What does the built-in function sum do with sum(list, [])?

Question

When I want to unfold a list, I found a way like below:

>>> a = [[1, 2], [3, 4], [5, 6]]
>>> a
[[1, 2], [3, 4], [5, 6]]
>>> sum(a,         


        

Answer 1

First of all, never use sum for concatenating/flattening lists because it's of quadratic time and hence not efficient at all compare to the other ways around. It actually uses a schlemiel the painter algorithm.

The sum function calls the __add__ attribute of the start on each iteration with all the items of an iterable that's been passed as the first argument.

For example :

>>> [].__add__([2,3])
[2, 3]
#OR
>>> [] + [1,2,3]
[1, 2, 3]

And in this case the result would be a concatenated list of your input lists. From an algorithmic perspective it does the followings:

>>> a = [[1, 2], [3, 4], [5, 6]]
>>> start = []
>>> for i in a:
...     start += i
... 
>>> start
[1, 2, 3, 4, 5, 6]

Not that you can call the sum function on any sequence of objects that have an __add__ attribute, but note that since the default start argument is 0 if your object is not an integer it will raise an TypeError. In that case you need to specify a proper start for the function.

>>> class newObj(object):
...    def  __init__(self,val):
...         self.val = val
...    def __add__(self,item):
...        return '{}_____{}'.format(self.val,item)
... 
>>> 
>>> start=newObj('new_obj')
>>> 
>>> start
<__main__.newObj object at 0x7f75f9241c50>
>>> 
>>> start + 5
'new_obj_____5'
>>> 
>>> 
>>> sum(['1','2','3'],start)
'new_obj_____123'