A multidimensional list like
sum adds a sequence together using the
+ operator. e.g
sum([1,2,3]) == 6. The 2nd parameter is an optional start value which defaults to 0. e.g.
sum([1,2,3], 10) == 16.
In your example it does
 + [1,2] + [3,4] where
+ on 2 lists concatenates them together. Therefore the result is
The empty list is required as the 2nd paramter to
sum because, as mentioned above, the default is for
sum to add to 0 (i.e.
0 + [1,2] + [3,4]) which would result in unsupported operand type(s) for +: 'int' and 'list'
This is the relevant section of the help for
sum(sequence[, start]) -> value
Returns the sum of a sequence of numbers (NOT strings) plus the value of parameter 'start' (which defaults to 0).
As wallacoloo comented this is not a general solution for flattening any multi dimensional list. It just works for a list of 1D lists due to the behavior described above.
For a way to flatten 1 level of nesting see this recipe from the itertools page:
def flatten(listOfLists): "Flatten one level of nesting" return chain.from_iterable(listOfLists)
Note that the recipe returns an
itertools.chain object (which is iterable) and the other question's answer returns a
generator object so you need to wrap either of these in a call to
list if you want the full list rather than iterating over it. e.g.