python - How do I sum tuples in a list where the first value is the same?

Question

I have a list of stocks and positions as tuples. Positive for buy, negative for sell. Example:

p = [('AAPL', 50), ('AAPL', -50), ('RY', 100), ('RY', -43)]

How can I sum the positions of stocks, to get the current holdings?

result = [('AAPL', 0), ('RY', 57)]

Answer 1

How about this? You can read about collections.defaultdict.

>>> from collections import defaultdict
>>> testDict = defaultdict(int)
>>> p = [('AAPL', 50), ('AAPL', -50), ('RY', 100), ('RY', -43)]
>>> for key, val in p:
        testDict[key] += val


>>> testDict.items()
[('AAPL', 0), ('RY', 57)]