CODEWITHSUNDEEP
pythonpython-3.x
Michael Schlottke-Lakemper
Michael Schlottke-Lakemper

Reputation: 9321

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

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.

Upvotes: 1

Views: 59

Answers (3)

Gryphius
Gryphius

Reputation: 78886

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...

Upvotes: 2

bruno desthuilliers
bruno desthuilliers

Reputation: 77902

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...

Upvotes: 2

Martijn Pieters
Martijn Pieters

Reputation: 1121834

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()

e.g. 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.

Upvotes: 1

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