with ignored

[Barrett Lewis]

>
> However, ignored() is actually implemented as a generator function
>
with the @contextmanager decorator shortcut.  This decorator takes a
> generator function and wraps it up as a class with the necessary
> __enter__ and __exit__ methods.  The __enter__ method in this case
> calls the .next() method of the generator and returns after it yields
> once.


I looked up the source to the decorator
found here:http://hg.python.org/cpython/file/406b47c64480/Lib/contextlib.py for
anyone interested.
It appears that they are using yield so that they can call it again and
force it to throw the stop iteration exception.



> The __exit__ method calls it again -- or it calls the .throw()
> method if an exception was passed in -- and expects the generator to
> exit as a result.
>
>
And here to go with what I said above, they want to be able to use the
generators throw ability to capture any exception and throw it to the
generator, but then why do they *need* the generator to not iterate again?
Or in other words, why when they create the context manager via
_GeneratorContextManager on line 33, is there the clause in __exit__ to
have stop iteration happen or else throw a runtime error?

I am thinking this is because it needs the yield to be over so that it can
return the control flow to normal, and that subroutine will be done. But I
am new to the whole subroutine paradigm so I could be misunderstanding
this.

Also, if the value was none, why are they instantiating a new exception
with value = type()? How could there be a type for the exception without
the exception? (From what I understand type is the error type, and value is
the error object).


So from the perspective of the generator it does its context setup (in
> this case, setting up a try block) prior to the yield, and then does
> the cleanup (in this case, selectively catching and suppressing the
> exceptions) after the yield.
>
This part makes sense logically. And I like the simplicity. Now I just need
to make sure I understand how the yield semantics are allowing this
construct to work.

Thanks for the reply Ian!

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