Avoid loading any data for reprs (pydata#7203)

* Avoid loading any data for reprs

* Update test_formatting.py

* Update whats-new.rst

* Update doc/whats-new.rst

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

* specify dtype to avoid os issues

* Update xarray/tests/test_formatting.py

* Update whats-new.rst

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

doc/whats-new.rst

@@ -35,6 +35,10 @@ New Features
 Breaking changes
 ~~~~~~~~~~~~~~~~
 
+- ``repr(ds)`` may not show the same result because it doesn't load small,
+  lazy data anymore. Use ``ds.head().load()`` when wanting to see just a sample
+  of the data. (:issue:`6722`, :pull:`7203`).
+  By `Jimmy Westling <https://github.com/illviljan>`_.
 - Many arguments of plotmethods have been made keyword-only.
 - ``xarray.plot.plot`` module renamed to ``xarray.plot.dataarray_plot`` to prevent
   shadowing of the ``plot`` method. (:issue:`6949`, :pull:`7052`).

xarray/core/formatting.py

@@ -579,7 +579,7 @@ def short_data_repr(array):
         return short_numpy_repr(array)
     elif is_duck_array(internal_data):
         return limit_lines(repr(array.data), limit=40)
-    elif array._in_memory or array.size < 1e5:
+    elif array._in_memory:
         return short_numpy_repr(array)
     else:
         # internal xarray array type

xarray/tests/test_formatting.py

@@ -575,17 +575,28 @@ def test_large_array_repr_length() -> None:
 
 @requires_netCDF4
 def test_repr_file_collapsed(tmp_path) -> None:
-    arr = xr.DataArray(np.arange(300), dims="test")
-    arr.to_netcdf(tmp_path / "test.nc", engine="netcdf4")
+    arr_to_store = xr.DataArray(np.arange(300, dtype=np.int64), dims="test")
+    arr_to_store.to_netcdf(tmp_path / "test.nc", engine="netcdf4")
 
     with xr.open_dataarray(tmp_path / "test.nc") as arr, xr.set_options(
         display_expand_data=False
     ):
-        actual = formatting.array_repr(arr)
+        actual = repr(arr)
         expected = dedent(
             """\
         <xarray.DataArray (test: 300)>
-        array([  0,   1,   2, ..., 297, 298, 299])
+        [300 values with dtype=int64]
+        Dimensions without coordinates: test"""
+        )
+
+        assert actual == expected
+
+        arr_loaded = arr.compute()
+        actual = arr_loaded.__repr__()
+        expected = dedent(
+            """\
+        <xarray.DataArray (test: 300)>
+        0 1 2 3 4 5 6 7 8 9 10 11 12 ... 288 289 290 291 292 293 294 295 296 297 298 299
         Dimensions without coordinates: test"""
         )
 
@@ -699,3 +710,18 @@ def test__element_formatter(n_elements: int = 100) -> None:
     )
     actual = intro + values
     assert expected == actual
+
+
+def test_lazy_array_wont_compute() -> None:
+    from xarray.core.indexing import LazilyIndexedArray
+
+    class LazilyIndexedArrayNotComputable(LazilyIndexedArray):
+        def __array__(self, dtype=None):
+            raise NotImplementedError("Computing this array is not possible.")
+
+    arr = LazilyIndexedArrayNotComputable(np.array([1, 2]))
+    var = xr.DataArray(arr)
+
+    # These will crash if var.data are converted to numpy arrays:
+    var.__repr__()
+    var._repr_html_()