Pycharm won’t start the debugger with a clean interpreter

You get something like

Traceback (most recent call last):
  File "$HOME/.local/share/JetBrains/Toolbox/apps/pycharm-professional/plugins/python/helpers/pydev/_pydevd_bundle/pydevd_cython_wrapper.py", line 8, in <module>
    from _pydevd_bundle_ext import pydevd_cython as mod
ModuleNotFoundError: No module named '_pydevd_bundle_ext'

which means the debugger can’t find the Cython extension. Normally you get asked for that, but apparently sometimes you don’t. In that case, you can manually install it with

pip install setuptools cython # just in case your env doesn't have  them
python ~/.local/share/JetBrains/Toolbox/apps/pycharm-professional/plugins/python/helpers/pydev/setup_cython.py build_ext -i

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