Combiner signature

[rogiervandergeer]

The combiner signature has been changed to combine(*parents, **kwargs)
to combine(parents, **kwargs). This was done mainly to prevent users accidentally passing too many parents. E.g., if one has a combiner function like:

def combiner(mom, dad, x=1, y=0):
    return (mom+dad)/x + y

and one passes three parents and no kwargs the function is called as combiner(*parents), which results in the third parent ending up as x. This only throws an error when x is also passed (and #76 is solved). Passing all parents as a sequence to a single argument solves this problem.

[rogiervandergeer]

@koaning can you check if the error messages you get with the new signature are any better?

[koaning]

import random 
from evol import Population, Evolution

pop = Population([i for i in range(100)], lambda x:x)
evo = (Evolution()
      .survive(n=4)
      .breed(parent_picker=lambda x: [random.choice(x)],
             combiner=lambda parents: (parents[0]+parents[1])/2))

pop.evolve(evo, n=10)

Results in

IndexError: tuple index out of range

Am thinking about what might be a nice change for offspring_generator in order to facilitate errors like this.

[koaning]

I mean ... we could do a try/catch here such that we can return a very descriptive error message. Something like

Something went wrong in the breed step. This is most likely due to the parent_picker that is passed does not have the number of parents picked that is required by the combiner. The parent_picker selected <X> parents when the error happened.

It feels like it might be a nice thing to do. When we resolve this we also need to re-generate the documentation.

[koaning]

i actually prefer the resulting syntax too

[rogiervandergeer]

Catching (a subset of) exceptions thrown by the breeder and then raising another exception is tricky since you never know what went wrong inside the function. We could however augment the message from any exception thrown by adding a string like "the function was called with parents".