Side effects in warnings are suppressed by the xdoctest runner.

[Erotemic]

It looks like in xdoctest/doctest_example the:

        with warnings.catch_warnings(record=True) as self.warn_list:

Might be preventing warnings from printing. In xdev I'm testing:

import warnings


def make_warnings_print_tracebacks():
    """
    Makes warnings show tracebacks when displayed.

    Applies a monkeypatch to warnings.formatwarning that includes a traceback
    in the displayed warning message.

    NOTE:
        Use the context manager `:class:WarningsWithTracebacks` instead, or
        :func:`WarningsWithTracebacks.apply()` if global modification is
        needed.
    """
    WarningsWithTracebacks.apply()


class WarningsWithTracebacks:
    """
    Makes warnings show tracebacks when displayed.

    Applies a monkeypatch to warnings.formatwarning that includes a traceback
    in the displayed warning message.

    CommandLine:
        xdoctest -m xdev.tracebacks WarningsWithTracebacks:0
        xdoctest -m xdev.tracebacks WarningsWithTracebacks:1

    Example:
        >>> from xdev.tracebacks import *  # NOQA
        >>> import warnings
        >>> with WarningsWithTracebacks():
        ...     warnings.warn('This is a test warning')
        <string>:1: UserWarning: This is a test warning
        Traceback (most recent call last):
        ...

    Example:
        >>> from xdev.tracebacks import *  # NOQA
        >>> import warnings
        >>> warnings.warn('This warning1 has no traceback')
        >>> with WarningsWithTracebacks():
        >>>     warnings.warn('This warning2 has a traceback')
        >>> warnings.warn('This warning3 has no traceback')

    Example:
        >>> import warnings
        >>> def my_function():
        ...     with WarningsWithTracebacks():
        ...         warnings.warn('This is a test warning')
        >>> def driver():
        >>>     my_function()
        >>> driver()
    """
    _enter_count = 0

    def __init__(self, thread_safe=True):
        if thread_safe:
            import threading
            self._lock = threading.Lock()
        else:
            from contextlib import nullcontext
            self._lock = nullcontext()

    def __enter__(self):
        with self._lock:
            # Apply patching if it's the first entry in this thread
            if self._enter_count == 0:
                print("ENTER")
                self._orig_formatwarning = warnings.formatwarning
                warnings.formatwarning = self._monkeypatch_formatwarning_tb
            else:
                print("NOT ENTER")
            self._enter_count += 1

    def __exit__(self, exc_type, exc_val, exc_tb):
        self._enter_count -= 1
        if self._enter_count == 0:
            warnings.formatwarning = self._orig_formatwarning

    def _monkeypatch_formatwarning_tb(self, *args, **kwargs):
        print("MONKEY MONKEY MONKEY")
        import traceback
        s = self._orig_formatwarning(*args, **kwargs)
        if len(s.strip()):
            tb = traceback.format_stack()
            s += ''.join(tb[:-1])
        return s

    @classmethod
    def apply(cls):
        """
        Apply this warnings traceback patch globaly. It will not be undone.
        """
        cls().__enter__()


def make_warnings_print_tracebacks2():
    """
    Makes warnings show tracebacks when displayed.

    Applies a monkeypatch to warnings.formatwarning that includes a traceback
    in the displayed warning message.

    CommandLine:
        xdoctest -m xdev.tracebacks make_warnings_print_tracebacks2

    Example:
        >>> make_warnings_print_tracebacks2()
        >>> warnings.warn('This is a test warning')
    """
    import warnings
    import traceback
    _orig_formatwarning = warnings.formatwarning
    warnings._orig_formatwarning = _orig_formatwarning
    def _monkeypatch_formatwarning_tb(*args, **kwargs):
        s = _orig_formatwarning(*args, **kwargs)
        if len(s.strip()):
            tb = traceback.format_stack()
            s += ''.join(tb[:-1])
        return s
    warnings.formatwarning = _monkeypatch_formatwarning_tb


if __name__ == '__main__':
    """
    CommandLine:
        python ~/code/xdev/xdev/tracebacks.py
    """

    def main():
        # make_warnings_print_tracebacks2()
        # warnings.warn('This is a test warning')
        warnings.warn('This warning1 has no traceback')
        with WarningsWithTracebacks():
            warnings.warn('This warning2 has a traceback')
        warnings.warn('This warning3 has no traceback')

    main()

And running via xdoctest does not print MONKEY MONKEY MONKEY, but using the regular main does. Simply hacking out that line does cause the warning traceback to show.

It looks like this might be an issue witch catch_warnings. It seems like it patches warnings.showwarning, which causes formatwarning to never be called.

[Erotemic]

This seems like xdoctest might need to implement its own version of catch_warnings. I think through this idea in this thread: https://discuss.python.org/t/capturing-custom-formatwarning-with-warnings-catch-warnings-record-true/83807