Question

I guess the subject sounds pretty stupid, so I’ll show some code:

def foo(**kwargs):
    # How can you detect the difference between (**{}) and ()?
    pass
foo(**{})
foo()

Is there any way to detect inside of foo, how the method was called?

Update 1

Because there were some comments why you possible want to do something, I’ll try to explain some background.

super(MyClass, self).foo(*args, **kwargs) sucks – a lot of wasteful duplication. I want to write ‘self.super()’. In this case, just call the super class and hand over all parameters that the calling method got as well. Works like a charm.

Now the problematic magic part:

I want to say ‘self.super(something)’ and in this case, only ‘something’ is passed to the super method. Works for most cases.

This is where it breaks:

def foo(self, fnord=42, *args, **kwargs):
    self.super(*args, **kwargs)

So the super method should get the arguments that the calling method – however if *args, **kwargs are empty, currently the library can not detect this condition and passed all arguments including ‘fnord’…

Of course I could use self.super.foo(*args, **kwargs) as an alternative syntax but that’s lame 🙂

PS: Yes, I know p3k’s super, but still not nice and it does not work with Python 2.x…

Update 2

Actually even Python’s ast module removes the **{} (ast.parse('foo(**{})')) so it looks like this happens so early in the parsing process that you can not get this information later on…

So in the end I have either to give up on that specific problem (raising an AmbiguousSyntaxError) or to use text parsing as proposed by ~unutbu. Re-thinking my approach, the latter might actually feasable because I only need to know if it is self.super(\s*), self.super(\S+).

Answer 1

This hack only works with CPython.

import traceback

def foo(**kwargs):
    # stack is a list of 4-tuples: (filename, line number, function name, text)
    # see http://docs.python.org/library/traceback.html#module-traceback

    (filename,line_number,function_name,text)=traceback.extract_stack()[-2]
    print('foo was called: %s'%text)

foo(**{})
# foo was called: foo(**{})
foo()
# foo was called: foo()

As an example of how this might be useful:

def pv(var):
    (filename,line_number,function_name,text)=traceback.extract_stack()[-2]
    print('%s: %s'%(text[text.find('(')+1:-1],var))

x=5
pv(x)
# x: 5

Notice that pv is called with just the value x, but
it prints both the “name” of the variable (as it was called), and the value of a variable. Sometimes I use this when debugging and am too lazy to write out the full print statement.