Prevent f-string merge quote changes with nested quotes (#4498)

CHANGES.md

@@ -17,6 +17,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix/remove string merging changing f-string quotes on f-strings with internal quotes
+  (#4498)
 - Remove parentheses around sole list items (#4312)
 - Collapse multiple empty lines after an import into one (#4489)
 

docs/the_black_code_style/future_style.md

@@ -137,10 +137,11 @@ foo(
 
 _Black_ will split long string literals and merge short ones. Parentheses are used where
 appropriate. When split, parts of f-strings that don't need formatting are converted to
-plain strings. User-made splits are respected when they do not exceed the line length
+plain strings. f-strings will not be merged if they contain internal quotes and it would
-limit. Line continuation backslashes are converted into parenthesized strings.
+change their quotation mark style. User-made splits are respected when they do not
-Unnecessary parentheses are stripped. The stability and status of this feature is
+exceed the line length limit. Line continuation backslashes are converted into
-tracked in [this issue](https://github.com/psf/black/issues/2188).
+parenthesized strings. Unnecessary parentheses are stripped. The stability and status of
+this feature istracked in [this issue](https://github.com/psf/black/issues/2188).
 
 (labels/wrap-long-dict-values)=
 

src/black/trans.py

@@ -794,6 +794,8 @@ def _validate_msg(line: Line, string_idx: int) -> TResult[None]:
                 - The set of all string prefixes in the string group is of
                   length greater than one and is not equal to {"", "f"}.
                 - The string group consists of raw strings.
+                - The string group would merge f-strings with different quote types
+                  and internal quotes.
                 - The string group is stringified type annotations. We don't want to
                   process stringified type annotations since pyright doesn't support
                   them spanning multiple string values. (NOTE: mypy, pytype, pyre do
@@ -820,6 +822,8 @@ def _validate_msg(line: Line, string_idx: int) -> TResult[None]:
 
                 i += inc
 
+        QUOTE = line.leaves[string_idx].value[-1]
+
         num_of_inline_string_comments = 0
         set_of_prefixes = set()
         num_of_strings = 0
@@ -842,6 +846,19 @@ def _validate_msg(line: Line, string_idx: int) -> TResult[None]:
 
             set_of_prefixes.add(prefix)
 
+            if (
+                "f" in prefix
+                and leaf.value[-1] != QUOTE
+                and (
+                    "'" in leaf.value[len(prefix) + 1 : -1]
+                    or '"' in leaf.value[len(prefix) + 1 : -1]
+                )
+            ):
+                return TErr(
+                    "StringMerger does NOT merge f-strings with different quote types"
+                    "and internal quotes."
+                )
+
             if id(leaf) in line.comments:
                 num_of_inline_string_comments += 1
                 if contains_pragma_comment(line.comments[id(leaf)]):

tests/data/cases/preview_fstring.py

@@ -0,0 +1,2 @@
+# flags: --unstable
+f"{''=}" f'{""=}'

tests/data/cases/preview_long_strings.py

@@ -882,7 +882,7 @@ def foo():
 
 log.info(
     "Skipping:"
-    f" {desc['db_id']} {foo('bar',x=123)} {'foo' != 'bar'} {(x := 'abc=')} {pos_share=} {desc['status']} {desc['exposure_max']}"
+    f' {desc["db_id"]} {foo("bar",x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
 )
 
 log.info(
@@ -902,7 +902,7 @@ def foo():
 
 log.info(
     "Skipping:"
-    f" {'a' == 'b' == 'c' == 'd'} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']} {desc['exposure_max']}"
+    f' {"a" == "b" == "c" == "d"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
 )
 
 log.info(

tests/data/cases/preview_long_strings__regression.py

@@ -552,6 +552,7 @@ async def foo(self):
 }
 
 # Regression test for https://github.com/psf/black/issues/3506.
+# Regressed again by https://github.com/psf/black/pull/4498
 s = (
     "With single quote: ' "
     f" {my_dict['foo']}"
@@ -1239,9 +1240,15 @@ async def foo(self):
 }
 
 # Regression test for https://github.com/psf/black/issues/3506.
-s = f"With single quote: '  {my_dict['foo']} With double quote: \"  {my_dict['bar']}"
+# Regressed again by https://github.com/psf/black/pull/4498
+s = (
+    "With single quote: ' "
+    f" {my_dict['foo']}"
+    ' With double quote: " '
+    f' {my_dict["bar"]}'
+)
 
 s = (
     "Lorem Ipsum is simply dummy text of the printing and typesetting"
-    f" industry:'{my_dict['foo']}'"
+    f' industry:\'{my_dict["foo"]}\''
 )