The "args" command in pdb crashes when an argument cannot be printed. Fortunately, this is easy to fix. For version 3.3.3: In function Pdb.do_args (lib/pdb.py, line 1120) Change line 1131 self.message('%s = %r' % (name, dict[name])) to try: r = repr(dict[name]) except: r = "(Cannot print object)" self.message('%s = %s' % (name, r))

Why is the object not printable? Can you provide a reproducer? Also, the bare except is buggy, it would catch things like KeyboardInterrupt that should not be caught there. 'except Exception' would be appropriate in this case.

Thanks for your reaction. The object is not printable, since I was debugging an __init__ of an object, and some fields where being initialized: class foo: def __init__(self): foo.bar = "hello" def repr(self): return foo.bar I tried to make a useable patch file (with neater layout and using Exeption), see attach.

Oops. Here the correct example: >>> class foo: ... def __init__(self): ... foo.bar = "hello" ... def __repr__(self): return foo.bar ... >>> pdb.runcall(foo) > <stdin>(3)__init__() (Pdb) a Traceback (most recent call last): File ".\pdb.py", line 1132, in do_args self.message('%s = %r' % (name, dict[name])) File "<stdin>", line 4, in __repr__ AttributeError: type object 'foo' has no attribute 'bar' During handling of the above exception, another exception occurred: Traceback (most recent call last): File "<stdin>", line 1, in <module> File ".\pdb.py", line 1580, in runcall return Pdb().runcall(*args, **kwds) File "C:\Python33\lib\bdb.py", line 439, in runcall res = func(*args, **kwds) File "<stdin>", line 3, in __init__ File "<stdin>", line 3, in __init__ File "C:\Python33\lib\bdb.py", line 47, in trace_dispatch return self.dispatch_line(frame) File "C:\Python33\lib\bdb.py", line 65, in dispatch_line self.user_line(frame) File ".\pdb.py", line 266, in user_line self.interaction(frame, None) File ".\pdb.py", line 345, in interaction self._cmdloop() File ".\pdb.py", line 318, in _cmdloop self.cmdloop() File "C:\Python33\lib\cmd.py", line 138, in cmdloop stop = self.onecmd(line) File ".\pdb.py", line 411, in onecmd return cmd.Cmd.onecmd(self, line) File "C:\Python33\lib\cmd.py", line 217, in onecmd return func(arg) File ".\pdb.py", line 1134, in do_args self.message('%s = *** repr failed: %s ***' % (name,)) TypeError: not enough arguments for format string At least, I expect pdb to not crash, but a clearer error (as in the patch) is nice to have.

I am not good at this. Sorry for the mess. Here is a good example, and a good patch: >>> class foo: ... def __init__(self): ... foo.bar = "hello" ... def __repr__(self): return foo.bar ... >>> pdb.runcall(foo) Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'pdb' is not defined >>> import pdb >>> pdb.runcall(foo) > <stdin>(3)__init__() (Pdb) a Traceback (most recent call last): File "<stdin>", line 1, in <module> File "C:\Python33\lib\pdb.py", line 1577, in runcall return Pdb().runcall(*args, **kwds) File "C:\Python33\lib\bdb.py", line 439, in runcall res = func(*args, **kwds) File "<stdin>", line 3, in __init__ File "<stdin>", line 3, in __init__ File "C:\Python33\lib\bdb.py", line 47, in trace_dispatch return self.dispatch_line(frame) File "C:\Python33\lib\bdb.py", line 65, in dispatch_line self.user_line(frame) File "C:\Python33\lib\pdb.py", line 266, in user_line self.interaction(frame, None) File "C:\Python33\lib\pdb.py", line 345, in interaction self._cmdloop() File "C:\Python33\lib\pdb.py", line 318, in _cmdloop self.cmdloop() File "C:\Python33\lib\cmd.py", line 138, in cmdloop stop = self.onecmd(line) File "C:\Python33\lib\pdb.py", line 411, in onecmd return cmd.Cmd.onecmd(self, line) File "C:\Python33\lib\cmd.py", line 217, in onecmd return func(arg) File "C:\Python33\lib\pdb.py", line 1131, in do_args self.message('%s = %r' % (name, dict[name])) File "<stdin>", line 4, in __repr__ AttributeError: type object 'foo' has no attribute 'bar'

There is at least one other place (do_break) where this same problem could crop up. Unittest handles this by having a 'safe_repr' function. pdb doesn't need the same function unittest does, but it could do something similar, and then use %s and this function in the places where it currently uses repr to print an arbitrary object: def safe_repr(obj): try: return repr(obj) except Exception: return object.__repr__(obj)

Maybe we could use Pdb._getval_except(arg, frame=None) in the routine do_args. If I understand the code, do_args does quite some work to get the value of name in the context of the current frame, maybe just calling self._getval_except(name, frame=self.curframe) plus or minus some code would do the job? I guess the code would actually become shorter... I'll try to figure it out.

I did figure it out. It almost works, except when a argument lost its value, and the same name exists in the global context. To be more specific: I simplified do_args to the following code (that obviously ugly by explicitly evaluating repr in context, but that is not the point) def do_args(self, arg): """a(rgs) Print the argument list of the current function. Modified by Jurjen """ co = self.curframe.f_code n = co.co_argcount if co.co_flags & 4: n = n+1 if co.co_flags & 8: n = n+1 for i in range(n): name = co.co_varnames[i] expr = 'repr(%s)' % (name,) self.message('%s = %s' % (name, self._getval_except(expr))) At it works perfectly, except for this little surprise: >>> bar = "BAR" >>> def foo(bar): ... del bar ... return 5 >>> pdb.runcall(f, 10) > <stdin>(2)f() -> del bar (Pdb) a bar = 5 (Pdb) n > <stdin>(3)f() -> return 5 (Pdb) a bar = 'BAR' ##### Huh? Expected undefined I'll leave it to the experts to patch this in proper Python coding style. So, the conclusion is we need a way to safely evaluate the call to repr() in context, with self.curframe_locals[co.co_varnames[i]] as argument. I did not find a good supporting routine for that elsewhere in pdb.

> There is at least one other place (do_break) where this same problem could crop up. Also in do_retval. And the exception is silently ignored in do_p and do_pp when repr() fails, which is not correct. A solution could be to have a message_safe method to be used in such cases. For example, substitute in do_args: self.message('%s = %r' % (name, dict[name])) with: self.message_safe('%s = %r', name, dict[name]) def message_safe(self, fmt, *args): try: print(fmt % args, file=self.stdout) except Exception: exc_info = sys.exc_info()[:2] self.error(traceback.format_exception_only(*exc_info)[-1].strip())

Commands that silently fail when an object is not printable: p, pp Commands that crash when an object is not printable: args, retval Python 3: display Python 2: on a 'return' trace event when the return value is not printable The attached script illustrates all these cases.

p/pp commands were fixed in this commit: https://github.com/python/cpython/commit/6544b2532df