I actually don't get the warnings when I run test_idle under regrtest. But what I did was followed a hunch and added a check for warnings.showwarning() and sure enough, it's been left modified by IDLE: PyShell replaces the function as a side-effect of import. Not sure how you want to fix it. Using warnings.catch_warnings() during import would solve it, but I don't know if that's how you want to go in case that showwarnings replacement is needed.