How do I resolve the error "KeyError: 'Missing constraint index'"?

The error message "KeyError: 'Missing constraint index'" appears when you try to add a single constraint using the method Model.addConstrs(). This is used for adding multiple constraints to a model with a single method call. 

To add a single constraint, use Model.addConstr() instead.

Example

>>> model = gp.Model()
>>> x = model.addVar()
>>> y = model.addVar()
>>> model.addConstrs(x + y <= 1) # try adding a single constraint
Traceback (most recent call last):
File "src/gurobipy/model.pxi", line 3536, in gurobipy.Model.addConstrs
File "src/gurobipy/model.pxi", line 223, in gurobipy.Model.__genexpr_key
AttributeError: 'TempConstr' object has no attribute 'gi_frame'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "src/gurobipy/model.pxi", line 3538, in gurobipy.Model.addConstrs
KeyError: 'Missing constraint index'

Instead, use Model.addConstr() (without the 's' in the end):

>>> model.addConstr(x + y <= 1)
<gurobi.Constr *Awaiting Model Update*>

The plural version Model.addConstrs() can be used as follows when adding multiple constraints (with a similar structure):

>>> z = model.addVars(5)
>>> model.addConstrs(z[i] + z[i+1] <= 1 for i in range(4))
{0: <gurobi.Constr *Awaiting Model Update*>, 1: <gurobi.Constr *Awaiting Model Update*>, 
2: <gurobi.Constr *Awaiting Model Update*>, 3: <gurobi.Constr *Awaiting Model Update*>}