Idiom for flattening a shallow nested list: how does it work?

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 expe

Answer 1

You should read this as:

for y in l:
    for x in y:
        yield x

That's the generator version, but all comprehensions have the same basic syntax: while the x is put up front, the rest of the expression is still read left-to-right. I was confused by this at first too, expecting it to be the other way around, but it makes sense once you add filtering expressions:

>>> l = [[1,2,3,4,5], [1,"foo","bar"], [2,3]]
>>> [x for y in l
...    if len(y) < 4
...    for x in y
...    if isinstance(x, int)]
[1, 2, 3]

Now imagine having to write this entire thing backwards:

[x if isinstance(x, int)
   for x in y
   if len(y) < 4
   for y in l]

That would be confusing even to veteran Prolog programmers, not to mention the people maintaining Python parsers :)

The current syntax also matches that in Haskell, which inspired list comprehensions in the first place.