How to get the index of function parameter list comprehension

January 22, 2024

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*>}

>Solution :

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.