gh-82045: Correct and deduplicate "isprintable" docs; add test. (GH-130118)

We had the definition of what makes a character "printable" documented in three places, giving two different definitions.
The definition in the comment on `_PyUnicode_IsPrintable` was inverted; correct that.

With that correction, the two definitions turn out to be equivalent -- but to confirm that, you have to go look up, or happen to know, that those are the only five "Other" categories and only three "Separator" categories in the Unicode character database.  That makes it hard for the reader to tell whether they really are the same, or if there's some subtle difference in the intended semantics.

Fix that by cutting the C API docs' and the C comment's copies of the subtle details, in favor of referring to the Python-level docs. That ensures it's explicit that these are all meant to agree, and also lets us concentrate improvements to the wording in one place.

Speaking of which, borrow some ideas from the C comment, along with other tweaks, to hopefully add a bit more clarity to that one newly-centralized copy in the docs.

Also add a thorough test that the implementation agrees with this definition.

Author:    Greg Price <[email protected]>

Co-authored-by: Greg Price <[email protected]>
(cherry picked from commit 3402e13)

Author: StanFromIreland

Doc/c-api/unicode.rst

@@ -256,13 +256,8 @@ the Python configuration.
 
 .. c:function:: int Py_UNICODE_ISPRINTABLE(Py_UCS4 ch)
 
-   Return ``1`` or ``0`` depending on whether *ch* is a printable character.
-   Nonprintable characters are those characters defined in the Unicode character
-   database as "Other" or "Separator", excepting the ASCII space (0x20) which is
-   considered printable.  (Note that printable characters in this context are
-   those which should not be escaped when :func:`repr` is invoked on a string.
-   It has no bearing on the handling of strings written to :data:`sys.stdout` or
-   :data:`sys.stderr`.)
+   Return ``1`` or ``0`` depending on whether *ch* is a printable character,
+   in the sense of :meth:`str.isprintable`.
 
 
 These APIs can be used for fast direct character conversions:

Doc/library/stdtypes.rst

@@ -1876,13 +1876,19 @@ expression support in the :mod:`re` module).
 
 .. method:: str.isprintable()
 
-   Return ``True`` if all characters in the string are printable or the string is
-   empty, ``False`` otherwise.  Nonprintable characters are those characters defined
-   in the Unicode character database as "Other" or "Separator", excepting the
-   ASCII space (0x20) which is considered printable.  (Note that printable
-   characters in this context are those which should not be escaped when
-   :func:`repr` is invoked on a string.  It has no bearing on the handling of
-   strings written to :data:`sys.stdout` or :data:`sys.stderr`.)
+   Return true if all characters in the string are printable, false if it
+   contains at least one non-printable character.
+
+   Here "printable" means the character is suitable for :func:`repr` to use in
+   its output; "non-printable" means that :func:`repr` on built-in types will
+   hex-escape the character.  It has no bearing on the handling of strings
+   written to :data:`sys.stdout` or :data:`sys.stderr`.
+
+   The printable characters are those which in the Unicode character database
+   (see :mod:`unicodedata`) have a general category in group Letter, Mark,
+   Number, Punctuation, or Symbol (L, M, N, P, or S); plus the ASCII space 0x20.
+   Nonprintable characters are those in group Separator or Other (Z or C),
+   except the ASCII space.
 
 
 .. method:: str.isspace()

Lib/test/test_str.py

@@ -853,6 +853,15 @@ def test_isprintable(self):
         self.assertTrue('\U0001F46F'.isprintable())
         self.assertFalse('\U000E0020'.isprintable())
 
+    @support.requires_resource('cpu')
+    def test_isprintable_invariant(self):
+        for codepoint in range(sys.maxunicode + 1):
+            char = chr(codepoint)
+            category = unicodedata.category(char)
+            self.assertEqual(char.isprintable(),
+                             category[0] not in ('C', 'Z')
+                             or char == ' ')
+
     def test_surrogates(self):
         for s in ('a\uD800b\uDFFF', 'a\uDFFFb\uD800',
                   'a\uD800b\uDFFFa', 'a\uDFFFb\uD800a'):

Objects/clinic/unicodeobject.c.h

@@ -142,18 +142,10 @@ int _PyUnicode_IsNumeric(Py_UCS4 ch)
     return (ctype->flags & NUMERIC_MASK) != 0;
 }
 
-/* Returns 1 for Unicode characters to be hex-escaped when repr()ed,
-   0 otherwise.
-   All characters except those characters defined in the Unicode character
-   database as following categories are considered printable.
-      * Cc (Other, Control)
-      * Cf (Other, Format)
-      * Cs (Other, Surrogate)
-      * Co (Other, Private Use)
-      * Cn (Other, Not Assigned)
-      * Zl Separator, Line ('\u2028', LINE SEPARATOR)
-      * Zp Separator, Paragraph ('\u2029', PARAGRAPH SEPARATOR)
-      * Zs (Separator, Space) other than ASCII space('\x20').
+/* Returns 1 for Unicode characters that repr() may use in its output,
+   and 0 for characters to be hex-escaped.
+
+   See documentation of `str.isprintable` for details.
 */
 int _PyUnicode_IsPrintable(Py_UCS4 ch)
 {

Objects/unicodectype.c

@@ -12016,15 +12016,14 @@ unicode_isidentifier_impl(PyObject *self)
 /*[clinic input]
 str.isprintable as unicode_isprintable
 
-Return True if the string is printable, False otherwise.
+Return True if all characters in the string are printable, False otherwise.
 
-A string is printable if all of its characters are considered printable in
-repr() or if it is empty.
+A character is printable if repr() may use it in its output.
 [clinic start generated code]*/
 
 static PyObject *
 unicode_isprintable_impl(PyObject *self)
-/*[clinic end generated code: output=3ab9626cd32dd1a0 input=98a0e1c2c1813209]*/
+/*[clinic end generated code: output=3ab9626cd32dd1a0 input=4e56bcc6b06ca18c]*/
 {
     Py_ssize_t i, length;
     int kind;