Python 3: warnings displayed multiple times despite default 'once' filter

[embray]

(I suspect this has come up before but I couldn't find an existing issue for it.)

The expected default behavior in Python for handling warnings is such that once a specific warning has been displayed once, it should not be displayed again (unless the warning filters are updated). E.g.:

Python 3.6.1 (default, Feb 21 2018, 12:59:49)
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import warnings
>>> def foo():
...     warnings.warn('foo')
...
>>> foo()
__main__:2: UserWarning: foo
>>> foo()
>>>

However, in IPython CLI this behavior is not observed (probably same for the notebook but I haven't tested):

Python 3.6.1 (default, Feb 21 2018, 12:59:49)
Type "copyright", "credits" or "license" for more information.

IPython 5.5.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: import warnings

In [2]: def foo():
   ...:     warnings.warn('foo')
   ...:

In [3]: foo()
.../local/bin/ipython3:2: UserWarning: foo
  # EASY-INSTALL-ENTRY-SCRIPT: 'ipython==5.5.0','console_scripts','ipython3'

In [4]: foo()
.../local/bin/ipython3:2: UserWarning: foo
  # EASY-INSTALL-ENTRY-SCRIPT: 'ipython==5.5.0','console_scripts','ipython3'

I have documented the cause of this behavior here, and I believe it is more of a CPython bug than an IPython bug. But it's possible there's a workaround that could be applied, or at the very least I thought the issue should be noted.

[embray]

Incidentally it seems this behavior is broken on Python 2 as well, but in that case I'm not sure what the reason is...

[embray]

Incidentally it seems this behavior is broken on Python 2 as well, but in that case I'm not sure what the reason is...

I see, this is #6611. So that's intentional. I don't agree with the solution though--it's even more brute-force than the Python 3 bug since it basically breaks the intended behavior (although I understand why a workaround was needed...)

On second thought, that only impacts code that was entered interactively. It won't affect code from modules so I'm okay with that. Then we're just left with the Python 3 issue which, unfortunately, impacts warnings regardless where they come from....

[Carreau]

Hum, I can see how this can be annoying in the REPL.

We switched the default on purpose, as you noted in #6611, and #6680. Would a configuration option to disable this behavior suits you ?

If there is an upstream issue with Python, the fix will depends on the cost/benefits ratio.

[embray]

I believe a configuration option would be useful here. While I understand the rationale behind #6680 it would also be nice to be able to get back more "standard" behavior w.r.t. warnings, even if it's not the default.

That would do nothing to help with the upstream issue, but it would be good to have either way.