[Twisted-Python] Sequential use of asynchronous calls

[Maarten ter Huurne]

Hi,

Reading through the implementation of inlineCallbacks, I found some more 
differences, but they are small.

In my implementation the decorator has a check whether the thing it is 
decorating is really a generator. Without this check, you will get an 
exception only when you start using the decorated function. I think it is 
usually better to detect errors at an early stage. However, the check uses 
compiler.consts.CO_GENERATOR, which is not documented in the Python Library 
Reference, so using it might be risky.

My implementation only catches StandardError being raised by the generator. 
That is a bad idea: it should certainly catch custom Exceptions as well. It 
should probably catch everything, like inlineCallbacks does, so the 
asynchronous equivalents of "finally" have a chance to clean things up on 
system exit.

inlineCallbacks has protection against a Deferred being returned which has 
already fired. My implementation treats that case the same as a Deferred that 
will be handled by the reactor and will therefore recurse.

The way the "throw" call is done is slightly different:

http://twistedmatrix.com/trac/browser/trunk/twisted/internet/defer.py#L724

    result = g.throw(result.type, result.value, result.tb)

My implementation:

    traceback = result.getTracebackObject() \
        if hasattr(result, 'getTracebackObject') else None
    d = self.gen.throw(result.type, result.getErrorMessage(), traceback)

The "value" part is different: result.getErrorMessage() returns the error 
message string, while result.value contains the exception object, 
however "throw" is designed to act like "raise" and will accept both uses.

Also the "traceback" part is different: if the "tb" field is None, 
getTracebackObject will construct a traceback object if possible. I don't 
know if that extra feature will ever be useful in this particular use case.

To summarize:
InlineCallbacks seems to be better than my implementation, except perhaps:
- it might be useful to add a check whether the function to be decorated is 
really a generator
- result.getTracebackObject() may or may not be better than result.tb

Bye,
		Maarten
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