Fixing some argument bugs in pymc3.py.

The patch !293 (merged) introduced a bug which meant that a step_kwargs argument had to provided if there was a step argument, this patch fixes this by introducing an option if nuts_kwargs/step_kwargs is None:. It also allows the user to specify the draws argument in default_kwargs instead of the init (sorry for two patches!) @matthew-pitkin could you check if these are okay ?

bilby/core/sampler/pymc3.py

@@ -62,12 +62,12 @@ class Pymc3(MCMCSampler):
         live_plot_kwargs=None, compute_convergence_checks=True, use_mmap=False)
 
     def __init__(self, likelihood, priors, outdir='outdir', label='label',
-                 use_ratio=False, plot=False, draws=1000,
+                 use_ratio=False, plot=False,
                  skip_import_verification=False, **kwargs):
         Sampler.__init__(self, likelihood, priors, outdir=outdir, label=label,
                          use_ratio=use_ratio, plot=plot,
                          skip_import_verification=skip_import_verification, **kwargs)
-        self.draws = draws
+        self.draws = self.__kwargs['draws']
         self.chains = self.__kwargs['chains']
 
     @staticmethod
@@ -401,7 +401,6 @@ class Pymc3(MCMCSampler):
                             sms = self.step_method[key]
                         else:
                             sms = [self.step_method[key]]
-
                         for sm in sms:
                             if sm.lower() not in step_methods:
                                 raise ValueError("Using invalid step method '{}'".format(self.step_method[key]))
@@ -443,10 +442,15 @@ class Pymc3(MCMCSampler):
                             if curmethod == 'nuts':
                                 if nuts_kwargs is not None:
                                     args = nuts_kwargs
-                                else:
+                                elif step_kwargs is not None:
                                     args = step_kwargs.get('nuts', {})
+                                else:
+                                    args = {}
                             else:
-                                args = step_kwargs.get(curmethod, {})
+                                if step_kwargs is not None:
+                                    args = step_kwargs.get('curmethod', {})
+                                else:
+                                    args = {}
                             self.kwargs['step'].append(pymc3.__dict__[step_methods[curmethod]](vars=[self.pymc3_priors[key]], **args))
                     else:
                         curmethod = self.step_method[key].lower()
@@ -454,10 +458,15 @@ class Pymc3(MCMCSampler):
                         if curmethod == 'nuts':
                             if nuts_kwargs is not None:
                                 args = nuts_kwargs
-                            else:
+                            elif step_kwargs is not None:
                                 args = step_kwargs.get('nuts', {})
+                            else:
+                                args = {}
                         else:
-                            args = step_kwargs.get(curmethod, {})
+                            if step_kwargs is not None:
+                                args = step_kwargs.get(curmethod, {})
+                            else:
+                                args = {}
                         self.kwargs['step'].append(pymc3.__dict__[step_methods[curmethod]](vars=[self.pymc3_priors[key]], **args))
         else:
             with self.pymc3_model:
@@ -470,8 +479,10 @@ class Pymc3(MCMCSampler):
                         if curmethod == 'nuts':
                             if nuts_kwargs is not None:
                                 args = nuts_kwargs
-                            else:
+                            elif step_kwargs is not None:
                                 args = step_kwargs.get('nuts', {})
+                            else:
+                                args = {}
                         else:
                             args = step_kwargs.get(curmethod, {})
                         compound.append(pymc3.__dict__[step_methods[curmethod]](**args))
@@ -483,8 +494,10 @@ class Pymc3(MCMCSampler):
                         if curmethod == 'nuts':
                             if nuts_kwargs is not None:
                                 args = nuts_kwargs
-                            else:
+                            elif step_kwargs is not None:
                                 args = step_kwargs.get('nuts', {})
+                            else:
+                                args = {}
                         else:
                             args = step_kwargs.get(curmethod, {})
                         self.kwargs['step'] = pymc3.__dict__[step_methods[curmethod]](**args)