The only purpose of the module_globals argument of warnings.warn_explicit() is to get the source line of the warning from the module loader when the file cannot be read. The C implementation computes it in get_source_line(), but then throws it away.

call_show_warning() passes None in the line slot of WarningMessage and ignores its own sourceline argument:

    msg = PyObject_CallFunctionObjArgs(warnmsg_cls, message, category,
            filename, lineno_obj, Py_None, Py_None,
            source ? source : Py_None, module,
            NULL);

sourceline is only used in the fallback show_warning() path, taken when warnings._showwarnmsg is not available.

As a result the source line is lost, while the pure Python implementation, which seeds linecache instead, displays it:

$ cat > spam.py <<EOF
import warnings
def f():
    warnings.warn_explicit('eggs', UserWarning, 'bar', 1, module_globals=globals())
EOF
$ ./python -c 'import spam; spam.f()'
bar:1: UserWarning: eggs
$ ./python -c 'import sys; sys.modules["_warnings"] = None; import spam; spam.f()'
bar:1: UserWarning: eggs
  import warnings

The None was added in 914cde8 together with the source argument, so the C implementation has never passed the source line.

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