Defer importing `devtools` in `sitecustomize.py`.

[kalekundert]

It's convenient to be able to use debug() as if it were a builtin function, but importing devtools directly in sitecustomize.py is a pretty significant performance hit for every invocation of python that doesn't use debug(). Specifically, this import takes 100-200 ms, which is a noticeable delay for interactive sessions and short-running scripts:

$ python -X importtime -c 'import devtools'

While it would probably be possible to make devtools faster to import, the best solution is simply to avoid importing devtools unless it will be used. It took me a while to figure out how to do this, but in the end the code is pretty clear and succinct, and I think it will be useful to others:

# sitecustomize.py
class lazy_debug:
    @property
    def __call__(self):
        from devtools import debug
        return debug

__builtins__['debug'] = lazy_debug()

This PR updates the README file to include the above snippet.

[codecov[bot]]

Codecov Report

Merging #49 into master will not change coverage. The diff coverage is n/a.

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[samuelcolvin]

I hadn't noticed this, thanks so much for pointing it out.

The main problem with this approach is that other tools attached to debug like debug.timer don't work with this approach. See here.

I know 99.9% of debug usage is just debug(), indeed I had to look up these other tools as even I had forgotten exactly where they were. But I still think it's confusing to disable them like this.

There's also an example in the docs that isn't updated.

The other problem is that debug import could be speeded up a lot, see https://github.com/samuelcolvin/pydantic/issues/1127 for a discussion of something similar.

In summary I think we should:

• remove all import/exports from devtools/__init__.py
• see how much we can speed up from devtools import debug by deferring other package imports.
• add devtools/lazy.py module with only contains a thin wrapper around the standard debug instance, so sitecustomize.py can just contain

from devtools.lazy import lazy_debug
__builtins__['debug'] = lazy_debug

[samuelcolvin]

See #50 - I think if we can improve the import time from 100-200ms to 10-20ms, perhaps we don't need a lazy import workaround?

[kalekundert]

That all sounds really good, especially the idea of adding a devtools.lazy module. I'd personally want to keep using a lazy loader even if import time got down to 10-20 ms, just because I feel like everything in sitecustomize.py should be as fast as possible, but you're right that the difference would be imperceptible at that point.

I didn't know about debug.timer() and friends, although they seem useful. I added a __getattr__() method to the lazy wrapper to properly expose those methods as well.

[samuelcolvin]

okay, happy to accept this PR, but could you update the duplicate code in the docs docs/examples/sitecustomize.py?

[kalekundert]

Sorry, you mentioned that earlier and I didn't notice. Should be up to date now.

[samuelcolvin]

I think this is solved by #50, I'll make a release soon.

[alexmojaki]

FWIW here's how I solve a similar problem in birdseye: https://github.com/alexmojaki/birdseye/blob/master/birdseye/__init__.py

It's a lazy object similar to this PR but it's part of the library itself, not just a sitecustomize recipe. It's worked great for me.