diff --git a/Lib/test/test_zipfile.py b/Lib/test/test_zipfile.py
index 2851051425bf12..687e43df780d65 100644
--- a/Lib/test/test_zipfile.py
+++ b/Lib/test/test_zipfile.py
@@ -1856,11 +1856,14 @@ def test_comments(self):
self.assertEqual(zipf.comment, b"an updated comment")
# check that comments are correctly shortened in append mode
+ # and the file is indeed truncated
with zipfile.ZipFile(TESTFN,mode="w") as zipf:
zipf.comment = b"original comment that's longer"
zipf.writestr("foo.txt", "O, for a Muse of Fire!")
+ original_zip_size = os.path.getsize(TESTFN)
with zipfile.ZipFile(TESTFN,mode="a") as zipf:
zipf.comment = b"shorter comment"
+ self.assertTrue(original_zip_size > os.path.getsize(TESTFN))
with zipfile.ZipFile(TESTFN,mode="r") as zipf:
self.assertEqual(zipf.comment, b"shorter comment")
diff --git a/Lib/zipfile.py b/Lib/zipfile.py
index 915698f9e05889..816f8582bbf6d5 100644
--- a/Lib/zipfile.py
+++ b/Lib/zipfile.py
@@ -1918,6 +1918,8 @@ def _write_end_record(self):
centDirSize, centDirOffset, len(self._comment))
self.fp.write(endrec)
self.fp.write(self._comment)
+ if self.mode == "a":
+ self.fp.truncate()
self.fp.flush()
def _fpclose(self, fp):
diff --git a/Misc/NEWS.d/next/Library/2020-04-03-16-13-59.bpo-40105.hfM2c0.rst b/Misc/NEWS.d/next/Library/2020-04-03-16-13-59.bpo-40105.hfM2c0.rst
new file mode 100644
index 00000000000000..f71a7a1e697a48
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-04-03-16-13-59.bpo-40105.hfM2c0.rst
@@ -0,0 +1,2 @@
+ZipFile truncates files to avoid corruption when a shorter comment is provided
+in append ("a") mode. Patch by Jan Mazur.
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