Raise an informative error message when object array has mixed types (p…

…ydata#4700)

Co-authored-by: Mathias Hauser <mathause@users.noreply.github.com>
Co-authored-by: Maximilian Roos <5635139+max-sixty@users.noreply.github.com>

xarray/conventions.py

@@ -52,16 +52,32 @@ def _var_as_tuple(var: Variable) -> T_VarTuple:
     return var.dims, var.data, var.attrs.copy(), var.encoding.copy()
 
 
-def _infer_dtype(array, name: T_Name = None) -> np.dtype:
-    """Given an object array with no missing values, infer its dtype from its
-    first element
-    """
+def _infer_dtype(array, name=None):
+    """Given an object array with no missing values, infer its dtype from all elements."""
     if array.dtype.kind != "O":
         raise TypeError("infer_type must be called on a dtype=object array")
 
     if array.size == 0:
         return np.dtype(float)
 
+    native_dtypes = set(np.vectorize(type, otypes=[object])(array.ravel()))
+    if len(native_dtypes) > 1 and native_dtypes != {bytes, str}:
+        raise ValueError(
+            "unable to infer dtype on variable {!r}; object array "
+            "contains mixed native types: {}".format(
+                name, ", ".join(x.__name__ for x in native_dtypes)
+            )
+        )
+
+    native_dtypes = set(np.vectorize(type, otypes=[object])(array.ravel()))
+    if len(native_dtypes) > 1 and native_dtypes != {bytes, str}:
+        raise ValueError(
+            "unable to infer dtype on variable {!r}; object array "
+            "contains mixed native types: {}".format(
+                name, ", ".join(x.__name__ for x in native_dtypes)
+            )
+        )
+
     element = array[(0,) * array.ndim]
     # We use the base types to avoid subclasses of bytes and str (which might
     # not play nice with e.g. hdf5 datatypes), such as those from numpy

xarray/tests/test_conventions.py

@@ -495,6 +495,18 @@ def test_encoding_kwarg_fixed_width_string(self) -> None:
         pass
 
 
+@pytest.mark.parametrize(
+    "data",
+    [
+        np.array([["ab", "cdef", b"X"], [1, 2, "c"]], dtype=object),
+        np.array([["x", 1], ["y", 2]], dtype="object"),
+    ],
+)
+def test_infer_dtype_error_on_mixed_types(data):
+    with pytest.raises(ValueError, match="unable to infer dtype on variable"):
+        conventions._infer_dtype(data, "test")
+
+
 class TestDecodeCFVariableWithArrayUnits:
     def test_decode_cf_variable_with_array_units(self) -> None:
         v = Variable(["t"], [1, 2, 3], {"units": np.array(["foobar"], dtype=object)})