[Python-Dev] Negative times behaviour in itertools.repeat for Python maintenance releases (2.7, 3.3 and maybe 3.4) (original) (raw)

Steven D'Aprano steve at pearwood.info
Mon Jan 27 13:56:51 CET 2014

On Mon, Jan 27, 2014 at 04:29:04AM -0800, Larry Hastings wrote:

The code currently has semantics that cannot be accurately represented in a Python signature. We could do one of three things:

1) Do nothing, and don't allow inspect.Signature to produce a signature for the function. This is the status quo. 2) Change the semantics of the function in a non-backwards-compatible way so that we can represent its signature accurately with an inspect.Signature object. For example, "change the function so that providing times=-1 as a keyword argument behaves the same as providing times=-1 as a positional-only argument" is such an incompatible change. Another is "change the function to not accept keyword arguments at all". 3) Change the semantics of the function in a backwards-compatible way so that we can represent its supported signature accurately with an inspect.Signature object. Allow continued use of the old semantics for a full deprecation cycle (two major versions) if not longer. For example, "change the times argument to have a default of None, and change the logic so that times=None means it repeats forever" would be such an approach. For 3.3 and 3.4, I suggest that only 1) makes sense.

Are you rejecting the idea that the current behaviour is an out and out buggy, and therefore fixing these things can and should occur in a bug-fix release?

As far as I can see, the only piece of evidence that the given behaviour isn't a bug is that the signature says "object [, times]" rather than "object, times=None". That's not conclusive: I've often written signatures using [ ] to indicate optional arguments without specifying the default value in the signature.

As it stands now, the documentation is internally contradictory. In one part of the documentation, it gives a clear indication that "times is None" should select the repeat forever behaviour. In another part of the documentation, it fails to mention that None is an acceptable value to select the repeat forever behaviour.

For 3.5 I prefer 3), specifically the "times=None" approach, as that's how the function has been documented as working since the itertools module was first introduce in 2.3. And I see functions as having accurate signatures as a good thing.

I'm against 2), as I'm against removing functionality simply for purity's sakes. Removing functionality breaks code. So it's best reserved for critical problems like security issues. I cite the thread we just had in python-dev, subject line "Deprecation policy", as an excellent discussion and summary of this topic.

I'm confused... you seem to be saying that you are against changing the behaviour of repeat so that:

repeat(x, -1)

and 

repeat(x, times=-1)

behave the same. Is that actually what you mean, or have I misunderstood?

Are there any other functions in the standard library where the behaviour differs depending on whether an argument is given positionally or by keyword?

-- Steven

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