Rename fetch_files_to_be_fetched -> fetch, force option

bagit.py

@@ -588,9 +588,12 @@ def files_to_be_fetched(self):
         for _, _, filename in self.fetch_entries():
             yield filename
 
-    def fetch_files_to_be_fetched(self):
+    def fetch(self, force=False):
         """
         Fetches files from the fetch.txt
+
+        Arguments:
+            force (boolean): Fetch files even when they are present in the data directory
         """
         proxy_handler = ProxyHandler() # will default to adhere to *_proxy env vars
         opener = build_opener(proxy_handler)
@@ -599,7 +602,7 @@ def fetch_files_to_be_fetched(self):
             if not fnmatch_any(url, self.fetch_url_whitelist):
                 raise BagError(_("Malformed URL in fetch.txt: %s, matches none of the whitelisted URL patterns %s") % (url, self.fetch_url_whitelist))
             expected_size = -1 if expected_size == '-' else int(expected_size)
-            if filename in self.payload_files():
+            if filename in self.payload_files() and not force:
                 LOGGER.info(_("File already fetched: %s"), filename)
                 continue
             req = Request(url)

test.py

@@ -1130,7 +1130,30 @@ def test_fetching_payload_file(self):
         with self.assertRaises(bagit.BagError):
             self.bag.validate()
         # re-fetch it
-        self.bag.fetch_files_to_be_fetched()
+        self.bag.fetch()
+        # should be valid again
+        self.bag.validate()
+        self.assertEqual(len(self.bag.compare_fetch_with_fs()), 0, 'complete')
+
+    def test_force_fetching(self):
+        test_payload = 'loc/2478433644_2839c5e8b8_o_d.jpg'
+        with open(j(self.tmpdir, "fetch.txt"), "w") as fetch_txt:
+            print("https://github.com/LibraryOfCongress/bagit-python/raw/master/test-data/%s %s data/%s" % (
+                  test_payload, 139367, test_payload), file=fetch_txt)
+        self.bag.save(manifests=True)
+        # now replace one payload file with an empty string
+        with open(j(self.tmpdir, "data", test_payload), 'w') as payload:
+            payload.write('')
+        # should be invalid now
+        with self.assertRaisesRegexp(bagit.BagError, "^Payload-Oxum validation failed."):
+            self.bag.validate()
+        # non-forcefully downloading should not help
+        self.bag.fetch()
+        # should **still* be invalid now
+        with self.assertRaisesRegexp(bagit.BagError, "^Payload-Oxum validation failed."):
+            self.bag.validate()
+        # fetch with force
+        self.bag.fetch(force=True)
         # should be valid again
         self.bag.validate()
         self.assertEqual(len(self.bag.compare_fetch_with_fs()), 0, 'complete')