Add parens around implicit string concatenations where increases readability

CHANGES.md

@@ -26,6 +26,8 @@
   normalized as expected (#3168)
 - When using `--skip-magic-trailing-comma` or `-C`, trailing commas are stripped from
   subscript expressions with more than 1 element (#3209)
+- Implicitly concatenated strings inside a list, set, or function call are now wrapped
+  inside parentheses (#3162)
 - Fix a string merging/split issue when a comment is present in the middle of implicitly
   concatenated strings on its own line (#3227)
 

src/black/__init__.py

@@ -498,8 +498,10 @@ def main(  # noqa: C901
             user_level_config = str(find_user_pyproject_toml())
             if config == user_level_config:
                 out(
-                    "Using configuration from user-level config at "
-                    f"'{user_level_config}'.",
+                    (
+                        "Using configuration from user-level config at "
+                        f"'{user_level_config}'."
+                    ),
                     fg="blue",
                 )
             elif config_source in (

src/black/mode.py

@@ -178,8 +178,10 @@ class Mode:
     def __post_init__(self) -> None:
         if self.experimental_string_processing:
             warn(
-                "`experimental string processing` has been included in `preview`"
-                " and deprecated. Use `preview` instead.",
+                (
+                    "`experimental string processing` has been included in `preview`"
+                    " and deprecated. Use `preview` instead."
+                ),
                 Deprecated,
             )
 

src/black/parsing.py

@@ -30,9 +30,11 @@
     # Either our python version is too low, or we're on pypy
     if sys.version_info < (3, 7) or (sys.version_info < (3, 8) and not _IS_PYPY):
         print(
-            "The typed_ast package is required but not installed.\n"
-            "You can upgrade to Python 3.8+ or install typed_ast with\n"
-            "`python3 -m pip install typed-ast`.",
+            (
+                "The typed_ast package is required but not installed.\n"
+                "You can upgrade to Python 3.8+ or install typed_ast with\n"
+                "`python3 -m pip install typed-ast`."
+            ),
             file=sys.stderr,
         )
         sys.exit(1)

src/black/trans.py

@@ -1043,6 +1043,43 @@ def _get_max_string_length(self, line: Line, string_idx: int) -> int:
         max_string_length = self.line_length - offset
         return max_string_length
 
+    @staticmethod
+    def _prefer_paren_wrap_match(LL: List[Leaf]) -> Optional[int]:
+        """
+        Returns:
+            string_idx such that @LL[string_idx] is equal to our target (i.e.
+            matched) string, if this line matches the "prefer paren wrap" statement
+            requirements listed in the 'Requirements' section of the StringParenWrapper
+            class's docstring.
+                OR
+            None, otherwise.
+        """
+        # The line must start with a string.
+        if LL[0].type != token.STRING:
+            return None
+
+        matching_nodes = [
+            syms.listmaker,
+            syms.dictsetmaker,
+            syms.testlist_gexp,
+            syms.arglist,
+        ]
+        # If the string is an immediate child of a list/set/tuple literal, or
+        # an argument of a function call...
+        if (
+            parent_type(LL[0]) in matching_nodes
+            or parent_type(LL[0].parent) in matching_nodes
+        ):
+            # And the string is surrounded by commas (or is the first/last child)...
+            prev_sibling = LL[0].prev_sibling
+            next_sibling = LL[0].next_sibling
+            if (not prev_sibling or prev_sibling.type == token.COMMA) and (
+                not next_sibling or next_sibling.type == token.COMMA
+            ):
+                return 0
+
+        return None
+
 
 def iter_fexpr_spans(s: str) -> Iterator[Tuple[int, int]]:
     """
@@ -1138,6 +1175,9 @@ class StringSplitter(BaseStringSplitter, CustomSplitMapMixin):
     def do_splitter_match(self, line: Line) -> TMatchResult:
         LL = line.leaves
 
+        if self._prefer_paren_wrap_match(LL) is not None:
+            return TErr("Line needs to be wrapped in parens first.")
+
         is_valid_index = is_valid_index_factory(LL)
 
         idx = 0
@@ -1583,8 +1623,7 @@ def _get_string_operator_leaves(self, leaves: Iterable[Leaf]) -> List[Leaf]:
 
 class StringParenWrapper(BaseStringSplitter, CustomSplitMapMixin):
     """
-    StringTransformer that splits non-"atom" strings (i.e. strings that do not
-    exist on lines by themselves).
+    StringTransformer that wraps strings in parens and then splits at the LPAR.
 
     Requirements:
         All of the requirements listed in BaseStringSplitter's docstring in
@@ -1604,6 +1643,11 @@ class StringParenWrapper(BaseStringSplitter, CustomSplitMapMixin):
             OR
         * The line is a dictionary key assignment where some valid key is being
         assigned the value of some string.
+            OR
+        * The line starts with an "atom" string that prefers to be wrapped in
+        parens. It's preferred to be wrapped when it's is an immediate child of
+        a list/set/tuple literal or an argument of a function call, AND the
+        string is surrounded by commas (or is the first/last child).
 
     Transformations:
         The chosen string is wrapped in parentheses and then split at the LPAR.
@@ -1628,6 +1672,9 @@ class StringParenWrapper(BaseStringSplitter, CustomSplitMapMixin):
         changed such that it no longer needs to be given its own line,
         StringParenWrapper relies on StringParenStripper to clean up the
         parentheses it created.
+
+        For "atom" strings that prefers to be wrapped in parens, it requires
+        StringSplitter to hold the split until the string is wrapped in parens.
     """
 
     def do_splitter_match(self, line: Line) -> TMatchResult:
@@ -1644,6 +1691,7 @@ def do_splitter_match(self, line: Line) -> TMatchResult:
             or self._assert_match(LL)
             or self._assign_match(LL)
             or self._dict_match(LL)
+            or self._prefer_paren_wrap_match(LL)
         )
 
         if string_idx is not None:

tests/data/preview/cantfit.py

@@ -79,10 +79,14 @@
 )
 # long arguments
 normal_name = normal_function_name(
-    "but with super long string arguments that on their own exceed the line limit so"
-    " there's no way it can ever fit",
-    "eggs with spam and eggs and spam with eggs with spam and eggs and spam with eggs"
-    " with spam and eggs and spam with eggs",
+    (
+        "but with super long string arguments that on their own exceed the line limit"
+        " so there's no way it can ever fit"
+    ),
+    (
+        "eggs with spam and eggs and spam with eggs with spam and eggs and spam with"
+        " eggs with spam and eggs and spam with eggs"
+    ),
     this_is_a_ridiculously_long_name_and_nobody_in_their_right_mind_would_use_one_like_it=0,
 )
 string_variable_name = "a string that is waaaaaaaayyyyyyyy too long, even in parens, there's nothing you can do"  # noqa

tests/data/preview/comments7.py

@@ -226,39 +226,53 @@ class C:
             # metadata_version errors.
             (
                 {},
-                "None is an invalid value for Metadata-Version. Error: This field is"
-                " required. see"
-                " https://packaging.python.org/specifications/core-metadata",
+                (
+                    "None is an invalid value for Metadata-Version. Error: This field"
+                    " is required. see"
+                    " https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
             (
                 {"metadata_version": "-1"},
-                "'-1' is an invalid value for Metadata-Version. Error: Unknown Metadata"
-                " Version see"
-                " https://packaging.python.org/specifications/core-metadata",
+                (
+                    "'-1' is an invalid value for Metadata-Version. Error: Unknown"
+                    " Metadata Version see"
+                    " https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
             # name errors.
             (
                 {"metadata_version": "1.2"},
-                "'' is an invalid value for Name. Error: This field is required. see"
-                " https://packaging.python.org/specifications/core-metadata",
+                (
+                    "'' is an invalid value for Name. Error: This field is required."
+                    " see https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
             (
                 {"metadata_version": "1.2", "name": "foo-"},
-                "'foo-' is an invalid value for Name. Error: Must start and end with a"
-                " letter or numeral and contain only ascii numeric and '.', '_' and"
-                " '-'. see https://packaging.python.org/specifications/core-metadata",
+                (
+                    "'foo-' is an invalid value for Name. Error: Must start and end"
+                    " with a letter or numeral and contain only ascii numeric and '.',"
+                    " '_' and '-'. see"
+                    " https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
             # version errors.
             (
                 {"metadata_version": "1.2", "name": "example"},
-                "'' is an invalid value for Version. Error: This field is required. see"
-                " https://packaging.python.org/specifications/core-metadata",
+                (
+                    "'' is an invalid value for Version. Error: This field is required."
+                    " see https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
             (
                 {"metadata_version": "1.2", "name": "example", "version": "dog"},
-                "'dog' is an invalid value for Version. Error: Must start and end with"
-                " a letter or numeral and contain only ascii numeric and '.', '_' and"
-                " '-'. see https://packaging.python.org/specifications/core-metadata",
+                (
+                    "'dog' is an invalid value for Version. Error: Must start and end"
+                    " with a letter or numeral and contain only ascii numeric and '.',"
+                    " '_' and '-'. see"
+                    " https://packaging.python.org/specifications/core-metadata"
+                ),
             ),
         ],
     )