Groupby crash on a single level

:

@@ -0,0 +1,95 @@
+name: pandas-dev
+channels:
+  - https://conda.anaconda.org/conda-forge/
+dependencies:
+  # required
+  - numpy>=1.15
+  - python=3.6
+  - python-dateutil>=2.6.1
+  - pytz
+
+  # benchmarks
+  - asv
+
+  # building
+  - cython>=0.29.13
+
+  # code checks
+  - black=19.10b0
+  - cpplint
+  - flake8
+  - flake8-comprehensions>=3.1.0  # used by flake8, linting of unnecessary comprehensions
+  - flake8-rst>=0.6.0,<=0.7.0  # linting of code blocks in rst files
+  - isort  # check that imports are in the right order
+  - mypy=0.730
+  - pycodestyle  # used by flake8
+
+  # documentation
+  - gitpython  # obtain contributors from git for whatsnew
+  - sphinx
+  - numpydoc>=0.9.0
+
+  # documentation (jupyter notebooks)
+  - nbconvert>=5.4.1
+  - nbsphinx
+  - pandoc
+  # Dask and its dependencies
+  - dask-core
+  - toolz>=0.7.3
+  - fsspec>=0.5.1
+  - partd>=0.3.10
+  - cloudpickle>=0.2.1
+
+  # web (jinja2 is also needed, but it's also an optional pandas dependency)
+  - markdown
+  - feedparser
+  - pyyaml
+  - requests
+
+  # testing
+  - boto3
+  - botocore>=1.11
+  - hypothesis>=3.82
+  - moto  # mock S3
+  - pytest>=5.0.1
+  - pytest-cov
+  - pytest-xdist>=1.21
+  - seaborn
+  - statsmodels
+
+  # unused (required indirectly may be?)
+  - ipywidgets
+  - nbformat
+  - notebook>=5.7.5
+  - pip
+
+  # optional
+  - blosc
+  - bottleneck>=1.2.1
+  - ipykernel
+  - ipython>=5.6.0
+  - jinja2  # pandas.Styler
+  - matplotlib>=2.2.2  # pandas.plotting, Series.plot, DataFrame.plot
+  - numexpr>=2.6.8
+  - scipy>=1.1
+
+  # optional for io
+  - beautifulsoup4>=4.6.0  # pandas.read_html
+  - fastparquet>=0.3.2  # pandas.read_parquet, DataFrame.to_parquet
+  - html5lib  # pandas.read_html
+  - lxml  # pandas.read_html
+  - openpyxl<=3.0.1  # pandas.read_excel, DataFrame.to_excel, pandas.ExcelWriter, pandas.ExcelFile
+  - pyarrow>=0.13.1  # pandas.read_parquet, DataFrame.to_parquet, pandas.read_feather, DataFrame.to_feather
+  - pyqt>=5.9.2  # pandas.read_clipboard
+  - pytables>=3.4.2  # pandas.read_hdf, DataFrame.to_hdf
+  - python-snappy  # required by pyarrow
+  - s3fs  # pandas.read_csv... when using 's3://...' path
+  - sqlalchemy  # pandas.read_sql, DataFrame.to_sql
+  - xarray  # DataFrame.to_xarray
+  - xlrd  # pandas.read_excel, DataFrame.to_excel, pandas.ExcelWriter, pandas.ExcelFile
+  - xlsxwriter  # pandas.read_excel, DataFrame.to_excel, pandas.ExcelWriter, pandas.ExcelFile
+  - xlwt  # pandas.read_excel, DataFrame.to_excel, pandas.ExcelWriter, pandas.ExcelFile
+  - odfpy  # pandas.read_excel
+  - pyreadstat  # pandas.read_spss
+  - pip:
+    - git+https://github.com/pandas-dev/pandas-sphinx-theme.git@master

doc/source/whatsnew/v1.0.0.rst

@@ -824,7 +824,7 @@ Groupby/resample/rolling
 - Bug in :meth:`DataFrameGroupBy.rolling().quantile()` ignoring ``interpolation`` keyword argument (:issue:`28779`)
 - Bug in :meth:`DataFrame.groupby` where ``any``, ``all``, ``nunique`` and transform functions would incorrectly handle duplicate column labels (:issue:`21668`)
 - Bug in :meth:`DataFrameGroupBy.agg` with timezone-aware datetime64 column incorrectly casting results to the original dtype (:issue:`29641`)
--
+- Bug in :meth:`DataFrame.groupby` when using axis=1 and having a single level columns index (:issue:`30208`)
 
 Reshaping
 ^^^^^^^^^

pandas/core/groupby/grouper.py

@@ -491,8 +491,11 @@ def get_grouper(
                     raise ValueError("multiple levels only valid with MultiIndex")
 
             if isinstance(level, str):
-                if obj.index.name != level:
-                    raise ValueError(f"level name {level} is not the name of the index")
+                if obj._get_axis(axis).name != level:
+                    raise ValueError(
+                        f"level name {level} is not the name "
+                        f"of the {obj._get_axis_name(axis)}"
+                    )
             elif level > 0 or level < -1:
                 raise ValueError("level > 0 or level < -1 only valid with MultiIndex")
 

pandas/tests/groupby/test_grouping.py

@@ -501,15 +501,17 @@ def test_groupby_level(self, sort, mframe, df):
         with pytest.raises(ValueError, match=msg):
             df.groupby(level=1)
 
-    def test_groupby_level_index_names(self):
+    def test_groupby_level_index_names(self, axis):
         # GH4014 this used to raise ValueError since 'exp'>1 (in py2)
         df = DataFrame({"exp": ["A"] * 3 + ["B"] * 3, "var1": range(6)}).set_index(
             "exp"
         )
-        df.groupby(level="exp")
-        msg = "level name foo is not the name of the index"
+        if axis in (1, "columns"):
+            df = df.T
+        df.groupby(level="exp", axis=axis)
+        msg = f"level name foo is not the name of the {df._get_axis_name(axis)}"
         with pytest.raises(ValueError, match=msg):
-            df.groupby(level="foo")
+            df.groupby(level="foo", axis=axis)
 
     @pytest.mark.parametrize("sort", [True, False])
     def test_groupby_level_with_nas(self, sort):