Make isinstance(obj, FuncNode) work!!

[thorwhalen]

We needed the is_func_node facade because isinstance(obj, FuncNode) alone had unexpected behaviors. I think the facade (maybe a better thought out one?) could be useful either way, but I don't like the fact that we don't know why isinstance(obj, FuncNode) doesn't work the expected way, and shouldn't leave this under the carpet (because other developers will fall into the trap as well!).

Here's one of the syndroms of the problem.
Note: You can only reproduce this error if you use the older is_func_node, which first tries to use isinstance(obj, FuncNode).
The code for this is below.

from meshed import DAG, FuncNode
import i2

def foo(x, y):
    return x + y

dag1 = DAG([foo])
assert str(i2.Sig(dag1.func_nodes[0].func)) == '(x, y)'
# well, yeah, because:
assert dag1.func_nodes[0].func == foo
                
dag2 = DAG([FuncNode(foo)])
assert str(i2.Sig(dag2.func_nodes[0].func)) == '(scope)'
# That's because... 
assert isinstance(dag2.func_nodes[0].func, FuncNode)

But DAG construction is supposed to take FuncNodes as is. The code says so, but if we use isinstance(obj, FuncNode to determine if obj is a FuncNode, it will fail, in the case of the code above. Putting a break point in the is_func_node code, we see that id(type(obj)) != id(FuncNode). Somehow, it has a different FuncNode than the one we constructed the DAG with?

The is_func_node code

def is_func_node(obj) -> bool:
    """
    >>> is_func_node(FuncNode(lambda x: x))
    True
    >>> is_func_node("I am not a FuncNode: I'm a string")
    False
    """
    # A weaker check than an isinstance(obj, FuncNode), which fails when we're
    # developing (therefore changing) FuncNode definition (without relaunching python
    # kernel). This is to be used instead, at least during development times
    # TODO: Replace with isinstance(obj, FuncNode) is this when development stabalizes
    # return isinstance(obj, FuncNode)
    cls = type(obj)
    if cls is not type:
        try:
            return isinstance(obj, FuncNode)
        except Exception:
            return any(getattr(x, "__name__", "") == "FuncNode" for x in cls.mro())
    else:
        return False