Don't use .get() with defaultdicts

[correctmost]

Type of Changes

	Type
✓	🔨 Refactoring

Description

ASTWalker.walk was calling get ~1.42 million times when linting yt-dlp. This is a small speed-up that also helps unclutter profiler output.

timeit data

$ python -m timeit -s 'from collections import defaultdict; visit_events = defaultdict(list)' -n 1000000 'x=visit_events.get("x", ())'
1000000 loops, best of 5: 50.7 nsec per loop

$ python -m timeit -s 'from collections import defaultdict; visit_events = defaultdict(list)' -n 1000000 'x=visit_events["x"]'
1000000 loops, best of 5: 29.4 nsec per loop

Stats

Before

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
  2123486    0.529    0.000    0.529    0.000 {method 'get' of 'dict' objects}

Command	Mean [s]	Min [s]	Max [s]	Relative
pylint --recursive=y .	33.597 ± 0.157	33.444	33.839	1.00

After

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   697806    0.204    0.000    0.204    0.000 {method 'get' of 'dict' objects}

Command	Mean [s]	Min [s]	Max [s]	Relative
pylint --recursive=y .	33.536 ± 0.134	33.308	33.731	1.00

[github-actions]

🤖 According to the primer, this change has no effect on the checked open source code. 🤖🎉

This comment was generated for commit 8a1ec04