bpo-42112: ZipInfo.from_file removes only needed slashes

Lib/test/test_zipfile.py

@@ -2037,6 +2037,16 @@ def test_decompress_without_3rd_party_library(self):
             with zipfile.ZipFile(zip_file) as zf:
                 self.assertRaises(RuntimeError, zf.extract, 'a.txt')
 
+    def test_reading_archive_member_starts_with_slash(self):
+        member_path = '/folder/file.txt'
+        member_data = b'hello world'
+        with open(TESTFN, "wb") as f:
+            f.write(member_data)
+        with zipfile.ZipFile(TESTFN2, 'w') as zf:
+            zf.write(TESTFN, member_path)
+        with zipfile.ZipFile(TESTFN2) as zf:
+            self.assertEqual(zf.read(member_path), member_data)
+
     def tearDown(self):
         unlink(TESTFN)
         unlink(TESTFN2)

Lib/zipfile.py

@@ -510,7 +510,10 @@ def from_file(cls, filename, arcname=None, *, strict_timestamps=True):
         if arcname is None:
             arcname = filename
         arcname = os.path.normpath(os.path.splitdrive(arcname)[1])
-        while arcname[0] in (os.sep, os.altsep):
+        while (arcname[0] in (os.sep, os.altsep) and
+               arcname[1] in (os.sep, os.altsep)):
+            arcname = arcname[1:]
+        if arcname[0] in (os.sep, os.altsep) and os.sep not in arcname[1:]:
             arcname = arcname[1:]
         if isdir:
             arcname += '/'

Misc/NEWS.d/next/Library/2020-10-22-07-07-17.bpo-42112.Z_ED9N.rst

@@ -0,0 +1,2 @@
+Fix unnecessary removing of a leading slash in :meth:`zipfile.ZipInfo.from_file`
+when member's archive name is in the form of `/directory/filename`.