I'm running 3.6.4 on Mac OS X 10.13.2, bash shell. Doing:

$ pydoc modules

causes:

Please wait a moment while I gather a list of all available modules...

Fatal Python error: PyThreadState_Get: no current thread
Abort trap: 6

Reproduced this several times with the same result.

I think this was my mistake; when I used pydoc3 instead of pydoc it ran to completion. Please feel free to close this; sorry for the noise.

Glad you solved the immediate problem.  Under the covers, pydoc (either pydoc2 or pydoc3) tries to import all the modules associated with the interpreter instance it's running under - from the Python standard library plus any additional vendor-supplied modules and and any you install (with pip et al).  It seems likely that the Python 2 instance associated with the "pydoc" on your shell path (not "pydoc3" and Python 3) has at least one broken module installed.  It may be a case of a third-party module being compiled against one version of Python 2.7 (say, a Homebrew version) and being executed with another (say the Apple-supplied system Python 2.7).  See, for example, https://stackoverflow.com/questions/35640529/what-does-fatal-python-error-pythreadstate-get-no-current-thread-mean