Allow special MB models to work even without t*

oggm/core/climate.py

@@ -541,10 +541,7 @@ def mb_climate_on_height(gdir, heights, *, time_range=None, year_range=None):
                                        "None if using a winter_prcp_factor")
 
         # Have we decided on a factor yet?
-        try:
-            df = gdir.read_json('local_mustar')
-        except FileNotFoundError:
-            df = {}
+        df = gdir.read_json('local_mustar', allow_empty=True)
         prcp_fac = df.get('glacier_prcp_scaling_factor')
         if prcp_fac is None:
             # Then decide and store
@@ -924,10 +921,7 @@ def _fallback_local_t_star(gdir):
 
     """
     # Scalars in a small dict for later
-    try:
-        df = gdir.read_json('local_mustar')
-    except FileNotFoundError:
-        df = {}
+    df = gdir.read_json('local_mustar', allow_empty=True)
     df['rgi_id'] = gdir.rgi_id
     df['t_star'] = np.nan
     df['bias'] = np.nan
@@ -1062,10 +1056,7 @@ def local_t_star(gdir, *, ref_df=None, tstar=None, bias=None,
                                           '{:.2f}'.format(gdir.rgi_id, mustar))
 
     # Scalars in a small dict for later
-    try:
-        df = gdir.read_json('local_mustar')
-    except FileNotFoundError:
-        df = {}
+    df = gdir.read_json('local_mustar', allow_empty=True)
     df['rgi_id'] = gdir.rgi_id
     df['t_star'] = int(tstar)
     df['bias'] = bias
@@ -1574,10 +1565,7 @@ def mu_star_calibration_from_geodetic_mb(gdir,
     gdir.write_json(out, 'climate_info')
 
     # Store diagnostics
-    try:
-        df = gdir.read_json('local_mustar')
-    except FileNotFoundError:
-        df = {}
+    df = gdir.read_json('local_mustar', allow_empty=True)
     df['rgi_id'] = gdir.rgi_id
     df['t_star'] = np.nan
     df['bias'] = 0

oggm/core/massbalance.py

@@ -633,7 +633,8 @@ def __init__(self, gdir, mu_star=None, bias=None,
             you want to use (the default is to use the calibrated value)
         y0 : int, optional, default: tstar
             the year at the center of the period of interest. The default
-            is to use tstar as center.
+            is to use tstar as center. If t_star is not available, raises
+            an error.
         halfsize : int, optional
             the half-size of the time window (window size = 2 * halfsize + 1)
         filename : str, optional
@@ -651,7 +652,10 @@ def __init__(self, gdir, mu_star=None, bias=None,
 
         if y0 is None:
             df = gdir.read_json('local_mustar')
-            y0 = df['t_star']
+            y0 = df.get('t_star', np.NaN)
+            if not np.isfinite(y0):
+                raise InvalidParamsError('t_star has not been set for this '
+                                         'glacier. Please set `y0` explicitly')
 
         # This is a quick'n dirty optimisation
         try:
@@ -851,7 +855,10 @@ def __init__(self, gdir, mu_star=None, bias=None,
 
         if y0 is None:
             df = gdir.read_json('local_mustar')
-            y0 = df['t_star']
+            y0 = df.get('t_star', np.NaN)
+            if not np.isfinite(y0):
+                raise InvalidParamsError('t_star has not been set for this '
+                                         'glacier. Please set `y0` explicitly')
 
         self.mbmod = PastMassBalance(gdir, mu_star=mu_star, bias=bias,
                                      filename=filename,
@@ -950,7 +957,10 @@ def __init__(self, gdir, mu_star=None, bias=None,
             else:
                 if y0 is None:
                     df = gdir.read_json('local_mustar')
-                    y0 = df['t_star']
+                    y0 = df.get('t_star', np.NaN)
+                    if not np.isfinite(y0):
+                        raise InvalidParamsError('t_star has not been set for this '
+                                                 'glacier. Please set `y0` explicitly')
                 self.years = np.arange(y0 - halfsize, y0 + halfsize + 1)
         else:
             self.rng = None
@@ -1247,7 +1257,11 @@ def __init__(self, gdir, fls=None, mu_star=None,
                     fl.rgi_id != gdir.rgi_id) and (_y0 is None):
 
                 df = gdir.read_json('local_mustar', filesuffix='_' + fl.rgi_id)
-                kwargs['y0'] = df['t_star']
+                y0 = df.get('t_star', np.NaN)
+                if not np.isfinite(y0):
+                    raise InvalidParamsError('t_star has not been set for this '
+                                             'glacier. Please set `y0` explicitly')
+                kwargs['y0'] = y0
 
             self.flowline_mb_models.append(
                 mb_model_class(gdir, mu_star=fl.mu_star, bias=fl_bias,

oggm/tests/test_models.py

@@ -1065,10 +1065,16 @@ def test_hef_mu_star_calibration_from_geodetic_mb(self, hef_gdir):
 
         np.testing.assert_allclose(ref_mb_geodetic, mb_modelled.mean())
         np.testing.assert_allclose(pf, 3.35713, rtol=1e-4)
+
+        with pytest.raises(InvalidParamsError):
+            # This does not work
+            mb = massbalance.ConstantMassBalance(gdir)
+
         cfg.PARAMS['use_winter_prcp_factor'] = False
         cfg.PARAMS['prcp_scaling_factor'] = prev_fac
 
 
+
 class TestModelFlowlines():
 
     def test_rectangular(self):

oggm/utils/_workflow.py

@@ -3067,7 +3067,7 @@ def write_pickle(self, var, filename, use_compression=None, filesuffix=''):
         with _open(fp, 'wb') as f:
             pickle.dump(var, f, protocol=4)
 
-    def read_json(self, filename, filesuffix=''):
+    def read_json(self, filename, filesuffix='', allow_empty=False):
         """Reads a JSON file located in the directory.
 
         Parameters
@@ -3076,6 +3076,8 @@ def read_json(self, filename, filesuffix=''):
             file name (must be listed in cfg.BASENAME)
         filesuffix : str
             append a suffix to the filename (useful for experiments).
+        allow_empty : bool
+            if True, does not raise an error if the file is not there.
 
         Returns
         -------
@@ -3087,8 +3089,15 @@ def read_json(self, filename, filesuffix=''):
             return self._read_deprecated_climate_info()
 
         fp = self.get_filepath(filename, filesuffix=filesuffix)
-        with open(fp, 'r') as f:
-            out = json.load(f)
+        if allow_empty:
+            try:
+                with open(fp, 'r') as f:
+                    out = json.load(f)
+            except FileNotFoundError:
+                out = {}
+        else:
+            with open(fp, 'r') as f:
+                out = json.load(f)
         return out
 
     def write_json(self, var, filename, filesuffix=''):