How to reference the correct file when printing a warning in python?

Question

I just started to use the warnings module in python. I'm trying to understand how to reference the correct line in a given file when I issue a warning.

I'm writing a module where some functions can issue warnings, but I would like all warnings to point to the line where they are issued in scripts where the module was imported, rather than to a line where they are issued in the module itself. I think I get how the stacklevel argument of warnings.warn works, but I don't see how to use it with functions of my module that are also used internally, since their stack level might vary.

I will try to make myself clear using an example. Say I write the following module, that I call testmodule.py.

import warnings

def f():
    warnings.warn('This is a test warning.', stacklevel=2)

def g():
    f()

Then I write the following script.

import testmodule as test

test.f()
test.g()

If I run this script, the output is

"script_filename":3: UserWarning: This is a test warning.
  test.f()
"module_filename":7: UserWarning: This is a test warning.
  f()

where "script_filename" and "module_filename" are the actual names of the files where I saved the script and the module on my computer.

In the above example, both warnings correctly identify the line where f is called. However, the warning issued when calling test.g points inside of testmodule, so it is not useful to a user of the module. Is there a way to make both of these warnings point to the line where they are issued in the script, and not in the module itself?

saint-jaeger · Accepted Answer · 2020-11-12 15:50:32Z

1

With these small changes, I got the exact behaviour you are asking for. Adding arguments to g() may not be what you wanted, but I think this is an answer, and may help you understand.

import warnings

def f(slevel=2):
    warnings.warn('This is a test warning.', stacklevel=slevel)

def g(i):
    f(i)

import testmodule  as test

test.f()
test.g(3)

Outputs

script_filename:3: UserWarning: This is a test warning.
  test.f()
script_filename:4: UserWarning: This is a test warning.
  test.g(3)

answered Nov 12, 2020 at 15:50

saint-jaeger

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2 Comments

Vincent Over a year ago

Thanks for your answer! It seems to be an improvement, but it is not really what I'm looking for. To use this solution, a user of the module would have to know not only that f will issue a warning, but also how deep f is nested when using g. So it would not really be helpful to a user of the module.

Vincent Over a year ago

But it would indeed be helpful if g didn't take any arguments, but called f(3). I will try to work something out from that. Thank you for your help!

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