Why does groupby work like this?

Question

Flask

I decided to use groupby to group by a common feature (the id of the related object).

The result was the following code

for k, v in groupby(order.proposal.proposal_products, key=lambda prod_cp: prod_cp.request_product_id):
     lv = list(v)
     print(type(k), k, lv)

Result

<class 'int'> 25 [<ProposalProduct 11>]
<class 'int'> 28 [<ProposalProduct 12>]
<class 'int'> 24 [<ProposalProduct 10>]
<class 'int'> 28 [<ProposalProduct 14>]

Why are the keys duplicated?

Answer 1

I figured it out. For groupby to work correctly, the array must be sorted.