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

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hugovk merged 4 commits into
python:mainfrom
brittanyrey:b-perf-pdb-lazy-imports
Sep 8, 2026
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gh-156774: Speed up pdb startup with asyncio guard#156775
hugovk merged 4 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 asyncio guard
import pdb -28.8%
breakpoint() → continue -28.5%
python -m pdb script -26.0%
pdb post-mortem -24.6%

@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 gh-156774: Speed up pdb startup with 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%
@brittanyrey

brittanyrey commented Sep 1, 2026

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PR + issue are fully rescoped @hugovk

hugovk
hugovk approved these changes Sep 2, 2026
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Thanks!

Comment thread Lib/pdb.py Outdated
maurycy
maurycy reviewed Sep 2, 2026
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Comment thread Misc/NEWS.d/next/Library/2026-08-31-11-40-00.gh-issue-156774.Pd8Lz1.rst Outdated
@hugovk
hugovk merged commit 024b6bc into python:main Sep 8, 2026
51 checks passed
@brittanyrey
brittanyrey deleted the b-perf-pdb-lazy-imports branch September 8, 2026 15:53
maurycy added a commit to maurycy/cpython that referenced this pull request Sep 10, 2026
@maurycy
Merge branch 'main' into tachyon-native-callable
e028ff5 
* main: (158 commits)
  pythongh-156924: Try reifying lazy imports in `ForwarRef.evaluate()` (python#156940)
  pythongh-156233: Fix typos and code snippet errors in the docs (python#156475)
  pythongh-156837: Refer to yield expressions in generator function definitions (pythonGH-156863)
  pythongh-155292: Skip updating unicodedata with mismatched interpreter (pythonGH-157066)
  pythongh-157170: Document the scope of global curses settings (pythonGH-157207)
  pythongh-155966: Correct handling of `math.tanpi` poles (python#155980)
  pythongh-157170: Restore use_env() after test_use_prescr_screen in test_curses (pythonGH-157171)
  pythongh-156910: fix deadlock in type_set_abstractmethods under free-threading (python#156948)
  pythongh-155648: Write the empty and placeholder IDLE tests (python#156260)
  pythongh-153569: centralize formatted-string state and source spans (python#156484)
  pythongh-157137: Mark the PEP 820 soft deprecations as 3.15, not `next` (python#157138)
  pythongh-152433: Use regular LoadLibrary in UWP for Windows system libs (pythonGH-156972)
  pythongh-121617: Fix Py_CLEAR() memcpy in C++: replace NULL with _Py_NULL (python#157188)
  pythongh-157135: Fix documentation errors in the `math.atan{2}pi` functions (python#157136)
  pythongh-121617: Fix Py_CLEAR() in C++: replace NULL with _Py_NULL (python#157067)
  pythongh-156774: Speed up pdb startup with asyncio guard (python#156775)
  pythongh-156109: Allow static, non-framework iOS builds (python#156110)
  pythongh-156780: Emscripten: add missing EM_JS_DEPS (python#156798)
  pythongh-152936: Make privileged functions available on Android (python#152977)
  pythongh-123018: Keep the libedit history file header when truncating (pythonGH-157165)
  ...
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@hugovk hugovk hugovk approved these changes
@johnslavik johnslavik johnslavik approved these changes
@gaogaotiantian gaogaotiantian Awaiting requested review from gaogaotiantian gaogaotiantian is a code owner
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@brittanyrey @hugovk @maurycy @johnslavik