homepage

This issue occurred at least in Python 2.7, I haven't checked in other versions.
When stepping on a return statement, pdb calls the return value __str__() method to display it at the end of the line. Only, it doesn't handle the potential exceptions raised within those functions.
An exemple would be:
import pdb
class A(object):
 def __new__(cls):
 pdb.set_trace()
 return super(A, cls).__new__()
 def __init__(self):
 self.value = "Foo"
 def __str__(self):
 return self.value
pdb.run("A()")
When using the step by step, pdb will be interrupted by an unhandled AttributeError.

I cannot reproduce the problem on python 2.7. The example runs
without problem after fixing the example with the
following changes:
 remove the call to pdb.set_trace(), the debugger is already
 started with a call to pdb.run()
 add the missing 'cls' parameter in the call to __new__() in:
 return super(A, cls).__new__()

Georg, if you agree that Xavier's analysis is correct, we can close this as not a bug.

Can't reproducible in py3 (3.8), IMHO can be closed.

Batuhan, thanks for the nudge. For the record ...
If __new__ were a normal instance method, then 'some_class.__new__()' would be a correct call, as 'some_class' would be passed to '__new__' as its first and perhaps only argument. But " __new__() is a static method (special-cased so you need not declare it as such)".
https://docs.python.org/3/reference/datamodel.html#object.__new__
Its special-casing makes it easy to forget that its first argument, a class, must be passed explicitly. Hence Xavier's comment.
In current 2.7.17 and 3.8 (for instance), with pdb left out of the picture, the missing cls argument results in
 TypeError: object.__new__(): not enough arguments
When I pdb.run('A()') new and step, it catches the error and refuses to advance.
> <pyshell#10>(3)__new__()
(Pdb) s
TypeError: object.__new__(): not enough arguments
> <pyshell#10>(3)__new__()