import gzip
import io

class EvilDecomp:
    def __init__(self, **kwargs):
        self.needs_input = False
        self.unused_data = b""
        self.eof = False
    def decompress(self, data, max_length):
        self.needs_input = True
        return b""

gzip.zlib._ZlibDecompressor = EvilDecomp

with gzip.GzipFile(
    fileobj=io.BytesIO(gzip.compress(b"hello world")), mode="rb"
) as f:
    f.read(1)

22:16:40.678647000PM CEST maurycy@gimel /Users/maurycy/src/cpython % ./python.exe meow.py
Traceback (most recent call last):
  File "/Users/maurycy/src/cpython/meow.py", line 18, in <module>
    f.read(1)
    ~~~~~~^^^
  File "/Users/maurycy/src/cpython/Lib/gzip.py", line 349, in read
    return self._buffer.read(size)
           ~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/maurycy/src/cpython/Lib/compression/_common/_streams.py", line 68, in readinto
    data = self.read(len(byte_view))
  File "/Users/maurycy/src/cpython/Lib/gzip.py", line 589, in read
    if buf == b"":
       ^^^
UnboundLocalError: cannot access local variable 'buf' where it is not associated with a value