Django - Pulp Optimisation AttributeError: 'str' object has no attribute 'hash' - django

Question

I am writing a script for pulp Optimisation to engage with my Django database. The problem contains a few thousand variables to be optimised and several hundred constraints which vary depending upon the values of a,b,c.
var_names[]
var_values{}
for _foo_ in list_1:
for _bar_ in list_2:
for _var_ in list_3:
for _eet_ list_4:
var_name = str(_foo_)+str(_bar_)+str(_var_)+str(_eet_)
var_names.append(var_name)
exec(str(_foo_)+str(_bar_)+str(_var_)+str(_eet_) + "= LpVariable("str(_foo_)+str(_bar_)+str(_var_)+str(_eet_)+", lowBound=0, cat='Integer')")
var_value = DataBase.objects.get(column_A = str(_foo_)+str(_var_)).value
var_values.append(var_value)
obj_func = LpAffineExpression([var_names[i],var_values[i] for in in range(len(var_names))])
problem = LpProblem(name="name", sense=LpMinimise)
#Example of constraints
exec("problem += (" + str(function(a1,b1,c1)_) +str(function(a1,b1,c2)) +" >= Database_2.objects.get(column_A = z1).value")
problem += obj_func
problem.sovle()
The code works in jupyter notebook when I load the database info as a dataframe. However, I keep receiving this following error code when using in Djagno:
File "/path/to/files/prob.py", line 1610, in <module>
problem.solve()
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1913, in solve
status = solver.actualSolve(self, **kwargs)
File "/path/to/files/lib/python3.9/site-packages/pulp/apis/coin_api.py", line 137, in actualSolve
return self.solve_CBC(lp, **kwargs)
File "/path/to/files/lib/python3.9/site-packages/pulp/apis/coin_api.py", line 153, in solve_CBC
vs, variablesNames, constraintsNames, objectiveName = lp.writeMPS(
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1782, in writeMPS
return mpslp.writeMPS(self, filename, mpsSense=mpsSense, rename=rename, mip=mip)
File "/path/to/files/lib/python3.9/site-packages/pulp/mps_lp.py", line 204, in writeMPS
constrNames, varNames, cobj.name = LpProblem.normalisedNames()
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1546, in normalisedNames
_variables = self.variables()
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1624, in variables
self.addVariables(list(self.objective.keys()))
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1614, in addVariables
self.addVariable(v)
File "/path/to/files/lib/python3.9/site-packages/pulp/pulp.py", line 1603, in addVariable
if variable.hash not in self._variable_ids:
AttributeError: 'str' object has no attribute 'hash'
I have the code stored as .py file which is called views.py.
I believe it may be an issue with the namespace of the LpVariable creations. I have tried to:
define the whole problem encapsulated as a function with no entry variable and returns a dict of the solution.
Define the problem as a class with problem.create and problem.solve as methods to create the variables and solve the function.
Update the exec() code to store variables in the globals diction.
exec(str(_foo_)+str(_bar_)+str(_var_)+str(_eet_) + "= LpVariable("str(_foo_)+str(_bar_)+str(_var_)+str(_eet_)+", lowBound=0, cat='Integer')", globals())
And alternately creating a local dict and executing the above code with locals(),local_dict .
Used LpVariable.dict
variables = LpVariable.dicts("variables", [(_foo_, _bar_, _var_, _eet_), for _foo_ in list1 for _bar_ in list2 for _var_ in list3 for _eet_ in list4], lowBound=o, cat="Integer")
This does create all the variables, however the function used in the constraints references the variables as per the name str(foo)+str(bar)+str(var)+str(eet) and not variable[i] , which then generates undefined variable errors.
As mentioned, this code does work in jupyter, I am just at a loss as to what the error may be a result of.

Answer 1

LpAffineExpression expects LpVariable, not var_names[i].
var_items = []
# exec(str(_foo_)+str(_bar_)+str(_var_)+str(_eet_) + "= LpVariable(" + str(_foo_)+str(_bar_)+str(_var_)+str(_eet_)+", lowBound=0, cat='Integer')")
# var_value = DataBase.objects.get(column_A = str(_foo_)+str(_var_)).value
# var_values.append(var_value)
var_value = LpVariable(var_name, lowBound=0, cat='Integer')
var_coeff = DataBase.objects.get(column_A = str(_foo_)+str(_var_)).value
var_items.append((var_value, var_coeff))
# obj_func = LpAffineExpression([var_names[i],var_values[i] for i in range(len(var_names))])
obj_func = LpAffineExpression(var_items)
Reference: https://coin-or.github.io/pulp/technical/pulp.html#pulp.LpAffineExpression