The following is my code:
import itertools
i = itertools.chain()
for a in [1, 2, 3]:
i = itertools.chain(i, (a for _ in range(2)))
print(list(i))
[3, 3, 3, 3, 3, 3]
Is there a way I can access the value of a when creating the generator, rather than when I iterate it in the print statement?
I'd like the output to be [1,1,2,2,3,3], ie, the value of a when the generator was created.
This is a trivial problem, but in my case I am iterating 1,000,000 rows in the outer loop, then in the inner loop generating 8 rows for each of those million, so I'm keen to keep it a generator.
Nb. The use case is I'm iterating a table in the outer loop, creating sub-objects for each row, passing the primary key to the sub-objects. The numbers are pretty large, so I want to build up the generator, then bulk insert after the loop (using Django's Model.objects.bulk_create(generator)). But by the time I call bulk_create the primary key is always set to the last row in the outer loop.
gen = itertools.chain()
for id in ParentModel.objects.all().value_list('id', flat=True)):
gen = itertools.chain(gen, (InnerModel(fk=id) for i in range(10000)))
InnerModel.objects.bulk_create(gen)
All the generated InnerModels point to the last OuterModel in the list.
If you don't mind wrapping the tuple into a lambda:
>>> import itertools
>>> i = itertools.chain()
>>> for a in [1, 2, 3]:
>>> i = itertools.chain(i, (lambda x: (x for _ in range(2)))(a))
>>> print(list(i))
[1, 1, 2, 2, 3, 3]
The idea is to copy the value of a of each iteration. lambda's argument can do that. In each iteration, a local variable x is created and assigned with a.
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