Today I learned how to issue user warnings like DeprecationWarnings or SyntaxWarnings.

Issue user warnings

Every once in a while I see a DeprecationWarning when I'm doing some coding. Last time I saw it, I think it was in some pandas code in my pandas and matplotlib tutorial with Pokémon. Today I needed to issue a similar warning and I set out to find out how that works.

Turns out there is a module in the standard library (obviously) that does this: warnings. Issuing a warning with the module warnings can be as simple as passing a string to the function warnings.warn:

>>> import warnings
>>> warnings.warn("Something wrong is not right!")
<stdin>:1: UserWarning: Something wrong is not right!

The module gives you a lot of flexibility, though, in terms of:

  • the types of warnings you issue;
  • the way the warnings are formatted;
  • how you can filter the warnings to make sure they only appear when you want them to;
  • where to write the warnings to;
  • and more!

The module warnings refers to the “types of warnings” as the categories of warnings. There are plenty of built-in warning categories, such as UserWarning (the default, as the example above shows), DeprecationWarning, SyntaxWarning, and others.

To change the category of warning you use, you can specify the category as the second parameter category:

>>> warnings.warn("Dang it!", category=RuntimeWarning)
<stdin>:1: RuntimeWarning: Dang it!

Now, a very interesting thing will happen if you run that exact same code again:

>>> warnings.warn("Dang it!", category=RuntimeWarning)
>>> # No output?!

That's because, by default, equal warnings are suppressed. (This is also customisable!) What you can do is reset the warnings:

# Only 1:
>>> for _ in range(5):
...     warnings.warn("Warning in a loop")
<stdin>:2: UserWarning: Warning in a loop

# All 5 warnings:
>>> for _ in range(5):
...     warnings.resetwarnings()
...     warnings.warn("Warning in a loop")
<stdin>:3: UserWarning: Warning in a loop
<stdin>:3: UserWarning: Warning in a loop
<stdin>:3: UserWarning: Warning in a loop
<stdin>:3: UserWarning: Warning in a loop
<stdin>:3: UserWarning: Warning in a loop

You can also specify the category by instantiating the appropriate class:

>>> warnings.warn(UserWarning("user warning test"))
<stdin>:1: UserWarning: user warning test
>>> warnings.warn(SyntaxWarning("syntax warning test"))
<stdin>:1: SyntaxWarning: syntax warning test
>>> warnings.warn(RuntimeWarning("runtime warning test"))
<stdin>:1: RuntimeWarning: runtime warning test

This is what I ended up using, together with the parameter stacklevel.

When a warning is posted, it shows the file and line number it came from. When you have wrapper functions around your warnings, you'll want to use the parameter stacklevel to make it so that the warnings refer to the place where you called your wrapper functions instead of having the warning refer to the wrapper function itself.

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