Scripting flow manipulations in mitmproxy is easy. There are a lot of great examples to learn from which help. The filter syntax is expressive and simplifies writing code to mangle flows. I've done body conversion [0], weird types of authorization testing, manipulated system update packages on the fly, all sorts of bizarre things. Auto-reloading scripts on change is such a time saver. If you want a python-based programmable HTTPS proxy for testing I have not found anything better.

[0]: https://carvesystems.com/news/proxying-wbxml-services/