Display data returned in ufunc error message

This makes debugging much easier!

xarray/core/computation.py

@@ -766,11 +766,9 @@ def func(*arrays):
         not isinstance(result_data, tuple) or len(result_data) != signature.num_outputs
     ):
         raise ValueError(
-            "applied function does not have the number of "
-            "outputs specified in the ufunc signature. "
-            "Result is not a tuple of {} elements: {!r}".format(
-                signature.num_outputs, result_data
-            )
+            f"applied function does not have the number of "
+            f"outputs specified in the ufunc signature. "
+            f"Result is not a tuple of {signature.num_outputs} elements:\n\n{result_data}"
         )
 
     objs = _all_of_type(args, Variable)
@@ -784,21 +782,21 @@ def func(*arrays):
         data = as_compatible_data(data)
         if data.ndim != len(dims):
             raise ValueError(
-                "applied function returned data with unexpected "
+                "applied function returned data with an unexpected "
                 f"number of dimensions. Received {data.ndim} dimension(s) but "
-                f"expected {len(dims)} dimensions with names: {dims!r}"
+                f"expected {len(dims)} dimensions with names: {dims!r}. The data returned "
+                f"was:\n\n{data!r}"
             )
 
         var = Variable(dims, data, fastpath=True)
         for dim, new_size in var.sizes.items():
             if dim in dim_sizes and new_size != dim_sizes[dim]:
                 raise ValueError(
-                    "size of dimension {!r} on inputs was unexpectedly "
-                    "changed by applied function from {} to {}. Only "
+                    f"size of dimension '{dim}' on inputs was unexpectedly "
+                    f"changed by applied function from {dim_sizes[dim]} to {new_size}. Only "
                     "dimensions specified in ``exclude_dims`` with "
-                    "xarray.apply_ufunc are allowed to change size.".format(
-                        dim, dim_sizes[dim], new_size
-                    )
+                    "xarray.apply_ufunc are allowed to change size. "
+                    "The data returned was:\n\n{data!r}"
                 )
 
         var.attrs = attrs

xarray/tests/test_computation.py

@@ -1666,7 +1666,10 @@ def identity(x):
     def tuple3x(x):
         return (x, x, x)
 
-    with pytest.raises(ValueError, match=r"number of outputs"):
+    with pytest.raises(
+        ValueError,
+        match=r"number of outputs.*Result is not a tuple of 2 elements:\n\n\[0 1 2 3 4 5 6 7 8 9\]",
+    ):
         apply_ufunc(identity, variable, output_core_dims=[(), ()])
 
     with pytest.raises(ValueError, match=r"number of outputs"):
@@ -1682,7 +1685,10 @@ def add_dim(x):
     def remove_dim(x):
         return x[..., 0]
 
-    with pytest.raises(ValueError, match=r"unexpected number of dimensions"):
+    with pytest.raises(
+        ValueError,
+        match=r"unexpected number of dimensions.*The data returned was:\n\n.*array\(\[\[0",
+    ):
         apply_ufunc(add_dim, variable, output_core_dims=[("y", "z")])
 
     with pytest.raises(ValueError, match=r"unexpected number of dimensions"):