Processing a list of lists

I have a list of lists, say:

    arr = [[1, 2], [1, 3], [1, 4]]

I would like to append 100 to each of the inner lists. Output for the above example would be:

    arr = [[1, 2, 100], [1, 3, 100], [1, 4, 100]]

I can of course do:

    for elem in arr:
开发者_开发问答        elem.append(100)

But is there something more pythonic that can be done? Why does the following not work:

    arr = [elem.append(100) for elem in arr]

The second version should be written like arr = [elem + [100] for elem in arr]. But the most pythonic way if you ask me is the first one. The for construct has it's own use, and it suits very well here.

Note that your code also works - the only thing you have to know is that there is no need to assign result to variable in such a case:

arr = [[1, 2], [1, 3], [1, 4]]
[x.append(100) for x in arr]

After execution arr will contain updated list [[1, 2, 100], [1, 3, 100], [1, 4, 100]], e.g it work like in-place update, but this not very common practice to do this.

The same result you will get in the next case:

map(lambda x: x.append(100), arr)

As was discussed you can use list comprehension or map to do this with assigning result to any variable:

res = map(lambda x: x + [100], arr)

You can do

[a + [100] for a in arr]

The reason why your append doesn't work is that append doesn't return the list, but rather None.

Of course, this is more resource intensive than just doing append - you end up making copies of everything.

This is the more pythonic way

 for elem in arr:
     elem.append(100)

but as an option you can also try this:

[arr[i].append(100) for i in range(len(arr))]
print arr # It will return [[1, 2, 100], [1, 3, 100], [1, 4, 100]]