[Python-Dev] PEP 553

Barry Warsaw barry at python.org
Mon Oct 2 18:03:25 EDT 2017

On Oct 2, 2017, at 17:36, Guido van Rossum <guido at python.org> wrote:

> I've seen your updates and it is now acceptable, except for *one* nit: in builtins.breakpoint() the pseudo code raises RuntimeError if sys.breakpointhook is missing or None. OTOH sys.breakpointhook() just issues a RuntimeWarning when something's wrong with the hook. Maybe builtins.breakpoint() should also just warn if it can't find the hook? Setting `sys.breakpointhook = None` might be the simplest way to programmatically disable breakpoints. Why not allow it?

Oh, actually the pseudocode doesn’t match the C implementation exactly in this regard.  Currently the C implementation is more like:

    def breakpoint(*args, **kws):
        import sys
        missing = object()
        hook = getattr(sys, 'breakpointhook', missing)
        if hook is missing:
            raise RuntimeError('lost sys.breakpointhook')
        return hook(*args, **kws)

The intent being, much like the other sys-hooks, that if PySys_GetObject("breakpointhook”) returns NULL, Something Bad Happened, so we have to set an error string and bail.  (PySys_GetObject() does not set an exception.)


>>> def foo():
...   print('yes')
...   breakpoint()
...   print('no')
>>> del sys.breakpointhook
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in foo
RuntimeError: lost sys.breakpointhook

Setting `sys.breakpoint = None` could be an interesting use case, but that’s not currently special in any way:

>>> sys.breakpointhook = None
>>> foo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in foo
TypeError: 'NoneType' object is not callable

I’m open to special-casing this if you think it’s useful.

(I’ll update the pseudocode in the PEP.)


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