Rely on array backend for string formatting (#6823)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: Illviljan

doc/whats-new.rst

@@ -40,6 +40,8 @@ Bug fixes
 - :py:attr:`DataArray.nbytes` now uses the ``nbytes`` property of the underlying array if available.
   (:pull:`6797`)
   By `Max Jones <https://github.com/maxrjones>`_.
+- Rely on the array backend for string formatting. (:pull:`6823`).
+  By `Jimmy Westling <https://github.com/illviljan>`_.
 - Fix incompatibility with numpy 1.20 (:issue:`6818`, :pull:`6821`)
   By `Michael Niklas <https://github.com/headtr1ck>`_.
 - Make FacetGrid.set_titles send kwargs correctly using `handle.udpate(kwargs)`.
@@ -88,6 +90,8 @@ New Features
 - Experimental support for wrapping any array type that conforms to the python
   `array api standard <https://data-apis.org/array-api/latest/>`_. (:pull:`6804`)
   By `Tom White <https://github.com/tomwhite>`_.
+- Allow string formatting of scalar DataArrays. (:pull:`5981`)
+  By `fmaussion <https://github.com/fmaussion>`_.
 
 Bug fixes
 ~~~~~~~~~

xarray/core/common.py

@@ -163,9 +163,22 @@ def _repr_html_(self):
             return f"<pre>{escape(repr(self))}</pre>"
         return formatting_html.array_repr(self)
 
-    def __format__(self: Any, format_spec: str) -> str:
-        # we use numpy: scalars will print fine and arrays will raise
-        return self.values.__format__(format_spec)
+    def __format__(self: Any, format_spec: str = "") -> str:
+        if format_spec != "":
+            if self.shape == ():
+                # Scalar values might be ok use format_spec with instead of repr:
+                return self.data.__format__(format_spec)
+            else:
+                # TODO: If it's an array the formatting.array_repr(self) should
+                # take format_spec as an input. If we'd only use self.data we
+                # lose all the information about coords for example which is
+                # important information:
+                raise NotImplementedError(
+                    "Using format_spec is only supported"
+                    f" when shape is (). Got shape = {self.shape}."
+                )
+        else:
+            return self.__repr__()
 
     def _iter(self: Any) -> Iterator[Any]:
         for n in range(len(self)):

xarray/tests/test_formatting.py

@@ -391,7 +391,10 @@ def test_diff_dataset_repr(self) -> None:
     def test_array_repr(self) -> None:
         ds = xr.Dataset(coords={"foo": [1, 2, 3], "bar": [1, 2, 3]})
         ds[(1, 2)] = xr.DataArray([0], dims="test")
-        actual = formatting.array_repr(ds[(1, 2)])
+        ds_12 = ds[(1, 2)]
+
+        # Test repr function behaves correctly:
+        actual = formatting.array_repr(ds_12)
         expected = dedent(
             """\
         <xarray.DataArray (1, 2) (test: 1)>
@@ -401,6 +404,14 @@ def test_array_repr(self) -> None:
 
         assert actual == expected
 
+        # Test repr, str prints returns correctly as well:
+        assert repr(ds_12) == expected
+        assert str(ds_12) == expected
+
+        # f-strings (aka format(...)) by default should use the repr:
+        actual = f"{ds_12}"
+        assert actual == expected
+
         with xr.set_options(display_expand_data=False):
             actual = formatting.array_repr(ds[(1, 2)])
             expected = dedent(
@@ -422,24 +433,27 @@ def test_array_repr_variable(self) -> None:
 
     @requires_dask
     def test_array_scalar_format(self) -> None:
-        var = xr.DataArray(0)
-        assert var.__format__("") == "0"
-        assert var.__format__("d") == "0"
-        assert var.__format__(".2f") == "0.00"
+        # Test numpy scalars:
+        var = xr.DataArray(np.array(0))
+        assert format(var, "") == repr(var)
+        assert format(var, "d") == "0"
+        assert format(var, ".2f") == "0.00"
 
-        var = xr.DataArray([0.1, 0.2])
-        assert var.__format__("") == "[0.1 0.2]"
-        with pytest.raises(TypeError) as excinfo:
-            var.__format__(".2f")
-        assert "unsupported format string passed to" in str(excinfo.value)
+        # Test dask scalars, not supported however:
+        import dask.array as da
 
-        # also check for dask
-        var = var.chunk(chunks={"dim_0": 1})
-        assert var.__format__("") == "[0.1 0.2]"
+        var = xr.DataArray(da.array(0))
+        assert format(var, "") == repr(var)
         with pytest.raises(TypeError) as excinfo:
-            var.__format__(".2f")
+            format(var, ".2f")
         assert "unsupported format string passed to" in str(excinfo.value)
 
+        # Test numpy arrays raises:
+        var = xr.DataArray([0.1, 0.2])
+        with pytest.raises(NotImplementedError) as excinfo:  # type: ignore
+            format(var, ".2f")
+        assert "Using format_spec is only supported" in str(excinfo.value)
+
 
 def test_inline_variable_array_repr_custom_repr() -> None:
     class CustomArray: