When to use list with sum in python

Question

Here are two examples:

sum(list(map(lambda x:x,range(10))))

and

sum(range(10))

The second example does not require a list(), but the first one does. Why?

How do I know when is list() a necessity? Similarly using list() for min() and max().

I am running python 3.3.5 with ipython 2.2.0. Here is what I see: enter image description here

print(sum) results in <built-in function sum> from python console and <function sum at 0x7f965257eb00> from ipythonNotebook. Looks like an issue with hidden imports in notebook.

sum(map(lambda x:x, range(10))) works just fine for me on Python 3.4.2. What version of Python are you running exactly? — Vaibhav Sagar
– Vaibhav Sagar, Commented Apr 14, 2015 at 1:14
from builtins import * solved the issue by overwriting the numpy.sum with builtins.sum. Thanks to everybody for your patience and help. — BiGYaN
– BiGYaN, Commented Apr 14, 2015 at 16:50

shx2 · Accepted Answer · 2015-04-14 01:21:27Z

3

Neither of the examples require the use of list. The sum builtin function works with any iterable, so converting the result of map to a list isn't necessary.

Just in case, make sure you are indeed using the builtin sum function. Doing something like from numpy import * would override that. (you can simply print sum and see what you get).

answered Apr 14, 2015 at 1:21

shx2

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2 Comments

BiGYaN Over a year ago

Neither of the examples required list() when i can from command line. probably this is an issue with some default imports in ipythonNotebook.

shx2 Over a year ago

@BiGYaN what's the output of print sum?

JCm · Accepted Answer · 2015-04-14 01:18:52Z

1

I guess the 1st one just enforces and expects the output of the map function to be a list because if there are multiple arguments, map() returns a list consisting of tuples containing the corresponding items from all iterables.

But either way base on your example, it would still work.

answered Apr 14, 2015 at 1:18

JCm

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