Hello Christophe,
In a .geo file, the ONELAB attributes that are not default ones (see _setStandardOptions in Common/GmshMessage.cpp) don't end up in the database: is this the expected behaviour?
If yes, what would be the way to define 'custom' attributes in a .geo/.dat file?
If not, here is a proposal for a fix, which define a new function _setOtherAttributes.
By the way, one could go even a step further, and remove all calls to p->setAttribute(...) in _setStandardOptions, since they should be handled by _setOtherAttributes.
As an illustration, here is a small MWE mwe_onelabAttributeGeo.zip.
When calling gmsh test.py, the variable L1 does not show the Ham attribute (defined in the .dat file), while the L2 variable does (defined in the .py file).
Cheers, Nicolas.