Fixed facet_grid with column named "key"

fixes #734

doc/changelog.qmd

@@ -61,6 +61,8 @@ title: Changelog
 - Fixed bug where `theme(legend_background=element_blank())`{.py} messed up
   the position of the legend, instead of only removing the background.
 
+- Fixed using `facet_grid` with a column named `key`. ({{< issue 734 >}})
+
 ### Enhancements
 
 - All `__all__` variables are explicitly assigned to help static typecheckers

plotnine/_utils/__init__.py

@@ -988,23 +988,15 @@ def resolution(x, zero=True):
 
 def cross_join(df1: pd.DataFrame, df2: pd.DataFrame) -> pd.DataFrame:
     """
-    Return a dataframe that is a cross between dataframes
-    df1 and df2
-
-    ref: https://github.com/pydata/pandas/issues/5401
+    Return a cross between df1 & df2 if each is not empty
     """
     if len(df1) == 0:
         return df2
 
     if len(df2) == 0:
         return df1
 
-    # Add as lists so that the new index keeps the items in
-    # the order that they are added together
-    all_columns = list(pd.Index(list(df1.columns) + list(df2.columns)))
-    df1["key"] = 1
-    df2["key"] = 1
-    return df1.merge(df2, on="key").loc[:, all_columns]
+    return df1.join(df2, how="cross")  # type: ignore
 
 
 def to_inches(value: float, units: str) -> float: