Checking for naive datetime now occurs in GzipFile.__init__ instead of GzipFile._write_gzip_header to avoid file staying open on raising ValueError.

Author: gchqdev227

Lib/gzip.py

@@ -225,6 +225,8 @@ def __init__(self, filename=None, mode=None,
                                              -zlib.MAX_WBITS,
                                              zlib.DEF_MEM_LEVEL,
                                              0)
+            if isinstance(mtime, datetime) and mtime.tzinfo is None:
+                raise ValueError("Refusing to write naive datetime to Gzip header")
             self._write_mtime = mtime
             self._buffer_size = _WRITE_BUFFER_SIZE
             self._buffer = io.BufferedWriter(_WriteBufferStream(self),
@@ -281,8 +283,6 @@ def _write_gzip_header(self, compresslevel):
         if mtime is None:
             mtime = time.time()
         elif isinstance(mtime, datetime):
-            if mtime.tzinfo is None:
-                raise ValueError("Refusing to write naive datetime to Gzip header")
             mtime = mtime.timestamp()
         write32u(self.fileobj, int(mtime))
         if compresslevel == _COMPRESS_LEVEL_BEST: