o', 'o'], ['o', 'x', 'o', 'x', 'o']], [['o', 'x', 'o', 'o', 'o']]]

You can also use map with enumerate:

for ind, ele in enumerate(l):
 l[ind] = list(map(list, ele))

You can also use the [:] syntax with a generator expression or list comprehension to change the original list:

l[:] = [list(map(list,ele)) for ele in l]
l[:] = (list(map(list,ele)) for ele in l)