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Previously mentioned in another post was Librosa with their onset detection method implementations. I will add some work by Sebastian Böck that I use often for personal and academic work. Onset detection methods in this link (https://github.com/CPJKU/SuperFlux) can be a good first step to compare against your implementation of the spectral flux.
There are also online & offline onset detection approaches available as part of the Python madmom library https://github.com/CPJKU/madmom as binaries and Python classes. The methods included in madmom have shown state of the art results in multiple Music Information Retrieval Evaluation eXchange (MIREX) campaigns in recent years. Hope that's useful to you.