bpo-35168: Make shlex.punctuation_chars read-only (#11631)

* bpo-35168: Documentation about shlex.punctuation_chars now states that it should be set in __init__.py

* bpo-35168: Convert shlex.punctuation_chars to read-only property

* Add NEWS.d entry

Doc/library/shlex.rst

@@ -113,7 +113,9 @@ The :mod:`shlex` module defines the following class:
    characters, those characters will be used as the punctuation characters.  Any
    characters in the :attr:`wordchars` attribute that appear in
    *punctuation_chars* will be removed from :attr:`wordchars`.  See
-   :ref:`improved-shell-compatibility` for more information.
+   :ref:`improved-shell-compatibility` for more information. *punctuation_chars*
+   can be set only upon :class:`~shlex.shlex` instance creation and can't be
+   modified later.
 
    .. versionchanged:: 3.6
       The *punctuation_chars* parameter was added.
@@ -317,8 +319,8 @@ variables which either control lexical analysis or can be used for debugging:
 
 .. attribute:: shlex.punctuation_chars
 
-   Characters that will be considered punctuation. Runs of punctuation
-   characters will be returned as a single token. However, note that no
+   A read-only property. Characters that will be considered punctuation. Runs of
+   punctuation characters will be returned as a single token. However, note that no
    semantic validity checking will be performed: for example, '>>>' could be
    returned as a token, even though it may not be recognised as such by shells.
 

Lib/shlex.py

@@ -55,7 +55,7 @@ class shlex:
             punctuation_chars = ''
         elif punctuation_chars is True:
             punctuation_chars = '();<>|&'
-        self.punctuation_chars = punctuation_chars
+        self._punctuation_chars = punctuation_chars
         if punctuation_chars:
             # _pushback_chars is a push back queue used by lookahead logic
             self._pushback_chars = deque()
@@ -65,6 +65,10 @@ class shlex:
             t = self.wordchars.maketrans(dict.fromkeys(punctuation_chars))
             self.wordchars = self.wordchars.translate(t)
 
+    @property
+    def punctuation_chars(self):
+        return self._punctuation_chars
+
     def push_token(self, tok):
         "Push a token onto the stack popped by the get_token method"
         if self.debug >= 1:

Lib/test/test_shlex.py

@@ -353,6 +353,13 @@ class ShlexTest(unittest.TestCase):
                 resplit = shlex.split(joined)
                 self.assertEqual(split_command, resplit)
 
+    def testPunctuationCharsReadOnly(self):
+        punctuation_chars = "/|$%^"
+        shlex_instance = shlex.shlex(punctuation_chars=punctuation_chars)
+        self.assertEqual(shlex_instance.punctuation_chars, punctuation_chars)
+        with self.assertRaises(AttributeError):
+            shlex_instance.punctuation_chars = False
+
 
 # Allow this test to be used with old shlex.py
 if not getattr(shlex, "split", None):

Misc/NEWS.d/next/Library/2019-01-22-09-23-20.bpo-35168.UGv2yW.rst

@@ -0,0 +1 @@
+:attr:`shlex.shlex.punctuation_chars` is now a read-only property.