[Python-ideas] Yielding through context managers

[Joshua Bartlett]

I'd like to propose adding the ability for context managers to catch and
handle control passing into and out of them via yield and generator.send()
/ generator.next().

For instance,

class cd(object):
    def __init__(self, path):
        self.inner_path = path

    def __enter__(self):
        self.outer_path = os.getcwd()
        os.chdir(self.inner_path)

    def __exit__(self, exc_type, exc_val, exc_tb):
        os.chdir(self.outer_path)

    def __yield__(self):
        self.inner_path = os.getcwd()
        os.chdir(self.outer_path)

    def __send__(self):
        self.outer_path = os.getcwd()
        os.chdir(self.inner_path)

Here __yield__() would be called when control is yielded through the with
block and __send__() would be called when control is returned via .send()
or .next(). To maintain compatibility, it would not be an error to leave
either __yield__ or __send__ undefined.

The rationale for this is that it's sometimes useful for a context manager
to set global or thread-global state as in the example above, but when the
code is used in a generator, the author of the generator needs to make
assumptions about what the calling code is doing. e.g.

def my_generator(path):
    with cd(path):
        yield do_something()
        do_something_else()

Even if the author of this generator knows what effect do_something() and
do_something_else() have on the current working directory, the author needs
to assume that the caller of the generator isn't touching the working
directory. For instance, if someone were to create two my_generator()
generators with different paths and advance them alternately, the resulting
behaviour could be most unexpected. With the proposed change, the context
manager would be able to handle this so that the author of the generator
doesn't need to make these assumptions.

Naturally, nested with blocks would be handled by calling __yield__ from
innermost to outermost and __send__ from outermost to innermost.

I rather suspect that if this change were included, someone could come up
with a variant of the contextlib.contextmanager decorator to simplify
writing generators for this sort of situation.

Cheers,

J. D. Bartlett
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