Find a way to determine inside function if method is called on the return value?

Question

My class looks like this:

class A:
    def __init__(self):
        self.bar = []

    ...

    @property
    def foo(self):
        return bar

Is there a way to find out inside foo whether a method will be called on its return value? I would like to be able to change the return value of foo depending on whether

a.foo.foobar()

or

a.foo

is called.

Answer 1

You could use a proxy class wrapping self.bar (or just self FWIW) in foo() ) and overload the proxy's __getattr__() or __getattribute__ methods (more tricky and can slow down your program quite a bit but well...).

Now the question is: what is your real problem ? There might be better / safer solutions...

Answer 2

for the fun of it...

#!/usr/bin/python
import traceback

def how_was_i_called():
    call=traceback.extract_stack(limit=2)[0][3]
    print "I was called like this: %s"%call

how_was_i_called()
try:
    how_was_i_called().foobar()
except AttributeError:
  pass

returns:

I was called like this: how_was_i_called()
I was called like this: how_was_i_called().foobar()

but please do not use hacks like this in real applications...

Answer 3

No, there is not. foo returns, and what happens with the return value after that is an entirely separate issue.

You could do this, for example:

 result = a.foo
 if some_condition:
      result.foobar()

eg accessing the foobar method on a.foo is an entirely separate expression that may or may not be executed. This could happen at a much later time too, or in a separate thread, or after serialising the object to disk, then loading it again, etc.

You can hook into attribute access on the returned object , but that'll be too late for your foo property to alter behaviour.