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gh-156774: Speed up pdb startup with asyncio guard - #156775

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brittanyrey wants to merge 3 commits into
python:mainfrom
brittanyrey:b-perf-pdb-lazy-imports
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gh-156774: Speed up pdb startup with asyncio guard#156775
brittanyrey wants to merge 3 commits into
python:mainfrom
brittanyrey:b-perf-pdb-lazy-imports

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@brittanyrey

@brittanyrey brittanyrey commented Sep 1, 2026 •
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summary

  • Guard expensive asyncio import on if the current program is using asyncio at all.

perf

Broken out impact of lazy imports + asyncio guard
Without it, only the initial import really improves:

Benchmark lazy imports only + asyncio guard
import pdb −41.3% −42.1%
breakpoint() → continue −5.1% −41.1%
python -m pdb script −3.9% −33.4%
pdb post-mortem −2.3% −33.3%
@brittanyrey
gh-999998: Speed up pdb startup with lazy imports
3c199f4 
Defer the eight imports that are not already pulled in by pdb's
required-eager imports (cmd, bdb, code, typing and the guarded
_pyrepl.utils), and lift five unguarded function-level imports to
module-scope lazy imports.

Guard Pdb._get_asyncio_task() on 'asyncio' in sys.modules. It runs on every
interaction(), so without the guard the deferred asyncio import is paid back
immediately on the first breakpoint and the lazy import buys nothing for an
actual debugging session. If asyncio was never imported there cannot be a
running task, so returning None is equivalent.

The guarded imports (readline, rlcompleter, _pyrepl) stay function-level:
they sit in try/except ImportError blocks, and hoisting them would move the
failure outside the guard.

import pdb              -42.1%
breakpoint() -> cont    -41.1%
python -m pdb script    -33.4%
pdb post-mortem         -33.3%
@bedevere-app bedevere-app Bot mentioned this pull request Sep 1, 2026
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@hugovk

hugovk commented Sep 1, 2026

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See #156567 for the lazy imports, your review welcome!

I was planning on doing the asyncio guard as a followup :) Shall we reduce this PR to just that?
@brittanyrey

brittanyrey commented Sep 1, 2026

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Sorry about that! I did a peek over the lazy imports label & missed your issue!

I'm glad to scope this down to just the asyncio gated change.
@brittanyrey brittanyrey changed the title gh-156774: Speed up pdb startup with lazy imports + asyncio guard Sep 1, 2026
@brittanyrey
pythongh-156774: Scope pdb change down to the asyncio import only
89fccc3 
Drop the other lazy imports and the lifted function-level imports, leaving
just the asyncio deferral and the _get_asyncio_task() guard. Those two carry
most of the win and keep the diff to five lines.

import pdb              -28.8% (was -42.1% with the full set)
breakpoint() -> cont    -28.5%
python -m pdb script    -26.0%
pdb post-mortem         -24.6%
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