Question

I found this bit of code in a module I am working on:

l = opaque_function()
thingys = [x for y in l for x in y]

I can’t read this. By experiment, I was able to determe that it is flattening a 2-level nested list, but the syntex is still opaque to me. It has obviously omitted some optional brackets.

>>> l = [[1,2],[3,4]]
>>> [x for y in l for x in y]
[1, 2, 3, 4]

My eyes want to parse it as either: [x for y in [l for x in y] ] or [ [x for y in l] for x in y ], but both of those fail due to y not being defined.

How should I be reading this?

(I suspect I will feel very embarassed when this is explained.)

Answer 1

From the list displays documentation:

When a list comprehension is supplied, it consists of a single expression followed by at least one for clause and zero or more for or if clauses. In this case, the elements of the new list are those that would be produced by considering each of the for or if clauses a block, nesting from left to right, and evaluating the expression to produce a list element each time the innermost block is reached.

Thus, your expression can be rewritten as:

thingys = []
for y in l:
    for x in y:
        thingys.append(x)