gh-117983: Defer import of threading for lazy module loading (#120233)

As noted in gh-117983, the import importlib.util can be triggered at
interpreter startup under some circumstances, so adding threading makes
it a potentially obligatory load.
Lazy loading is not used in the stdlib, so this removes an unnecessary
load for the majority of users and slightly increases the cost of the
first lazily loaded module.

An obligatory threading load breaks gevent, which monkeypatches the
stdlib. Although unsupported, there doesn't seem to be an offsetting
benefit to breaking their use case.

For reference, here are benchmarks for the current main branch:

```
❯ hyperfine -w 8 './python -c "import importlib.util"'
Benchmark 1: ./python -c "import importlib.util"
  Time (mean ± σ):       9.7 ms ±   0.7 ms    [User: 7.7 ms, System: 1.8 ms]
  Range (min … max):     8.4 ms …  13.1 ms    313 runs
```

And with this patch:

```
❯ hyperfine -w 8 './python -c "import importlib.util"'
Benchmark 1: ./python -c "import importlib.util"
  Time (mean ± σ):       8.4 ms ±   0.7 ms    [User: 6.8 ms, System: 1.4 ms]
  Range (min … max):     7.2 ms …  11.7 ms    352 runs
```

Compare to:

```
❯ hyperfine -w 8 './python -c pass'
Benchmark 1: ./python -c pass
  Time (mean ± σ):       7.6 ms ±   0.6 ms    [User: 5.9 ms, System: 1.6 ms]
  Range (min … max):     6.7 ms …  11.3 ms    390 runs
```

This roughly halves the import time of importlib.util.
This commit is contained in:
Chris Markiewicz 2024-07-03 16:50:46 -04:00 committed by GitHub
parent e245ed7d1e
commit 94f50f8ee6
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2 changed files with 5 additions and 1 deletions

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@ -13,7 +13,6 @@ from ._bootstrap_external import spec_from_file_location
import _imp
import sys
import threading
import types
@ -257,6 +256,9 @@ class LazyLoader(Loader):
def exec_module(self, module):
"""Make the module load lazily."""
# Threading is only needed for lazy loading, and importlib.util can
# be pulled in at interpreter startup, so defer until needed.
import threading
module.__spec__.loader = self.loader
module.__loader__ = self.loader
# Don't need to worry about deep-copying as trying to set an attribute

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@ -0,0 +1,2 @@
Defer the ``threading`` import in ``importlib.util`` until lazy loading is
used.