[ipython] Ipython setup and startup time

[neoniobium]

Startup time increased do to ipython import

Summary

Importing IPython increases startup of an Cmd2 app (on my system about 0.3–0.4 seconds).

Details

The following lines (A) (from cmd2/cmd2.py):

with contextlib.suppress(ImportError):
    from IPython import start_ipython

constitute about 1/3 (0.3–0.4s total on my system) of the total startup time for the getting_started example — what is noticeable.

I’m a bit puzzled by these lines, because start_ipython is only used in the do_ipy method of the Cmd class. However, inside this method, the function is imported again (B):

# Allow users to install ipython from a cmd2 prompt when needed and still have ipy command work
try:
    _dummy = start_ipython  # noqa: F823
except NameError:
    from IPython import start_ipython

At least on my linux system, removing lines (A) and only importing start_ipython (instead of (B)) doesn’t change anything, and it reduces the startup time as mentioned above.

I understand the comment in (B) that the intended purpose of this setup is that installing ipython from within a running cmd2 app (for example, via the !pip install ipython command) should make ipython usable without closing the app. Do I understand this correctly?

However, with or without the changes described above, this still seems possible for me—at least on Linux—when using a virtual environment with uv or pip directly.

Possible solution

The changes described above. However I cannot test whether they break anything on a different system like window, for example.

Additional note

I think it would be reasonable to provide ipython as its own optional dependency group in the pyproject.toml file.

[kmvanbrunt]

That first import is a relic of a bug fix in 2024.

The comment for it was:
When using gnureadline with Python 3.13, start_ipython needs to be imported before any readline-related stuff

During the conversion to prompt-toolkit, it looks like we removed the comment but left the import. I'm pretty sure we can remove it.

There is 1 remaining case where gnureadline could be imported and that's when we enable tab completion in the py shell. I'll set up a 3.13 test environment later and see what happens if we remove the first start_ipython import.

[kmvanbrunt]

For reference, here is the PR where that first start_ipython import was introduced.
#1358