Codemodder has extensive Java support, which Codegen does not support at the moment. Otherwise, my understanding of Codemodder is that it is focused on AST-level syntactical modifications. Codegen computes a richer graph datastructure, and this can be used for sophisticated modifications that depend on inheritance hierarchies, function usages, cross-file references and more.
Codemodder is written in Java, whereas you can write Codegen in a jupyter notebook or anywhere you can run Python.
Codemodder is written in Java, whereas you can write Codegen in a jupyter notebook or anywhere you can run Python.