import io
import gzip
class BadFile(io.BytesIO):
    first = False
    def write(self, data):
        if self.first:
            self.first = False
            raise OSError
def main():
    try:
        gzip.GzipFile(fileobj=BadFile(), mode="w")
    except OSError:
        pass
main()

./python -W error ../../demo.py
Exception ignored while calling deallocator <function GzipFile.__del__ at 0x7b14defe09e0>:
Traceback (most recent call last):
  File "/home/graingert/projects/cpython/Lib/gzip.py", line 458, in __del__
    warnings.warn("unclosed GzipFile",
ResourceWarning: unclosed GzipFile