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This is likely covered by existing/linked issues already, but wanted to leave it here nonetheless:
Given t-pdb.py:
```
import sys
def main():
 sys.stdout.write("main...\n")
 assert 0
if __name__ == "__main__":
 main()
```
Without the fix from the PR:
```
% python3.8 -m pdb -c cont t-pdb.py
main...
Traceback (most recent call last):
 File ".../pyenv/3.8.0/lib/python3.8/pdb.py", line 1702, in main
 pdb._runscript(mainpyfile)
 File ".../pyenv/3.8.0/lib/python3.8/pdb.py", line 1571, in _runscript
 self.run(statement)
 File ".../pyenv/3.8.0/lib/python3.8/bdb.py", line 587, in run
 exec(cmd, globals, locals)
 File "<string>", line 1, in <module>
 File ".../Vcs/cpython/t-pdb.py", line 1, in <module>
 import sys
 File ".../Vcs/cpython/t-pdb.py", line 6, in main
 assert 0
AssertionError
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> .../Vcs/cpython/t-pdb.py(6)main()
-> assert 0
(Pdb)
```
With the fix:
```
% /tmp/cpython-bisect/bin/python3.8 -m pdb -c cont t-pdb.py
main...
Traceback (most recent call last):
 File "/tmp/cpython-bisect/lib/python3.8/pdb.py", line 1703, in main
 pdb._runscript(mainpyfile)
 File "/tmp/cpython-bisect/lib/python3.8/pdb.py", line 1572, in _runscript
 self.run(statement)
 File "/tmp/cpython-bisect/lib/python3.8/bdb.py", line 583, in run
 exec(cmd, globals, locals)
 File "<string>", line 1, in <module>
 File ".../Vcs/cpython/t-pdb.py", line 10, in <module>
 main()
 File ".../Vcs/cpython/t-pdb.py", line 6, in main
 assert 0
AssertionError
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> .../Vcs/cpython/t-pdb.py(6)main()
-> assert 0
(Pdb)
```
As you can see the traceback in the fixed case contains `main()` correctly,
while it has `import sys` (the scripts first line) otherwise.
I can only repeat myself to ask for reviewing/merging https://github.com/python/cpython/pull/12419.