Is there anything safe in python?

20 January 2020

In the process of working on Hunter I have found many strange things from merely trying to do a repr on objects that are passed around. Code blowing up with an exception is the least of your concerns. Take a look at this:

class lazy(object):
    def __init__(self, fun, *args, **kwargs):
        self._fun = fun
        self._args = args
        self._kwargs = kwargs

    def __call__(self):
        return self.evaluate()

    def evaluate(self):
        return self._fun(*self._args, **self._kwargs)

    def __repr__(self):
        return repr(self())

Simply doing a repr on that will change the flow of the program, exactly what you don't want a debugging tool to do!

So then I tried something like:

def rudimentary_repr(obj):
    if isinstance(obj, dict):
        ...
    elif isinstance(obj, list):
        ...
    elif ...  # goes on for a while
        ...
    else:
        # give the not very useful
        # '<Something object at 0x123>'
        return object.__repr__(obj)

Add a simple depth check to deal with deep or infinite recursion and you're good right? I went for a simple depth check instead of pprint's recursion checker (that stores id of objects):

def rudimentary_repr(obj, maxdepth=5):
    if not maxdepth:
        return '...'
    newdepth = maxdepth - 1
    # then pass around newdepth, easy-peasy

At this point I thought the only real problem was how to reduce the number of branches and figure out on which objects it's safe to call repr (to avoid reimplementing __repr__ of everything interesting).

Then I added this, hoping this would save me lots of typing:

elif not hasattr(obj, '__dict__'):
    return repr(obj)

No __dict__ doesn't necessarily mean no state, but I hoped no one with do crummy stuff in __repr__ if they have an dict-less object.

But then I found this little fella:

class ApiModule(ModuleType):
    @property
    def __dict__(self):
        # force all the content of the module
        # to be loaded when __dict__ is read
        ...

And doubled down in the terrible idea of checking for a __dict__ (instead of hasattr(obj, '__dict__') I'd use hasdict(type(obj))):

def hasdict(obj_type, obj, tolerance=25):
    """
    A contrived mess to check that object
    doesn't have a __dit__ but avoid checking
    it if any ancestor is evil enough to
    explicitly define __dict__
    """
    ancestor_types = deque()
    while obj_type is not type and tolerance:
        ancestor_types.appendleft(obj_type)
        obj_type = type(obj_type)
        tolerance -= 1
    for ancestor in ancestor_types:
        __dict__ = getattr(ancestor, '__dict__', None)
        if __dict__ is not None:
            if '__dict__' in __dict__:
                return True
    return hasattr(obj, '__dict__')

I used that for a while until I came to the sad realization that you can't really trust anything. Behold:

class LazyObject(object):
    # Need to pretend to be the wrapped class, for the sake of objects that
    # care about this (especially in equality tests)
    __class__ = property(new_method_proxy(operator.attrgetter("__class__")))

What exactly is going on there? A simplified example to illustrate the problem:

>>> class Surprise(object):
...     @property
...     def __class__(self):
...         print('Boom!')
...
>>> p = Surprise()
>>> isinstance(p, dict)
Boom!
False

At this point it became clear that the hasdict idea wasn't going to fly for long so I ripped that out as well.

New plan:

  • Don't bother showing details for subclasses of builtin types (like dict, list etc). Subclasses could do any of the crazy things shown above.
  • Use type instead of isinstance. For example: to check if it's a Exception instance just check if BaseException is in type's MRO. As I'm typing this I realise someone could stick a descriptor into the args attribute, damn it. Perhaps getattr_static would solve it.
  • Use repr only on objects deemed to have a safe builtin type. Start with builtins, io, socket, _socket.

What I got now:

def safe_repr(obj, maxdepth=5):
    if not maxdepth:
        return '...'
    obj_type = type(obj)
    obj_type_type = type(obj_type)
    newdepth = maxdepth - 1

    # only represent exact builtins
    # (subclasses can have side-effects due to __class__ being
    # a property, __instancecheck__, __subclasscheck__ etc)
    if obj_type is dict:
        return '{%s}' % ', '.join('%s: %s' % (
            safe_repr(k, maxdepth),
            safe_repr(v, newdepth)
        ) for k, v in obj.items())
    elif obj_type is list:
        return '[%s]' % ', '.join(
            safe_repr(i, newdepth) for i in obj
        )
    elif obj_type is tuple:
        return '(%s%s)' % (
            ', '.join(safe_repr(i, newdepth) for i in obj),
            ',' if len(obj) == 1 else ''
        )
    elif obj_type is set:
        return '{%s}' % ', '.join(
            safe_repr(i, newdepth) for i in obj
        )
    elif obj_type is frozenset:
        return '%s({%s})' % (
            obj_type.__name__,
            ', '.join(safe_repr(i, newdepth) for i in obj)
        )
    elif obj_type is deque:
        return '%s([%s])' % (
            obj_type.__name__,
            ', '.join(safe_repr(i, newdepth) for i in obj)
        )
    elif obj_type in (Counter, OrderedDict, defaultdict):
        return '%s({%s})' % (
            obj_type.__name__,
            ', '.join('%s: %s' % (
                safe_repr(k, maxdepth),
                safe_repr(v, newdepth)
            ) for k, v in obj.items())
        )
    elif obj_type is types.MethodType:  # noqa
        self = obj.__self__
        name = getattr(obj, '__qualname__', None)
        if name is None:
            name = obj.__name__
        return '<%sbound method %s of %s>' % (
            'un' if self is None else '',
            name,
            safe_repr(self, newdepth)
        )
    elif obj_type_type is type and BaseException in obj_type.__mro__:
        return '%s(%s)' % (
            obj_type.__name__,
            ', '.join(safe_repr(i, newdepth) for i in obj.args)
        )
    elif obj_type_type is type and \
         obj_type is not InstanceType and \
         obj_type.__module__ in (builtins.__name__, 'io', 'socket', '_socket'):
        # hardcoded list of safe things. note that isinstance ain't used
        # (and we don't trust subclasses to do the right thing in __repr__)
        return repr(obj)
    else:
        return object.__repr__(obj)

The problematic code examples are taken out of popular projects like Celery, Pytest and Django but I don't think it matters who does it. What do you think?

This entry was tagged as debugging hunter python