Can an object inspect the name of the variable it's been assigned to?

Question

In Python, is there a way for an instance of an object to see the variable name it's assigned to? Take the following for example:

class MyObject(object):
    pass

x = MyObject()

Is it possible for MyObject to see it's been assigned to a variable name x at any point? Like in it's __init__ method?

Answer 1

Yes, it is possible*. However, the problem is more difficult than it seems upon first glance:

• There may be multiple names assigned to the same object.
• There may be no names at all.
• The same name(s) may refer to some other object(s) in a different namespace.

Regardless, knowing how to find the names of an object can sometimes be useful for debugging purposes - and here is how to do it:

import gc, inspect

def find_names(obj):
    frame = inspect.currentframe()
    for frame in iter(lambda: frame.f_back, None):
        frame.f_locals
    obj_names = []
    for referrer in gc.get_referrers(obj):
        if isinstance(referrer, dict):
            for k, v in referrer.items():
                if v is obj:
                    obj_names.append(k)
    return obj_names

If you're ever tempted to base logic around the names of your variables, pause for a moment and consider if redesign/refactor of code could solve the problem. The need to recover an object's name from the object itself usually means that underlying data structures in your program need a rethink.

_{* at least in Cpython}

Answer 2

As many others have said, it can't be done properly. However inspired by jsbueno's, I have an alternative to his solution.

Like his solution, I inspect the callers stack frame, which means it only works properly for Python-implemented callers (see note below). Unlike him, I inspect the bytecode of the caller directly (instead of loading and parsing the source code). Using Python 3.4+'s dis.get_instructions() this can be done with some hope of minimal compatibility. Though this is still some hacky code.

import inspect
import dis

def take1(iterator):
    try:
        return next(iterator)
    except StopIteration:
        raise Exception("missing bytecode instruction") from None

def take(iterator, count):
    for x in range(count):
        yield take1(iterator)

def get_assigned_name(frame):
    """Takes a frame and returns a description of the name(s) to which the
    currently executing CALL_FUNCTION instruction's value will be assigned.

    fn()                    => None
    a = fn()                => "a"
    a, b = fn()             => ("a", "b")
    a.a2.a3, b, c* = fn()   => ("a.a2.a3", "b", Ellipsis)
    """

    iterator = iter(dis.get_instructions(frame.f_code))
    for instr in iterator:
        if instr.offset == frame.f_lasti:
            break
    else:
        assert False, "bytecode instruction missing"
    assert instr.opname.startswith('CALL_')
    instr = take1(iterator)
    if instr.opname == 'POP_TOP':
        raise ValueError("not assigned to variable")
    return instr_dispatch(instr, iterator)

def instr_dispatch(instr, iterator):
    opname = instr.opname
    if (opname == 'STORE_FAST'              # (co_varnames)
            or opname == 'STORE_GLOBAL'     # (co_names)
            or opname == 'STORE_NAME'       # (co_names)
            or opname == 'STORE_DEREF'):    # (co_cellvars++co_freevars)
        return instr.argval
    if opname == 'UNPACK_SEQUENCE':
        return tuple(instr_dispatch(instr, iterator)
                     for instr in take(iterator, instr.arg))
    if opname == 'UNPACK_EX':
        return (*tuple(instr_dispatch(instr, iterator)
                     for instr in take(iterator, instr.arg)),
                Ellipsis)
    # Note: 'STORE_SUBSCR' and 'STORE_ATTR' should not be possible here.
    # `lhs = rhs` in Python will evaluate `lhs` after `rhs`.
    # Thus `x.attr = rhs` will first evalute `rhs` then load `a` and finally
    # `STORE_ATTR` with `attr` as instruction argument. `a` can be any 
    # complex expression, so full support for understanding what a
    # `STORE_ATTR` will target requires decoding the full range of expression-
    # related bytecode instructions. Even figuring out which `STORE_ATTR`
    # will use our return value requires non-trivial understanding of all
    # expression-related bytecode instructions.
    # Thus we limit ourselfs to loading a simply variable (of any kind)
    # and a arbitary number of LOAD_ATTR calls before the final STORE_ATTR.
    # We will represents simply a string like `my_var.loaded.loaded.assigned`
    if opname in {'LOAD_CONST', 'LOAD_DEREF', 'LOAD_FAST',
                    'LOAD_GLOBAL', 'LOAD_NAME'}:
        return instr.argval + "." + ".".join(
            instr_dispatch_for_load(instr, iterator))
    raise NotImplementedError("assignment could not be parsed: "
                              "instruction {} not understood"
                              .format(instr))

def instr_dispatch_for_load(instr, iterator):
    instr = take1(iterator)
    opname = instr.opname
    if opname == 'LOAD_ATTR':
        yield instr.argval
        yield from instr_dispatch_for_load(instr, iterator)
    elif opname == 'STORE_ATTR':
        yield instr.argval
    else:
        raise NotImplementedError("assignment could not be parsed: "
                                  "instruction {} not understood"
                                  .format(instr))

Note: C-implemented functions don't show up as Python stack frames and are thus hidden to this script. This will result in false positives. Consider Python function f() which calls a = g(). g() is C-implemented and calls b = f2(). When f2() tries to lookup up the assigned name, it will get a instead of b because the script is oblivious to C functions. (At least this is how I guess it will work :P )

Usage example:

class MyItem():
    def __init__(self):
        self.name = get_assigned_name(inspect.currentframe().f_back)

abc = MyItem()
assert abc.name == "abc"