ramakrishna reddy wrote:
> Hi All,
>
> I know
>
>> sum([1,2,3]) returns 6.
>
> But sum with [] 'empty list' as second parameter returns as below.
>
>> sum([[1,2,3], [3,4,5]], []) returns [1, 2, 3, 3, 4, 5]
> Can any one please explain this logic ? I searched in google but could not
> find the suitable answer.
sum() adds up whatever you feed it (unless it's a string). A simplified
pure-python implementation is
>>> def mysum(iterable, start=0):
... result = start
... for value in iterable:
... result = result + value
... return result
...
When you feed it a list of numbers
>>> mysum([1, 2, 3, 4])
10
it calculates the sum, when the list is empty it returns the start value
>>> mysum([])
0
...whatever it may be:
>>> mysum([], mysum)
<function mysum at 0x7fd5ee36a510>
>>> sum([], sum)
<built-in function sum>
When you feed it a list of lists it tries to add 0 + first_list and fails
>>> mysum([[1,2], [3,4]])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 4, in mysum
TypeError: unsupported operand type(s) for +: 'int' and 'list'
However when you provide a list as the start value that list and the lists
in the `iterable` argument are concatenated
>>> mysum([[1,2], [3,4]], [])
[1, 2, 3, 4]
>>> mysum([[1, "a"], [3, 4.0]], [42])
[42, 1, 'a', 3, 4.0]
...because concatenation is what the + operator is written to do
>>> ["foo", "bar", "baz"] + ["ham", "spam"]
['foo', 'bar', 'baz', 'ham', 'spam']
for Python's built-in lists.
If you feed your own objects to sum() you are completely free how you
implement + -- if you have it remove files from your hard drive then that's
what sum() will do when you try to calculate the sum:
>>> class Remove(list):
... def __radd__(self, other):
... for name in self:
... print("Deleting file", name)
...
>>> sum([Remove(["important.txt", "keepsafe.doc"]),
Remove(["nice_pic.jpg"])])
Deleting file important.txt
Deleting file keepsafe.doc
Deleting file nice_pic.jpg
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