pdb "args" crashes when an arg is not printable

[jneb]

BPO	20853
Nosy	@jneb, @bitdancer, @xdegaye, @akulakov
PRs	gh-65052: fixed PDB's args and retval commands to handle objects with broken repr() #28400
Files	pdb.patch: attempt to patch pdb.py pdb.patch: Simple patch to pdb to allow objects with bad repr safe_repr.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2014-03-05.10:45:57.026>
labels = ['type-feature', 'library']
title = 'pdb "args" crashes when an arg is not printable'
updated_at = <Date 2021-09-17.02:50:21.987>
user = 'https://github.com/jneb'

bugs.python.org fields:

activity = <Date 2021-09-17.02:50:21.987>
actor = 'andrei.avk'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2014-03-05.10:45:57.026>
creator = 'jneb'
dependencies = []
files = ['34369', '34370', '35326']
hgrepos = []
issue_num = 20853
keywords = ['patch']
message_count = 11.0
messages = ['212759', '212760', '213212', '213214', '213217', '213273', '213593', '213841', '213987', '218975', '401999']
nosy_count = 4.0
nosy_names = ['jneb', 'r.david.murray', 'xdegaye', 'andrei.avk']
pr_nums = ['28400']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue20853'
versions = ['Python 2.7', 'Python 3.3']

Linked PRs

• gh-65052: Prevent pdb from crashing when trying to display objects #110578
• [3.12] gh-65052: Prevent pdb from crashing when trying to display objects (GH-110578) #110734
• [3.11] gh-65052: Prevent pdb from crashing when trying to display objects (GH-110578) #111002

[jneb]

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))

[bitdancer]

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.

[jneb]

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.

[jneb]

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.

[jneb]

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'

[bitdancer]

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)

[jneb]

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.

[jneb]

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.

[xdegaye]

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())

[xdegaye]

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.

[akulakov]

p/pp commands were fixed in this commit:
6544b2532df