Calculate file hash on an initial portion, if specified

External downloaders may not support limiting the download to a specified size

test/test_download.py

@@ -37,6 +37,7 @@ from youtube_dl.utils import (
     UnavailableVideoError,
 )
 from youtube_dl.extractor import get_info_extractor
+from youtube_dl.downloader.common import FileDownloader
 
 RETRIES = 3
 
@@ -56,9 +57,10 @@ class YoutubeDL(youtube_dl.YoutubeDL):
         return super(YoutubeDL, self).process_info(info_dict)
 
 
-def _file_md5(fn):
+def _file_md5(fn, length=None):
     with open(fn, 'rb') as f:
-        return hashlib.md5(f.read()).hexdigest()
+        return hashlib.md5(
+            f.read(length) if length is not None else f.read()).hexdigest()
 
 
 defs = gettestcases()
@@ -224,7 +226,7 @@ def generator(test_case, tname):
                             (tc_filename, format_bytes(expected_minsize),
                                 format_bytes(got_fsize)))
                     if 'md5' in tc:
-                        md5_for_file = _file_md5(tc_filename)
+                        md5_for_file = _file_md5(tc_filename) if not params.get('test') else _file_md5(tc_filename, FileDownloader._TEST_FILE_SIZE)
                         self.assertEqual(tc['md5'], md5_for_file)
                 # Finally, check test cases' data again but this time against
                 # extracted data from info JSON file written during processing