Why can't pdb access a variable containing an exception?

Question

Sometime, I can't identify when or what's causing it, pdb will not help you with code like:

try:
    foo()
except Exception as e:
   import pdb; pdb.set_trace()

You end up with the usual prompt, but trying to access e will lead to:

(pdb) e
*** NameError: name 'e' is not defined.

It's not all the time of course, and it happens on linux, windows, my machine, my colleague machine...

Answer 1

In Python 3, the target of an except .. as target statement is cleared when the suite exits. From the try statement documentation:

When an exception has been assigned using as target, it is cleared at the end of the except clause. This is as if

except E as N:
    foo

was translated to

except E as N:
    try:
        foo
    finally:
        del N

This means the exception must be assigned to a different name to be able to refer to it after the except clause. Exceptions are cleared because with the traceback attached to them, they form a reference cycle with the stack frame, keeping all locals in that frame alive until the next garbage collection occurs.

Calling pdb.set_trace() effectively exits the block, so the implicit finally suite above is executed.

Bind the exception to a different name:

try:
    foo()
except Exception as e:
   exception = e
   import pdb; pdb.set_trace()

Demo:

>>> try:
...     foo()
... except Exception as e:
...    exception = e
...    import pdb; pdb.set_trace()
...
--Return--
> <stdin>(5)<module>()->None
(Pdb) e
*** NameError: name 'e' is not defined
(Pdb) exception
NameError("name 'foo' is not defined",)