[df] Allow reading a char column into a numpy array#22572
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Jun 11, 2026
In the AsNumpy operation values of the dataset are read into a ROOT::RVec collection of the corresponding column type. Subsequently, the raw data is accessed from the RVec and used to generate the array interface for a numpy array view on the collected data. When the column is of type char, and thus RDF would read values into a ROOT::RVec<char>, the raw data is accessed as a 'char *'. The Python bindings automatically convert 'char *' and 'const char *' to Python strings for full compatibility with existing functions (e.g. otherwise TObject::GetName would not return a string in Python). Thus, the array interface cannot be generated. This commit proposes to introduce a special behaviour in AsNumpy to automatically view the char column as an 'unsigned char' column. This in turn will not incur in the automatic conversion on the Python side. An array of 'unsigned char' is interpreted as a numpy array with dtype uint8. Since this is a decision which might be unexpected to some users, the commit also proposes to let the user know about it via a warning.
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In the AsNumpy operation values of the dataset are read into a ROOT::RVec collection of the corresponding column type. Subsequently, the raw data is accessed from the RVec and used to generate the array interface for a numpy array view on the collected data.
When the column is of type char, and thus RDF would read values into a ROOT::RVec, the raw data is accessed as a 'char *'. The Python bindings automatically convert 'char *' and 'const char *' to Python strings for full compatibility with existing functions (e.g. otherwise TObject::GetName would not return a string in Python). Thus, the array interface cannot be generated.
This commit proposes to introduce a special behaviour in AsNumpy to automatically view the char column as an 'unsigned char' column. This in turn will not incur in the automatic conversion on the Python side. An array of 'unsigned char' is interpreted as a numpy array with dtype uint8.
Since this is a decision which might be unexpected to some users, the commit also proposes to let the user know about it via a warning.
Fixes #22554