[Python-Dev] Re: threading issue: preventing starting twice (original) (raw)

Russell E. Owen rowen at cesmail.net
Thu Feb 26 15:17:17 EST 2004

In article <200402252140.i1PLeoT08847 at guido.python.org>, Guido van Rossum <guido at python.org> wrote:

> In the threading module, one must of course only start a thread once. I > have some questions about handling this gracefully. > > First of all, the documentation only says it is an error to start a > thread more than once. It does not say which exception is raised. On my > system I find it is AssertionError. So...is that going to be true on all > systems? Is the test based on "assert", in which case it will go away > for optimized code (and then what happens)?

Hey, it's open source! You can look for yourself. :-) Yes, the test is based on assert.

But if a feature is not documented, it could easily be changed in a future release with no warning. Thus it would be a mistake to assume that this error always raises AssertionError. (Also, since it is based on assert, the error will not even be raised if enables the optimizer?).

... There is isRunning(). Isn't that enough? I realize it doesn't distinguish between "not yet running" and "no longer running", but your application state should be enough to distinguish between the two, right?

I am trying to determine the state of a thread, and in particular determine if it's safe to try to start it. Thus isRunning is explicitly not enough.

The result is in my own code I end up writing a status layer around the thread, but I assume many other programmers have to do the same thing. It seems a rather roundabout way to do things when the threading Thread object ought to have a pretty good idea what state it is in.

-- Russell

More information about the Python-Dev mailing list