Nested list comprehension with if statement

Question

I'm trying to get my head around nested list comprehension and have read the excellent explanation here.

The problem I'm having translating is that I've an if clause in my inner loop and I can't see how to apply this to the func() step as I'm loosing the counter I get from enumerate() when I go from nested loops to list comprehension.

nested_list = [[{'a': 1, 'b': 2}, {'c': 3, 'd': 4}], [{'a': 5, 'b': 6}, {'c': 7, 'd': 8}]]
new_list = []
for c, x in enumerate(nested_list):
    for d, y in enumerate(x):
        if d == 1:
            new_list.append(y)
print(new_list)
[{'c': 3, 'd': 4}, {'c': 7, 'd': 8}]

Nested list comprehension might look something like

new_list = [if ??? y
               for x in nested_list
                   for y in x]

...but I can't see/think how to get the clause as I have no counter under the nested list comprehension.

Is there a way of achieving this or should I stick to the nested loops approach?

Answer 1

you can rewrite this as follows:

new_list = [y for x in nested_list for d, y in enumerate(x) if d == 1]

loops are in the "natural" order, and the condition is in the end. Include y only if d==1

one possible result (because dicts aren't ordered):

[{'c': 3, 'd': 4}, {'c': 7, 'd': 8}]

Note that in your case, it's simpler and more efficient (O(n) vs O(n**2)) to write:

new_list = [x[1] for x in nested_list]

the only difference is that if x is too short the latter code will break so maybe test length:

new_list = [x[1] for x in nested_list if len(x)>1]