BatchRunner's "variable parameters" is not strictly optional

[ericmjl]

I was trying to use BatchRunner to run multiple iterations of my model that had only fixed parameter sets. Initially, when reading the docs, I saw that the variable_parameters is specified as an optional keyword argument. However, if I do not pass in any variable parameters, then BatchRunner fails with the error:

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-54-793336723b8d> in <module>()
      8                         iterations=10,
      9                         max_steps=12 * 20,
---> 10                         model_reporters={'Projects': stage_counts})
     11 batch_run.run_all()

~/anaconda/envs/prog_forecast/lib/python3.6/site-packages/mesa/batchrunner.py in __init__(self, model_cls, variable_parameters, fixed_parameters, iterations, max_steps, model_reporters, agent_reporters, display_progress)
     81         """
     82         self.model_cls = model_cls
---> 83         self.variable_parameters = self._process_parameters(variable_parameters)
     84         self.fixed_parameters = fixed_parameters or {}
     85         self._include_fixed = len(self.fixed_parameters.keys()) > 0

~/anaconda/envs/prog_forecast/lib/python3.6/site-packages/mesa/batchrunner.py in _process_parameters(self, params)
    101         params = copy.deepcopy(params)
    102         bad_names = []
--> 103         for name, values in params.items():
    104             if (isinstance(values, str) or
    105                     not hasattr(values, "__iter__")):

AttributeError: 'NoneType' object has no attribute 'items'

I felt confused when I first encountered this error, as I was expecting BatchRunner to gracefully handle the variable_parameters=None scenario. It thus appeared as a bug to me.

If this is truly a bug, I think there's two different ways fixing this could go. If the intent is to make variable_parameters an optional keyword argument, then adding code to deal with its optionality would be good, e.g. some conditionals that deal with it. Otherwise, if the intent is to make variable_parameters a non-optional argument, it should not have a default value of None.

May I ask which of these design choices is the intended one? If it's the latter, I'm happy to put in a PR to make it non-optional. If it's the former, then I'd like to ask for guidance on putting in a PR, as there's probably a few places I'll need to add in conditionals that I wouldn't be fully aware of.