Packing a list of lists with struct.pack()

[Fredrik Lundh]

Panos Laganakos wrote:

> I have a list that includes lists of integers, in the form of:
> li = [[0, 1, 2], [3, 4, 5], ...]

> What can I do to get li as a single list of integers?
>
> I tried list comprehension in the form of:
> [([j for j in i]) for i in li]
>
> But that doesn't seem to work, any ideas?

you have it backwards: a nested list expression is like a nested
for loop, but with the innermost expression at the beginning.  a
for-loop would look like:

    for i in li:
        for j in i:
            ... do something with j ...

so the corresponding comprehension is

    [j for i in li for j in i]

which gives you the expected result.  when you pass this to pack,
you can use a generator expression instead:

    data = struct.pack("%di" % (len(li)*3), *(j for i in li for j in i))

</F>