Add support for PEP701

* fstrings are broken into several distinct token in py3.12, reattach
  them together as a singular string to preserve previous behavior.

Closes: #646
Signed-off-by: Alfred Wingate <parona@protonmail.com>

docs/release_notes.rst

@@ -4,6 +4,14 @@ Release Notes
 **pydocstyle** version numbers follow the
 `Semantic Versioning <http://semver.org/>`_ specification.
 
+
+Current development version
+---------------------------
+
+Bug Fixes
+
+* Add support for PEP-701 fixing fstring parsing in python3.12 (#656).
+
 6.3.0 - January 17th, 2023
 --------------------------
 

src/pydocstyle/parser.py

@@ -463,6 +463,7 @@ def leapfrog(self, kind, value=None):
                 return
             self.stream.move()
 
+
     def parse_docstring(self):
         """Parse a single docstring and return its value."""
         self.log.debug("parsing docstring, token is %s", self.current)
@@ -479,6 +480,27 @@ def parse_docstring(self):
             )
             self.stream.move()
             return docstring
+        if (sys.version_info.major, sys.version_info.minor) >= (
+            3,
+            12,
+        ) and self.current.kind == tk.FSTRING_START:
+            def fstring(string):
+                """Recursively parse fstring tokens to output it as one string."""
+                while self.current.kind != tk.FSTRING_END:
+                    self.stream.move()
+                    string += self.current.value
+                    if self.current.kind == tk.FSTRING_START:
+                        string = fstring(string)
+                        self.stream.move()
+                        string += self.current.value
+                return string
+            # Reattach fstring tokens together into a string to deal with PEP 701 in python3.12
+            start = self.current.start[0]
+            string = fstring(self.current.value)
+            end = self.current.end[0]
+            docstring = Docstring(string, start, end)
+            self.stream.move()
+            return docstring
         return None
 
     def parse_decorators(self):

src/tests/parser_test.py

@@ -114,6 +114,35 @@ def do_something(pos_param0, pos_param1, kw_param0="default"):
     assert str(function) == 'in public function `do_something`'
 
 
+def test_nested_fstring():
+    """Test parsing fstring with nested fstrings."""
+    parser = Parser()
+    code = CodeSnippet("""\
+        def do_something(pos_param0, pos_param1, kw_param0="default"):
+            f\"""Do something. {f"This is a nested fstring."}\"""
+            return None
+    """)
+    module = parser.parse(code, 'file_path')
+    assert module.is_public
+    assert module.dunder_all is None
+
+    function, = module.children
+    assert function.name == 'do_something'
+    assert function.decorators == []
+    assert function.children == []
+    assert function.docstring == 'f"""Do something. {f"This is a nested fstring."}"""'
+    assert function.docstring.start == 2
+    assert function.docstring.end == 2
+    assert function.kind == 'function'
+    assert function.parent == module
+    assert function.start == 1
+    assert function.end == 3
+    assert function.error_lineno == 2
+    assert function.source == code.getvalue()
+    assert function.is_public
+    assert str(function) == 'in public function `do_something`'
+
+
 def test_decorated_function():
     """Test parsing of a simple function with a decorator."""
     parser = Parser()