In Python, how does one catch warnings as if they were exceptions?

Question

A third-party library (written in C) that I use in my python code is issuing warnings. I want to be able to use the try except syntax to properly handle these warnings. Is there a way to do this?

How do you recognize "real warning"? I thought that in C you get real warning during compile. — Fenikso, Apr 13 '11 at 06:06
`warnings.filterwarnings` does exactly what you want, I don't understand what your issue with it is? — Rosh Oxymoron, Apr 13 '11 at 06:19
@Fenikso, @Rosh Oxymoron you were right. My mistake. `warnings.filterwarnigns('error')` does the job. I can't find the original answer that proposed this solution — Boris Gorelik, Apr 13 '11 at 06:37

niekas · Accepted Answer · 2022-12-15T09:46:38.490

255

To handle warnings as errors simply use this:

import warnings
warnings.filterwarnings("error")

After this you will be able to catch warnings same as errors, e.g. this will work:

try:
    some_heavy_calculations()
except RuntimeWarning:
    breakpoint()

You can also reset the behaviour of warnings by running:

warnings.resetwarnings()

P.S. Added this answer because the best answer in comments contains misspelling: filterwarnigns instead of filterwarnings.

edited Dec 15 '22 at 09:46

answered May 21 '15 at 08:46

niekas

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And if you just want to see a stack trace, the first two lines are all you need. – z0r Feb 23 '17 at 00:59
7

This is perfect. I just wanted my script to stop execution as soon as the warning was issued, so that I could print relevant debug information and fix the issue. – Praveen Apr 26 '17 at 15:53
2

You don't need the `filterwarnings` call in order to catch `Warnings`, at least in python 3. it just works. – naught101 Apr 03 '19 at 06:37
1

The accepted answer does not answer the OP's question. This answer does. This is the answer I was looking for when my search found this question. – Biggsy Jan 24 '20 at 10:39
1

Converting all warnings to errors can easily become problematic if you're using this within a larger script or software package. Can make things very hard to debug as well – Ben Jeffrey Nov 02 '20 at 14:35
2

Use warnings.resetwarnings() to reset when you're done – Nick Koprowicz Dec 13 '22 at 22:57
I am still seeing warnings after `warnings.resetwarnings()` in Jupyter notebook. It does not interrupt the program but they were not showing up earlier. It's all `DeprecationWarning`. Any clue? – close2zero Feb 02 '23 at 15:17

score 75 · Answer 2 · edited Apr 25 '18 at 18:14

75

To quote from the python handbook (27.6.4. Testing Warnings):

import warnings

def fxn():
    warnings.warn("deprecated", DeprecationWarning)

with warnings.catch_warnings(record=True) as w:
    # Cause all warnings to always be triggered.
    warnings.simplefilter("always")
    # Trigger a warning.
    fxn()
    # Verify some things
    assert len(w) == 1
    assert issubclass(w[-1].category, DeprecationWarning)
    assert "deprecated" in str(w[-1].message)

edited Apr 25 '18 at 18:14

Asclepius

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answered Apr 13 '11 at 06:13

Bobby Powers

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[Here](http://stackoverflow.com/a/15934081/461597) is an answer, that tells you how to use the `try` `except` syntax. – Unapiedra Oct 10 '14 at 13:12
2

This has the advantage, over niekas's answer, that if ``fnx`` returns something, you keep that result (and still can manage the warning). – Pietro Battiston Mar 22 '19 at 18:22
This does not answer the OP's question, which was about handling wanrings, not testing them. However, the answer by niekas below does show how to handle warnings. – Biggsy Jan 24 '20 at 10:42
Just a note that the above function will not work if your function only intermittently returns a warning because in the event that `fxn()` does not return a warning, then `w` will be an empty list. If `w` is an empty list (i.e. `[]`), then running the code will give you the following error: `IndexError: list index out of range`. If you're just looking to format or check properties of the captured warnings, then it's better to use a for-loop: `for x in w: print(f'{x.category.__name__}: {str(x.message)}')` – Steven M. Mortimer Jun 14 '20 at 21:29
1

This approach is useful if one wants to handle warnings without interrupting program execution. – normanius Mar 23 '21 at 12:01

azmeuk · Answer 3 · 2021-08-06T12:37:30.940

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If you just want your script to fail on warnings you can invoke python with the -W argument:

python -W error foobar.py

edited Aug 06 '21 at 12:37

answered Aug 22 '16 at 10:58

azmeuk

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To catch a specific warning type, e.g.: `python -W error::RuntimeWarning foobar.py` – Paul Price Dec 06 '21 at 20:50

score 24 · Answer 4 · answered Aug 26 '11 at 15:48

Here's a variation that makes it clearer how to work with only your custom warnings.

import warnings
with warnings.catch_warnings(record=True) as w:
    # Cause all warnings to always be triggered.
    warnings.simplefilter("always")

    # Call some code that triggers a custom warning.
    functionThatRaisesWarning()

    # ignore any non-custom warnings that may be in the list
    w = filter(lambda i: issubclass(i.category, UserWarning), w)

    if len(w):
        # do something with the first warning
        email_admins(w[0].message)

CharlesB · Answer 5 · 2021-05-20T11:18:28.030

12

Expanding on niekas answer, but using the catch_warnings context manager that resets the warnings behavior to default after context exit:

import warnings

with warnings.catch_warnings():
     warnings.simplefilter("error")
     # Code in this block will raise exception for a warning
# Code in this block will have default warning behaviour

edited May 20 '21 at 11:18

answered May 20 '21 at 09:23

CharlesB

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score 7 · Answer 6 · answered Jun 11 '22 at 23:14

Catching all warnings can be problematic. You can catch specific warnings. For example, I needed to catch a Pillow warning:

import warnings
warnings.filterwarnings("error", category=Image.DecompressionBombWarning)

def process_images():
  try:
    some_process()

  except Image.DecompressionBombWarning as e:
    print(e)

score 6 · Answer 7 · answered Jan 30 '18 at 18:32

6

In some cases, you need use ctypes to turn warnings into errors. For example:

str(b'test')  # no error
import warnings
warnings.simplefilter('error', BytesWarning)
str(b'test')  # still no error
import ctypes
ctypes.c_int.in_dll(ctypes.pythonapi, 'Py_BytesWarningFlag').value = 2
str(b'test')  # this raises an error

answered Jan 30 '18 at 18:32

Collin Anderson

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This answer is constructive simply for showing how to error only in certain warning types. For almost any large software project, if you do `warnings.simplefilter('error')` you won't get the traceback for the warning you saw in the logs, but instead get tracebacks from previously-filtered warnings. Using `simplefilter` is also the quickest way to arrive at your answer if you have some CLI invocation. – AlanSE Apr 12 '19 at 14:08

score 1 · Answer 8 · answered Jul 06 '21 at 11:22

1

Just for completeness, you can also export an env variable:

PYTHONWARNINGS=error /usr/bin/run_my_python_utility

answered Jul 06 '21 at 11:22

pmav99

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In Python, how does one catch warnings as if they were exceptions?

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