fix issue 302, where method extract_frames_as_images destory input frame list

Author: 1192119703jzx

mmif/utils/video_document_helper.py

@@ -84,18 +84,19 @@ def extract_frames_as_images(video_document: Document, framenums: List[int], as_
     video = capture(video_document)
     cur_f = 0
     tot_fcount = video_document.get_property(FRAMECOUNT_DOCPROP_KEY)
+    framenums_copy = framenums.copy()
     while True:
-        if not framenums or cur_f > tot_fcount:
+        if not framenums_copy or cur_f > tot_fcount:
             break
         ret, frame = video.read()
-        if cur_f == framenums[0]:
+        if cur_f == framenums_copy[0]:
             if not ret:
                 sec = convert(cur_f, 'f', 's', video_document.get_property(FPS_DOCPROP_KEY))
                 warnings.warn(f'Frame #{cur_f} ({sec}s) could not be read from the video {video_document.id}.')
                 cur_f += 1
                 continue
             frames.append(Image.fromarray(frame[:, :, ::-1]) if as_PIL else frame)
-            framenums.pop(0)
+            framenums_copy.pop(0)
         cur_f += 1
     return frames