Grouping and sum of a list of objects

[Otbredbaron]

Hi,

I have a list of objects with certain attributes: I would like to group them and then calculate the sum of one of their attributes.
I'll try to explain better with an example:

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from itertools import groupby from dataclasses import dataclass   @dataclass class Mine():     number: int     material: str     production: float     um: str     # Unit of measure     activity: bool   @dataclass class Total_Production():     material: str     tot_production: float     um: str     # Unit of measure   def by_material(p):     return p.material       def sum_by_material(mines):     grouped_prods = []     for key, group in groupby(sorted(mines, key=by_material), by_material):         tot = 0         for g in group:             tot += g.production         grouped_prods.append(Total_Production(key, tot, g.um))     return grouped_prods

This works fine, but I would like to calculate the sum using the relevant built-in function or in general to look for better ideas, I tried to write something like this:

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def sum_by_material(mines):     grouped_prods = []     for key, group in groupby(sorted(mines, key=by_material), by_material):         tot = sum(g.production for g in group)         grouped_prods.append(Total_Production(key, tot, ???))     return grouped_prods

But of course I lose the information about the unit of measure and I don't know how to add it.


Thank you in advance

[Gribouillis]

If you are ready to import the well known more_itertools library, you could write

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from more_itertools import spy   def sum_by_material(mines):     grouped_prods = []     for key, group in groupby(sorted(mines, key=by_material), by_material):         (mine,), group = spy(group)         tot = sum(g.production for g in group)         grouped_prods.append(Total_Production(key, tot, mine.um))     return grouped_prods

Note that your code assumes that all the mines with a given material use the same unit of measure.

You could also do this, without additional library

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def sum_by_material(mines):     grouped_prods = []     for key, group in groupby(sorted(mines, key=by_material), by_material):         mine = next(group)         tot = sum((g.production for g in group), mine.production)         grouped_prods.append(Total_Production(key, tot, mine.um))     return grouped_prods