Introspect calling object

Question

How do I introspect A's instance from within b.func() (i.e. A's instance's self):

class A():
    def go(self):
        b=B()
        b.func()

class B():
    def func(self):
        # Introspect to find the calling A instance here

Answer 1

In general we don't want that func to have access back to the calling instance of A because this breaks encapsulation. Inside of b.func you should have access to any args and kwargs passed, the state/attributes of the instance b (via self here), and any globals hanging around.

If you want to know about a calling object, the valid ways are:

1. Pass the calling object in as an argument to the function
2. Explicitly add a handle to the caller onto b instance sometime before using func, and then access that handle through self.

However, with that disclaimer out of the way, it's still worth knowing that Python's introspection capabilities are powerful enough to access the caller module in some cases. In the CPython implementation, here is how you could access the calling A instance without changing your existing function signatures:

class A:
    def go(self):
        b=B()
        b.func()

class B:
    def func(self):
        import inspect
        print inspect.currentframe().f_back.f_locals["self"]
        
if __name__ == "__main__":
    a = A()
    a.go()

Output:

<__main__.A instance at 0x15bd9e0>

This might be a useful trick to know about for debugging purposes. A similar technique is even used in stdlib logging, here, so that loggers are able to discover the source code/file name/line number/function name without needing to be explicitly passed that context. However, in normal use cases, it would not usually be a sensible design decision to access stack frames in the case that B.func actually needed to use A, because it's cleaner and easier to pass along the information that you need rather than to try and "reach back" to a caller.