make error message more clear

xarray/coding/common.py

@@ -113,7 +113,7 @@ def safe_setitem(dest, key: Hashable, value, name: T_Name = None):
     if key in dest:
         var_str = f" on variable {name!r}" if name else ""
         raise ValueError(
-            f"failed to prevent overwriting existing key {key} in attrs{var_str}. "
+            f"Key '{key}' already exists in attrs{var_str}, and will not be overwritten. "
             "This is probably an encoding field used by xarray to describe "
             "how a variable is serialized. To proceed, remove this key from "
             "the variable's attributes manually."

xarray/tests/test_coding_times.py

@@ -2088,7 +2088,7 @@ def test_timedelta_decode_via_dtype_invalid_encoding() -> None:
     attrs = {"dtype": "timedelta64[s]", "units": "seconds"}
     encoding = {"units": "foo"}
     encoded = Variable(["time"], [0, 1, 2], attrs=attrs, encoding=encoding)
-    with pytest.raises(ValueError, match="failed to prevent"):
+    with pytest.raises(ValueError, match=r"Key .* already exists"):
         conventions.decode_cf_variable("timedeltas", encoded)
 
 
@@ -2097,7 +2097,7 @@ def test_timedelta_encode_via_dtype_invalid_attribute(attribute) -> None:
     timedeltas = pd.timedelta_range(0, freq="D", periods=3)
     attrs = {attribute: "foo"}
     variable = Variable(["time"], timedeltas, attrs=attrs)
-    with pytest.raises(ValueError, match="failed to prevent"):
+    with pytest.raises(ValueError, match=r"Key .* already exists"):
         conventions.encode_cf_variable(variable)