how to remove an element from a nested list?

Question

if I have a nested list such as:

m=[[34,345,232],[23,343,342]]

if I write m.remove(345) it gives an error message saying the element is not in the list.

I want to know how to remove an element from the nested list, easily.

Answer 1

In [5]: m=[[34,345,232],[23,343,342]]

In [7]: [[ subelt for subelt in elt if subelt != 345 ] for elt in m] 
Out[7]: [[34, 232], [23, 343, 342]]

Note that remove(345) only removes the first occurrance of of 345 (if it exists). The above code removes all occurrances of 345.

Answer 2

There is no shortcut for this. You have to remove the value from every nested list in the container list:

for L in m:
    try:
        L.remove(345)
    except ValueError:
        pass

If you want similiar behavior like list.remove, use something like the following:

def remove_nested(L, x):
    for S in L:
        try:
            S.remove(x)
        except ValueError:
            pass
        else:
            break  # Value was found and removed
    else:
        raise ValueError("remove_nested(L, x): x not in nested list")