Show data by default in HTML repr for DataArray (#4182)

* Show data by default in HTML repr for DataArray

Fixes GH-4176

* add whats new for html repr

* fix test

doc/whats-new.rst

@@ -39,7 +39,9 @@ Breaking changes
   the default behaviour of :py:func:`open_mfdataset` has changed to use
   ``combine='by_coords'`` as the default argument value. (:issue:`2616`, :pull:`3926`)
   By `Tom Nicholas <https://github.com/TomNicholas>`_.
-
+- The ``DataArray`` and ``Variable`` HTML reprs now expand the data section by
+  default (:issue:`4176`)
+  By `Stephan Hoyer <https://github.com/shoyer>`_.
 
 Enhancements
 ~~~~~~~~~~~~

xarray/core/formatting_html.py

@@ -184,7 +184,7 @@ def dim_section(obj):
 def array_section(obj):
     # "unique" id to expand/collapse the section
     data_id = "section-" + str(uuid.uuid4())
-    collapsed = ""
+    collapsed = "checked"
     variable = getattr(obj, "variable", obj)
     preview = escape(inline_variable_array_repr(variable, max_width=70))
     data_repr = short_data_repr_html(obj)

xarray/tests/test_formatting_html.py

@@ -108,8 +108,8 @@ def test_summarize_attrs_with_unsafe_attr_name_and_value():
 def test_repr_of_dataarray(dataarray):
     formatted = fh.array_repr(dataarray)
     assert "dim_0" in formatted
-    # has an expandable data section
-    assert formatted.count("class='xr-array-in' type='checkbox' >") == 1
+    # has an expanded data section
+    assert formatted.count("class='xr-array-in' type='checkbox' checked>") == 1
     # coords and attrs don't have an items so they'll be be disabled and collapsed
     assert (
         formatted.count("class='xr-section-summary-in' type='checkbox' disabled >") == 2