gh-128201: Fix DeprecationWarning in Lib/test/test_pdb.py

[Eclips4]

• Issue: test_pdb raises a DeprecationWarning #128201

[gaogaotiantian]

Is this the correct way to fix it? I actually don't know what set policy does. Someone with asyncio expertise should check if this is needed or what's the modern way to write it. Replacing it with a "private" call might elimiate the warning but may not be the best way to fix this.

[hugovk]

If asyncio.set_event_loop_policy is deprecated, what should end users call instead? Presumably not asyncio._set_event_loop_policy?

Can we also call the replacement?

[Eclips4]

Initially I was thinking about a "default way" to fix that DeprecationWarning by rewriting code to avoid using deprecated things.
But after looking at #128024 I changed my mind.

This one was accidentally omitted in #128024:

• Line 2135 has been fixed
• Line 2256 has been fixed
• Line 2356 has been fixed

UPD: asyncio._set_event_loop_policy was added in #128024 for internal use in tests, I believe.
I think Kumar's plan is to eliminate use of asyncio._set_event_loop_policy in follow-up PR's.

[gaogaotiantian]

Okay I think the fix is okay at this point. However, is there an asyncio expert that can take a look at the testing code and see if this line is even needed? As there's no explicit code to change the policy. The testing code is pretty trivial, will a new_event_loop change the policy? Why do we need to set it back to default in the first place?

[kumaraditya303]

Okay I think the fix is okay at this point. However, is there an asyncio expert that can take a look at the testing code and see if this line is even needed? As there's no explicit code to change the policy. The testing code is pretty trivial, will a new_event_loop change the policy? Why do we need to set it back to default in the first place?

For pdb tests, asyncio.run could be used instead of asyncio.new_event_loop as that would take care of policy stuff internally.
AFAICS pdb doesn't mess around the policy stuff so migrating of all tests to use newer asyncio.run or asyncio.Runner seems worthwhile.