use local variable (#38850)

Author: ahmedabu98

sdks/python/apache_beam/io/fileio.py

@@ -313,9 +313,10 @@ def expand(self, pbegin) -> beam.PCollection[filesystem.FileMetadata]:
         fire_interval=self.interval)
 
     # match file pattern periodically
+    file_pattern = self.file_pattern
     match_files = (
         impulse
-        | 'GetFilePattern' >> beam.Map(lambda x: self.file_pattern)
+        | 'GetFilePattern' >> beam.Map(lambda x: file_pattern)
         | MatchAll(self.empty_match_treatment))
 
     # apply deduplication strategy if required