Preserve changes to frame locals when going up/down in the stack frame

[ccordoba12]

Description

This complements the excellent improvements on PR #13021 by preserving frame locals when going up/down in the stack. Below I show gifs of the previous and new behaviors using Spyder:

Before

After

Implementation

This was accomplished by saving a copy of all frame locals before the debugger starts and using them to set curframe_locals after every command run in the debugger. That prevents that accesses to f_locals (which happens in up/down) reset the locals set by the user.

Notes

• I draw inspiration for this from the patch present in https://bugs.python.org/issue9633, although it's very different.
• There's PEP 558 that should solve this problem the right way, but currently is marked for Python 3.11 and an implementation for it has been four years in the making.

[MrMino]

Unfortunately, this causes a regression - it makes it impossible to change the values of the variables in the local scope. The changes are only visible to the debugger, but not to the debugged code.

On master branch:

Python 3.8.6 (default, Jun 23 2021, 23:50:42) 
Type 'copyright', 'credits' or 'license' for more information
IPython 8.0.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: def print_my_var():
   ...:     my_var = '==== unchanged ===='
   ...:     print(my_var)
   ...: 

In [2]: %debug print_my_var()
NOTE: Enter 'c' at the ipdb>  prompt to continue execution.
> <string>(1)<module>()

ipdb> s
--Call--
> <ipython-input-1-5aa28ff21139>(1)print_my_var()
----> 1 def print_my_var():
      2     my_var = '==== unchanged ===='
      3     print(my_var)
      4 

ipdb> n
> <ipython-input-1-5aa28ff21139>(2)print_my_var()
      1 def print_my_var():
----> 2     my_var = '==== unchanged ===='
      3     print(my_var)
      4 

ipdb> n
> <ipython-input-1-5aa28ff21139>(3)print_my_var()
      1 def print_my_var():
      2     my_var = '==== unchanged ===='
----> 3     print(my_var)
      4 

ipdb> my_var = "==== changed ===="
ipdb> n
==== changed ====
--Return--
None
> <ipython-input-1-5aa28ff21139>(3)print_my_var()
      1 def print_my_var():
      2     my_var = '==== unchanged ===='
----> 3     print(my_var)
      4 

With the changes presented in this PR:

Python 3.8.6 (default, Jun 23 2021, 23:50:42) 
Type 'copyright', 'credits' or 'license' for more information
IPython 8.0.0.dev -- An enhanced Interactive Python. Type '?' for help.

In [1]: def print_my_var():
   ...:     my_var = '==== unchanged ===='
   ...:     print(my_var)
   ...: 

In [2]: %debug print_my_var()
NOTE: Enter 'c' at the ipdb>  prompt to continue execution.
> <string>(1)<module>()

ipdb> s
--Call--
> <ipython-input-1-5aa28ff21139>(1)print_my_var()
----> 1 def print_my_var():
      2     my_var = '==== unchanged ===='
      3     print(my_var)
      4 

ipdb> n
> <ipython-input-1-5aa28ff21139>(2)print_my_var()
      1 def print_my_var():
----> 2     my_var = '==== unchanged ===='
      3     print(my_var)
      4 

ipdb> n
> <ipython-input-1-5aa28ff21139>(3)print_my_var()
      1 def print_my_var():
      2     my_var = '==== unchanged ===='
----> 3     print(my_var)
      4 

ipdb> my_var = "==== changed ===="
ipdb> n
==== unchanged ====
--Return--
None
> <ipython-input-1-5aa28ff21139>(3)print_my_var()
      1 def print_my_var():
      2     my_var = '==== unchanged ===='
----> 3     print(my_var)
      4 

[ccordoba12]

Hey @MrMino, thanks a lot for checking that! I'll try to address that problem when I have some free time.

[Carreau]

+1 when this work. marking as for 7.x

[madbird1304]

@ccordoba12 , is there any need to copy this dict?
If there isn't, removing copy() could fix the problem mentioned by @MrMino (I've tested locally their example, and it worked as expected).

Suggested change

	stack_entry[0].f_locals.copy() for stack_entry in self.stack
	stack_entry[0].f_locals for stack_entry in self.stack

If you need to keep only certain subset of changes, I would suggest to wrap the original locals with a ChainMaps at some point.

[ccordoba12]

Could you clarify? What's a ChainMap?

[madbird1304]

[ccordoba12]

Ok, thanks a lot for the suggestion! I'll take a look at it as soon as Io have some free time.