Issue #25913: Leading <~ is optional now in base64.a85decode() with adobe=True.

Patch by Swati Jaiswal.

Lib/base64.py

@@ -367,10 +367,15 @@ def a85decode(b, *, foldspaces=False, adobe=False, ignorechars=b' \t\n\r\v'):
     """
     b = _bytes_from_decode_data(b)
     if adobe:
-        if not (b.startswith(_A85START) and b.endswith(_A85END)):
+        if not b.endswith(_A85END):
-            raise ValueError("Ascii85 encoded byte sequences must be bracketed "
+            raise ValueError(
-                             "by {!r} and {!r}".format(_A85START, _A85END))
+                "Ascii85 encoded byte sequences must end "
-        b = b[2:-2] # Strip off start/end markers
+                "with {!r}".format(_A85END)
+                )
+        if b.startswith(_A85START):
+            b = b[2:-2]  # Strip off start/end markers
+        else:
+            b = b[:-2]
     #
     # We have to go through this stepwise, so as to ignore spaces and handle
     # special short sequences

Lib/test/test_base64.py

@@ -494,6 +494,7 @@ class BaseXYTestCase(unittest.TestCase):
             eq(base64.a85decode(data, adobe=False), res, data)
             eq(base64.a85decode(data.decode("ascii"), adobe=False), res, data)
             eq(base64.a85decode(b'<~' + data + b'~>', adobe=True), res, data)
+            eq(base64.a85decode(data + b'~>', adobe=True), res, data)
             eq(base64.a85decode('<~%s~>' % data.decode("ascii"), adobe=True),
                res, data)
 
@@ -584,8 +585,6 @@ class BaseXYTestCase(unittest.TestCase):
                                       b"malformed", adobe=True)
         self.assertRaises(ValueError, base64.a85decode,
                                       b"<~still malformed", adobe=True)
-        self.assertRaises(ValueError, base64.a85decode,
-                                      b"also malformed~>", adobe=True)
 
         # With adobe=False (the default), Adobe framing markers are disallowed
         self.assertRaises(ValueError, base64.a85decode,

Misc/NEWS

@@ -76,6 +76,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #25913: Leading ``<~`` is optional now in base64.a85decode() with
+  adobe=True.  Patch by Swati Jaiswal.
+
 - Issue #26186: Remove an invalid type check in importlib.util.LazyLoader.
 
 - Issue #26367: importlib.__import__() raises SystemError like