Can I write a function that detect if it is called from 'with'-statement or not?

Question

More specifically, can I detect if a function is called as EXPR in with EXPR: BLOCK statement? I am trying to make myself familiar with the usage of with-statement in python. As a first step, I re-implemented an example, which generate marked up text, appeared in the reference of contextlib.contextmanager (ignoring exception handling for now).

class Markup(object):
    def __init__(self):
        self.tags = []
        self.append = self.tags.append
        self.pop = self.tags.pop

    def tag(self, name):
        self.append(name)
        return self

    def __enter__(self):
        print('<{}>'.format(self.tags[-1]))

    def __exit__(self, exc_type, exc_value, traceback):
        print('</{}>'.format(self.pop()))

>>> m = Markup()
>>> with m.tag('ELEMENT'):
...     print('sample text')
...
<ELEMENT>
sample text
</ELEMENT>

This works as expected. Then, I got to think if tag() can also generate empty elements:

>>> m = Markup()

# if a function appears as EXPR of "with EXPR: BLOCK", 'ELEMENT' is a container element. .
>>> with m.tag('ELEMENT'):
...     print('sample text')
...
<ELEMENT>
sample text
</ELEMENT> 

# in other cases, ELEMENT is an empty element.
>>> m.tag('ELEMENT')
<ELEMENT/>

To my naive eyes, it seems doable if callee can detect if it is called from with-statement or not. However, I do not know if this detection is possible or not. Is there such a way and, if there is, how?

Answer 1

You're not actually calling tag() "from" the with statement. You're calling tag(), then passing the return value from tag() to the with statement, which then calls __enter__ on that passed-in value, then runs the body, then calls __exit__.

So no, you can't detect the with statement before it's actually invoked (which is after tag() is called).