Support named escapes (\N{...}) in string processing

CHANGES.md

@@ -13,6 +13,8 @@
 - Fix incorrect custom breakpoint indices when string group contains fake f-strings
   (#2311)
 - Fix regression where `R` prefixes would be lowercased for docstrings (#2285)
+- Fix handling of named escapes (`\N{...}`) when `--experimental-string-processing` is
+  used (#2319)
 
 ## 21.5b2
 

src/black/trans.py

@@ -1243,6 +1243,40 @@ def more_splits_should_be_made() -> bool:
             last_line.comments = line.comments.copy()
             yield Ok(last_line)
 
+    def _get_nameescape_slices(self, string: str) -> List[Tuple[int, int]]:
+        """
+        Returns:
+            List of all ranges of @string which, if @string were to be split there,
+            would result in the splitting of an \\N{...} expression (which is NOT
+            allowed).
+        """
+        slices = []
+        # True - the previous backslash was unescaped
+        # False - the previous backslash was escaped *or* there was no backslash
+        previous_was_unescaped_backslash = False
+        it = iter(enumerate(string))
+        for idx, c in it:
+            if c == "\\":
+                previous_was_unescaped_backslash = not previous_was_unescaped_backslash
+                continue
+            if not previous_was_unescaped_backslash or c != "N":
+                previous_was_unescaped_backslash = False
+                continue
+            previous_was_unescaped_backslash = False
+
+            start = idx - 1  # the position of backslash before \N{...}
+            for idx, c in it:
+                if c == "}":
+                    end = idx
+                    break
+            else:
+                # malformed nameescape expression?
+                # should have been detected by AST parsing earlier...
+                raise RuntimeError(f"{self.__class__.__name__} LOGIC ERROR!")
+            slices.append((start, end))
+
+        return slices
+
     def _get_break_idx(self, string: str, max_break_idx: int) -> Optional[int]:
         """
         This method contains the algorithm that StringSplitter uses to
@@ -1307,6 +1341,16 @@ def breaks_fstring_expression(i: Index) -> bool:
 
             return False
 
+        nameescape_slices = self._get_nameescape_slices(string)
+
+        def breaks_nameescape_expression(i: Index) -> bool:
+            """
+            Returns:
+                True iff returning @i would result in the splitting of an
+                \\N{...} expression (which is NOT allowed).
+            """
+            return any(start <= i < end for start, end in nameescape_slices)
+
         def passes_all_checks(i: Index) -> bool:
             """
             Returns:
@@ -1330,6 +1374,7 @@ def passes_all_checks(i: Index) -> bool:
                 and is_not_escaped
                 and is_big_enough
                 and not breaks_fstring_expression(i)
+                and not breaks_nameescape_expression(i)
             )
 
         # First, we check all indices BELOW @max_break_idx.

tests/data/long_strings.py

@@ -207,6 +207,38 @@ def foo():
     " of it."
 )
 
+string_with_nameescape = (
+    "........................................................................ \N{LAO KO LA}"
+)
+
+string_with_nameescape = (
+    "........................................................................... \N{LAO KO LA}"
+)
+
+string_with_nameescape = (
+    "............................................................................ \N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    "...................................................................... \\\N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    "......................................................................... \\\N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    ".......................................................................... \\\N{LAO KO LA}"
+)
+
+string_with_escaped_nameescape = (
+    "........................................................................ \\N{LAO KO LA}"
+)
+
+string_with_escaped_nameescape = (
+    "........................................................................... \\N{LAO KO LA}"
+)
+
 
 # output
 
@@ -587,3 +619,43 @@ def foo():
     "This is a really long string that can't be merged because it has a likely pragma at the end"  # pylint: disable=some-pylint-check
     " of it."
 )
+
+string_with_nameescape = (
+    "........................................................................"
+    " \N{LAO KO LA}"
+)
+
+string_with_nameescape = (
+    "..........................................................................."
+    " \N{LAO KO LA}"
+)
+
+string_with_nameescape = (
+    "............................................................................"
+    " \N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    "......................................................................"
+    " \\\N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    "........................................................................."
+    " \\\N{LAO KO LA}"
+)
+
+string_with_nameescape_and_escaped_backslash = (
+    ".........................................................................."
+    " \\\N{LAO KO LA}"
+)
+
+string_with_escaped_nameescape = (
+    "........................................................................ \\N{LAO"
+    " KO LA}"
+)
+
+string_with_escaped_nameescape = (
+    "..........................................................................."
+    " \\N{LAO KO LA}"
+)

tests/data/long_strings__regression.py

@@ -514,6 +514,10 @@ async def foo(self):
 
 x = F"This is a long string which contains an f-expr that should not split {{{[i for i in range(5)]}}}."
 
+x = (
+    "\N{BLACK RIGHT-POINTING TRIANGLE WITH DOUBLE VERTICAL BAR}\N{VARIATION SELECTOR-16}"
+)
+
 
 # output
 
@@ -1142,3 +1146,7 @@ async def foo(self):
     "This is a long string which contains an f-expr that should not split"
     f" {{{[i for i in range(5)]}}}."
 )
+
+x = (
+    "\N{BLACK RIGHT-POINTING TRIANGLE WITH DOUBLE VERTICAL BAR}\N{VARIATION SELECTOR-16}"
+)