How to get the index of function parameter list comprehension

Question

Gurobipy can apparently read the index of a list comprehension formulated within the parentheses of a function. How does this work? Shouldn't this formulation pass a generator object to the function? How do you read the index from that?

    md = gp.Model()
    md.addConstrs(True for i in [1,2,5,3])

The output contains the indices that where used in the list comprehension formulation:

{1: <gurobi.Constr *Awaiting Model Update*>,
 2: <gurobi.Constr *Awaiting Model Update*>,
 5: <gurobi.Constr *Awaiting Model Update*>,
 3: <gurobi.Constr *Awaiting Model Update*>}

You mean how the addConstrs retrieve the [1,2,5,3] from gen expression? — user459872
– user459872, Commented Jan 22, 2024 at 11:28
Does this answer your question? Get index value in python list comprehension (for in preceded with variable) — Roronoa Zoro
– Roronoa Zoro, Commented Jan 22, 2024 at 11:39
@RoronoaZoro I don't think that is a duplicate. That question is more like "how can I modify my list comp to make the index accessible to me?". The question here is "how is it possible to get the underlying iterator without modifying the code of the comprehension?" — slothrop
– slothrop, Commented Jan 22, 2024 at 11:45

user459872 · Accepted Answer · 2024-01-22 11:51:47Z

6

I am not sure if I understand your question correctly, but if you are wondering how you can retrieve the iterator from generator expression, then that's by accessing <generator>.gi_frame.f_locals.

The gi_frame contains the frame object corresponds to the generator expression and it has f_locals attribute which denotes the local namespace seen by this frame.

>>> my_gen = (True for i in [1,2,5,3])
>>> type(my_gen)
<class 'generator'>
>>> my_gen.gi_frame.f_locals
{'.0': <tuple_iterator object at 0x1003cfa60>}
>>> my_gen.gi_frame.f_locals['.0']
<tuple_iterator object at 0x1003cfa60>
>>> list(my_gen.gi_frame.f_locals['.0'])
[1, 2, 5, 3]

You can even use the more direct API inspect.getgeneratorlocals.

>>> import inspect
>>> 
>>> inspect.getgeneratorlocals(my_gen)
{'.0': <tuple_iterator object at 0x1003cfc70>}

But please do note that:

CPython implementation detail: This function relies on the generator exposing a Python stack frame for introspection, which isn’t guaranteed to be the case in all implementations of Python. In such cases, this function will always return an empty dictionary.

edited Jan 22, 2024 at 11:51

answered Jan 22, 2024 at 11:38

user459872

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1 Comment

Riccardo Bucco Over a year ago

Amazing! Would you mind explain a bit more what those objects are?

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