[Python-Dev] 'hasattr' is broken by design

[Nick Coghlan]

On Tue, Aug 24, 2010 at 7:40 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> Properties are allowed to do whatever the heck they want. That doesn't mean
>> that you have to execute code to determine whether they exist or not.
>
> If you don't want to execute properties, do the lookup on the type,
> not the instance (obviously, you know the dance you need to do, since
> you've linked the code where you did it). Having apparently simple
> query operations like hasattr/getattr/len/etc execute arbitrary code
> is where much of the language's flexibility comes from, so I don't see
> how it can really be surprising when it happens.

Now, it may be worth considering an addition to the inspect module
that was basically:

def getattr_static(obj, attr):
    """Retrieve attributes without triggering dynamic lookup via the
descriptor protocol,
        __getattr__ or __getattribute__.

        Note: this function may not be able to retrieve all attributes
reported by dir(obj)
    """
    try:
        instance_dict = object.__getattribute__(obj, "__dict__")
    except AttributeError:
        pass
    else:
        if attr in instance_dict:
            return instance_dict[attr]
    for entry in getmro(obj.__class__):
        try:
            return entry.__dict__[attr]
        except AttributeError:
            pass

(not needing to deal with classic classes simplifies things a bit)

So, allowing for the fact that dir() may report attributes that can
only be found via dynamic lookup, your get_docstrings example could
become something like:

def get_docstrings(obj):
    try:
        members = dir(obj)
    except Exception:
        members = []
    for member in members:
        try:
            doc = getattr_static(obj, member).__doc__
        except AttributeError:
            doc = None
        yield member, doc

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia