Clarify apply_ufunc error message (Issue #2078) (#3119)

* Clarify error message (#2078)

* Update whats-new

* Add issue number to whats-new

* Update xarray/core/computation.py

Co-Authored-By: Deepak Cherian <[email protected]>

* black computation.py for formatting conventions

Author: rdrussotto

doc/whats-new.rst

@@ -80,6 +80,9 @@ Enhancements
 Bug fixes
 ~~~~~~~~~
 
+- Improve "missing dimensions" error message for :py:func:`~xarray.apply_ufunc` 
+  (:issue:`2078`). 
+  By `Rick Russotto <https://github.com/rdrussotto>`_.
 - :py:meth:`~xarray.DataArray.assign_coords` now supports dictionary arguments
   (:issue:`3231`).
   By `Gregory Gundersen <https://github.com/gwgundersen>`_.
@@ -106,7 +109,7 @@ Bug fixes
 - Fix error that arises when using open_mfdataset on a series of netcdf files
   having differing values for a variable attribute of type list. (:issue:`3034`)
   By `Hasan Ahmad <https://github.com/HasanAhmadQ7>`_.
-                               
+
 .. _whats-new.0.12.3:
 
 Documentation

xarray/core/computation.py

@@ -502,9 +502,10 @@ def broadcast_compat_data(variable, broadcast_dims, core_dims):
     missing_core_dims = [d for d in core_dims if d not in set_old_dims]
     if missing_core_dims:
         raise ValueError(
-            "operand to apply_ufunc has required core dimensions %r, but "
-            "some of these are missing on the input variable:  %r"
-            % (list(core_dims), missing_core_dims)
+            "operand to apply_ufunc has required core dimensions {}, but "
+            "some of these dimensions are absent on an input variable: {}".format(
+                list(core_dims), missing_core_dims
+            )
         )
 
     set_new_dims = set(new_dims)