Add support writing/reading categorical columns from pandas

root_pandas/readwrite.py

@@ -6,6 +6,7 @@
 import numpy as np
 from numpy.lib.recfunctions import append_fields
 from pandas import DataFrame, RangeIndex
+import pandas as pd
 from root_numpy import root2array, list_trees
 import fnmatch
 from root_numpy import list_branches
@@ -312,6 +313,15 @@ def convert_to_dataframe(array, start_index=None):
         assert len(columns) == len(df.columns), (columns, df.columns)
         df = df.reindex_axis(columns, axis=1, copy=False)
 
+    # Convert categorical columns back to categories
+    for c in df.columns:
+        match = re.match(r'^__rpCaT\*([^\*]+)\*(True|False)\*', c)
+        if match:
+            real_name, ordered = match.groups()
+            categories = c.split('*')[3:]
+            df[c] = pd.Categorical.from_codes(df[c], categories, ordered={'True': True, 'False': False}[ordered])
+            df.rename(index=str, columns={c: real_name}, inplace=True)
+
     return df
 
 
@@ -353,12 +363,25 @@ def to_root(df, path, key='my_ttree', mode='w', store_index=True, *args, **kwarg
     from root_numpy import array2root
     # We don't want to modify the user's DataFrame here, so we make a shallow copy
     df_ = df.copy(deep=False)
+
     if store_index:
         name = df_.index.name
         if name is None:
             # Handle the case where the index has no name
             name = ''
         df_['__index__' + name] = df_.index
+
+    # Convert categorical columns into something root_numpy can serialise
+    for col in df_.select_dtypes(['category']).columns:
+        name_components = ['__rpCaT', col, str(df_[col].cat.ordered)]
+        name_components.extend(df_[col].cat.categories)
+        if ['*' not in c for c in name_components]:
+            sep = '*'
+        else:
+            raise ValueError('Unable to find suitable separator for columns')
+        df_[col] = df_[col].cat.codes
+        df_.rename(index=str, columns={col: sep.join(name_components)}, inplace=True)
+
     arr = df_.to_records(index=False)
     array2root(arr, path, key, mode=mode, *args, **kwargs)