Should accidental array substitutions fail gracefully or does savetxt need a fix?

[pgk-vho]

Recent use-case that caused a minor headache that could maybe have a GPkit-side fix:

1. Solve a model with some Vectorized variables
2. Fix the value of a key variable to the optimal solution multiplied by a factor (m.substitutions.update({"velocity": 1.25*sol("velocity").magnitude}). In this instance the variable in question has a Vectorization of 1 so its optimal value is a single-value array.
3. Re-solve
4. Save new solution to text file using savetxt()

Everything works fine until step 4, when savetxt returns:

TypeError: ufunc 'isnan' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

Probing the initial solution reveals:

In [7]: sol("velocity")                                                                                                            
Out[7]: array([array([124.61993074])], dtype=object) <Unit('meter / second')>

Once I figured out the issue, the fix was easy (use .item()) but it feels like GPkit should either:

1. Flag the attempted substitution as invalid, or
2. Quietly "fix" the substitution, or
3. Be able to handle this kind of substitution in savetxt

MWE:

from gpkit import Variable, Model, Vectorize                                                            
                                                                                 
class System(Model):                                                             
    def setup(self):                                                             
        with Vectorize(1):                                                       
            self.Fleet = Fleet()                                                 
        constraints = [self.Fleet]                                               
        self.cost = sum(self.Fleet.z)                                            
        return constraints                                                       
                                                                                 
class Fleet(Model):                                                              
    def setup(self):                                                                                                                   
        with Vectorize(1):                                                       
            self.Vehicle = Vehicle()                                             
                                                                                 
        self.z = z = Variable("z")                                               
        constraints = [                                                          
            z >= sum(self.Vehicle.y**2),                                         
            self.Vehicle,                                                        
        ]                                                                        
        return constraints                                                       
                                                                                 
class Vehicle(Model):                                                            
    def setup(self):                                                             
        self.y = y = Variable("y")                                               
        constraints = [y >= 1]                                                   
        return constraints                                                       
                                                                                 
m = System()                                                                     
sol = m.solve()                                                                  
sol.savetxt("test.txt")                                                          
m.substitutions.update({"y": sol("y")})                 # :(
#m.substitutions.update({"y": sol("y").item()})         # :)
sol2 = m.solve()                                                                 
sol2.savetxt("test2.txt")                                                        
~                         

[bqpd]

Oh, yup, I should fix substitutions so that it doesn't result in an object-array solution dictionary.

[bqpd]

addressed in #1533