Example
List Comprehensions can use nested for loops. You can code any number of nested for loops within a list comprehension, and each for loop may have an optional associated if test. When doing so, the order of the for constructs is the same order as when writing a series of nested for statements. The general structure of list comprehensions looks like this:
[ expression for target1 in iterable1 [if condition1] for target2 in iterable2 [if condition2]... for targetN in iterableN [if conditionN] ]For example, the following code flattening a list of lists using multiple for statements:
data = [[1, 2], [3, 4], [5, 6]] output = [] for each_list in data: for element in each_list: output.append[element] print[output] # Out: [1, 2, 3, 4, 5, 6]can be equivalently written as a list comprehension with multiple for constructs:
Live Demo
In both the expanded form and the list comprehension, the outer loop [first for statement] comes first.
In addition to being more compact, the nested comprehension is also significantly faster.
The overhead for the function call above is about 140ns.
Inline ifs are nested similarly, and may occur in any position after the first for:
data = [[1], [2, 3], [4, 5]] output = [element for each_list in data if len[each_list] == 2 for element in each_list if element != 5] print[output] # Out: [2, 3, 4]Live Demo
For the sake of readability, however, you should consider using traditional for-loops. This is especially true when nesting is more than 2 levels deep, and/or the logic of the comprehension is too complex. multiple nested loop list comprehension could be error prone or it gives unexpected result.