Allow emcee to work with pre-release versions

The release version of emcee (2.2.1) is over two years old, however there are a couple of more recent pre-releases available on PyPI. These releases change the names of a few arguments, so in this PR I've made some changes that should allow the use of these newer versions of emcee.

bilby/core/sampler/emcee.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import, print_function
 
 import numpy as np
 from pandas import DataFrame
+from distutils.version import LooseVersion
 
 from ..utils import logger, get_progress_bar
 from .base_sampler import MCMCSampler, SamplerError
@@ -65,14 +66,58 @@ class Emcee(MCMCSampler):
 
     @property
     def sampler_function_kwargs(self):
+        import emcee
+
         keys = ['lnprob0', 'rstate0', 'blobs0', 'iterations', 'thin', 'storechain', 'mh_proposal']
-        return {key: self.kwargs[key] for key in keys}
+
+        # updated function keywords for emcee > v2.2.1
+        updatekeys = {'p0': 'initial_state',
+                      'lnprob0': 'log_prob0',
+                      'storechain': 'store'}
+
+        function_kwargs = {key: self.kwargs[key] for key in keys if key in self.kwargs}
+        function_kwargs['p0'] = self.pos0
+
+        if LooseVersion(emcee.__version__) > LooseVersion('2.2.1'):
+            if function_kwargs['mh_proposal'] is not None:
+                logger.warning("The 'mh_proposal' option is no longer used "
+                               "in emcee v{}, and will be ignored.".format(emcee.__version__))
+            del function_kwargs['mh_proposal']
+
+            for key in updatekeys:
+                if updatekeys[key] not in function_kwargs:
+                    function_kwargs[updatekeys[key]] = function_kwargs.pop(key)
+                else:
+                    del function_kwargs[key]
+
+        return function_kwargs
 
     @property
     def sampler_init_kwargs(self):
-        return {key: value
-                for key, value in self.kwargs.items()
-                if key not in self.sampler_function_kwargs}
+        import emcee
+
+        init_kwargs = {key: value
+                       for key, value in self.kwargs.items()
+                       if key not in self.sampler_function_kwargs}
+
+        init_kwargs['lnpostfn'] = self.lnpostfn
+        init_kwargs['dim'] = self.ndim
+
+        # updated init keywords for emcee > v2.2.1
+        updatekeys = {'dim': 'ndim',
+                      'lnpostfn': 'log_prob_fn'}
+
+        if LooseVersion(emcee.__version__) > LooseVersion('2.2.1'):
+            for key in updatekeys:
+                if key in init_kwargs:
+                    init_kwargs[updatekeys[key]] = init_kwargs.pop(key)
+
+            oldfunckeys = ['p0', 'lnprob0', 'storechain', 'mh_proposal']
+            for key in oldfunckeys:
+                if key in init_kwargs:
+                    del init_kwargs[key]
+
+        return init_kwargs
 
     @property
     def nburn(self):
@@ -107,9 +152,9 @@ class Emcee(MCMCSampler):
     def run_sampler(self):
         import emcee
         tqdm = get_progress_bar()
-        sampler = emcee.EnsembleSampler(dim=self.ndim, lnpostfn=self.lnpostfn, **self.sampler_init_kwargs)
+        sampler = emcee.EnsembleSampler(**self.sampler_init_kwargs)
         self._set_pos0()
-        for _ in tqdm(sampler.sample(p0=self.pos0, **self.sampler_function_kwargs),
+        for _ in tqdm(sampler.sample(**self.sampler_function_kwargs),
                       total=self.nsteps):
             pass
         self.result.sampler_output = np.nan