Intercept read access to a certain object attribute

Question

I have a library that accepts a file-like object but assumes that it has a valid .name unless that name is None. This causes it to break when I pass e.g. someproc.stdout as it has a name of <fdopen>. Unfortunately the attribute cannot be deleted or overwritten.

So I've written this simple wrapper to return None in case the name attribute is requested.

class InterceptAttrWrapper(object):
    def __init__(self, obj, attr, value):
        self.obj = obj
        self.attr = attr
        self.value = value

    def __getattribute__(self, name):
        ga = lambda attr: object.__getattribute__(self, attr)
        if name == ga('attr'):
            return ga('value')
        return getattr(ga('obj'), name)

This seems to work fine, I'm using it like this:

proc = subprocess.Popen(..., stdout=subprocess.PIPE)
some_func(InterceptAttrWrapper(proc.stdout, 'name', None))

However, I wonder if:

• My InterceptAttrWrapper object is missing something that could cause problems.
• There is a better way to access attributes within __getattribute__. That lambda I'm using right now looks pretty ugly.

Answer 1

Something to watch out for with your approach is that Python calls certain special methods without looking them up via __getattribute__. For example, the __iter__ method:

>>> w = InterceptAttrWrapper(open('test'), 'attr', 'value')
>>> for line in w: print line
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'InterceptAttrWrapper' object is not iterable

See the documentation on "Special method lookup for new-style classes". If this affects you, then you'll need to add to your wrapper class all the special methods that you need to support (in my example, __iter__, but when wrapping other objects one might need to implement __getitem__, __repr__, __hash__, or others).

Here's a slightly more readable way to get at the wrapper object's own data, together with support for intercepting multiple attributes:

class InterceptAttrWrapper(object):
    """
    Wrap an object and override some of its attributes.

    >>> import sys
    >>> stdin = InterceptAttrWrapper(sys.stdin, name = 'input')
    >>> stdin.mode # Pass through to original object.
    'r'
    >>> stdin.name # Intercept attribute and substitute value.
    'input'
    """
    def __init__(self, obj, **attrs):
        self._data = obj, attrs

    def __getattribute__(self, name):
        obj, attrs = object.__getattribute__(self, '_data')
        try:
            return attrs[name]
        except KeyError:
            return getattr(obj, name)