Question

I want to filter elements from a list of lists, and iterate over the elements of each element using a lambda. For example, given the list:

a = [[1,2,3],[4,5,6]]

suppose that I want to keep only elements where the sum of the list is greater than N. I tried writing:

filter(lambda x, y, z: x + y + z >= N, a)

but I get the error:

 <lambda>() takes exactly 3 arguments (1 given)

How can I iterate while assigning values of each element to x, y, and z? Something like zip, but for arbitrarily long lists.

thanks,

p.s. I know I can write this using: filter(lambda x: sum(x)…, a) but that’s not the point, imagine that these were not numbers but arbitrary elements and I wanted to assign their values to variable names.

Answer 1

Using lambda with filter is sort of silly when we have other techniques available.

In this case I would probably solve the specific problem this way (or using the equivalent generator expression)

>>> a = [[1, 2, 3], [4, 5, 6]]
>>> [item for item in a if sum(item) > 10]
[[4, 5, 6]]

or, if I needed to unpack, like

>>> [(x, y, z) for x, y, z in a if (x + y) ** z > 30]
[(4, 5, 6)]

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If I really needed a function, I could use argument tuple unpacking (which is removed in Python 3.x, by the way, since people don’t use it much): lambda (x, y, z): x + y + z takes a tuple and unpacks its three items as x, y, and z. (Note that you can also use this in def, i.e.: def f((x, y, z)): return x + y + z.)

You can, of course, use assignment style unpacking (def f(item): x, y, z = item; return x + y + z) and indexing (lambda item: item[0] + item[1] + item[2]) in all versions of Python.