pudb and pdb use literally the same code to implement user commands/variable assignments. It turns out that this is fine (and works in pudb and pdb) within the global scope, where variables are kept in a dict, but it very nearly can't be done inside a function, where variables are kept in the VM stack.

If you know otherwise, please let me know.

I see. It sounds like a hard problem.

As of Python 3.6.6 this seems to be resolved in pdb, it can change local variables.

Interesting. In that case, could you look into what pdb does to make that happen? We should be able to just steal what they do.

I don't see any obvious changes to bdb.py or pdb.py related to this.

@kgabor I can't reproduce this. I use this script to test, on Python 3.7.3rc1.

def f():
    i = 0

    while i < 10:
        i = i+1
        print(i)

f()

I use pdb to step into f until I'm in the middle of the loop, then say

(Pdb) s i = 20
> /home/andreas/tmp/zzz.py(5)f()
-> i = i+1
(Pdb) p i
1

which seems to contradict your statement.

I think this should be re-opened as it works in pdb.

When I test the code from #26 (comment), I can modify i just fine:

$ python -m pdb zzz.py
> .../zzz.py(1)<module>()
-> def f():
(Pdb) n
> .../zzz.py(8)<module>()
-> f()
(Pdb) s
--Call--
> .../zzz.py(1)f()
-> def f():
(Pdb) n
> .../zzz.py(2)f()
-> i = 0
(Pdb) n
> .../zzz.py(4)f()
-> while i < 10:
(Pdb) n
> .../zzz.py(5)f()
-> i = i+1
(Pdb) n
-> print(i)
(Pdb) i
1
(Pdb) i = 20
(Pdb) i
20
(Pdb) n
20
> .../zzz.py(4)f()
-> while i < 10:

This is on Python 3.10.8.

It looks like pdb achieves this by making a copy of f_locals https://github.com/python/cpython/blob/aa8b58cb33826bd2b1a1de631ebcd6a5353eecb5/Lib/pdb.py#L299. Otherwise, frame.f_locals recomputes the locals every time it is accessed, wiping any changes that were made to it.

I have an in progress PR for a similar change for pudb to #571. It requires more testing, and also some cleaning up for the different shells.