all variables are undefined in python debugger

Question

I'm facing a very strange issue on Python 3.6. In the middle of my code, I call import pdb; pdb.set_trace() to debug some code.

And then I'm not able to debug properly, for instance:

(Pdb) abc = 3
(Pdb) [abc for _ in range(2)]
*** NameError: name 'abc' is not defined
(Pdb) [abc, abc]
[3, 3]

It seems like whenever I use list comprehensions, there is an issue of variable not defined. However, if I call the debugger right after I open Python, I do not observe this behavior, everything runs fine.

Any ideas why I'm having this issue?

Answer 1

This happens because list comprehensions are (mostly) evaluated in a nested scope, and nested scopes created in pdb can't access the local variables of the scope being inspected. They can access globals, though, and when you launch pdb immediately after opening Python, you're running it in a global scope, so the abc you create is global.

This also happens with list comprehensions in exec and in class statements. Unfortunately, there isn't really a better workaround than "don't use list comprehensions there".