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Calling sum() on a list of timedelta objects results in TypeError: unsupported operand type(s) for +: 'int' and 'datetime.timedelta'
Here's a script that illustrates this behaviour: (also attached)
import datetime
x = [datetime.timedelta(1), datetime.timedelta(2)]
print(x[0] + x[1]) # datetime.timedelta(3)
print(sum(x)) # TypeError: unsupported operand type(s) for +: 'int' and 'datetime.timedelta'
The bug is present in all version of Python 2 and Python 3

`sum` has an optional `start` parameter, which defaults to 0, and is used as the first item to add. Since timedeltas and ints are not interoperable, that means you have to explicitly tell sum what to use.
The following code works:
>>> e=[datetime.timedelta(3), datetime.timedelta(5)]
>>> sum(e, datetime.timedelta(0))
datetime.timedelta(8)
This is not a bug, but maybe this could use a more descriptive error message? I don't know.

I don't think this warrants a special error message. If you get it, it is obvious you weren't yourself adding an int to a timedelta, so the next logical course of action should be to look at the docs for sum.

What is the reason for this start parameter? Why will sum not just use the first entry of the input sequence?
I think at least a error message that points programmers into the right direction was very helpful.