Prevent f-string merge quote changes with nested quotes

CHANGES.md

@@ -17,6 +17,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Fix/remove string merging changing f-string quotes (#4498)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

docs/the_black_code_style/future_style.md

@@ -132,10 +132,11 @@ 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
-limit. Line continuation backslashes are converted into parenthesized strings.
-Unnecessary parentheses are stripped. The stability and status of this feature is
-tracked in [this issue](https://github.com/psf/black/issues/2188).
+plain strings. f-strings will not be merged if it would change their quotation mark
+style. User-made splits are respected when they do not exceed the line length limit.
+Line continuation backslashes are converted into 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

@@ -806,6 +806,7 @@ 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.
                 - 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
@@ -832,6 +833,8 @@ 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
@@ -854,6 +857,11 @@ def _validate_msg(line: Line, string_idx: int) -> TResult[None]:
 
             set_of_prefixes.add(prefix)
 
+            if "f" in prefix and leaf.value[-1] != QUOTE:
+                return TErr(
+                    "StringMerger does NOT merge f-strings with different quote types."
+                )
+
             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

@@ -309,16 +309,12 @@ def foo():
 
 log.info(f"Skipping: {desc['db_id']=} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']=} {desc['exposure_max']=}")
 
-log.info(f'Skipping: {desc["db_id"]} {foo("bar",x=123)} {"foo" != "bar"} {(x := "abc=")} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
-
 log.info(f'Skipping: {desc["db_id"]} {desc["ms_name"]} {money=} {(x := "abc=")=} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
 
 log.info(f'Skipping: {desc["db_id"]} {foo("bar",x=123)=} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
 
 log.info(f'Skipping: {foo("asdf")=} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
 
-log.info(f'Skipping: {"a" == "b" == "c" == "d"} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
-
 log.info(f'Skipping: {"a" == "b" == "c" == "d"=} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}')
 
 log.info(f'Skipping: {  longer_longer_longer_longer_longer_longer_name   [ "db_id" ]      [ "another_key" ]   =  :  .3f }')
@@ -880,11 +876,6 @@ def foo():
     f" {desc['db_id']=} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']=} {desc['exposure_max']=}"
 )
 
-log.info(
-    "Skipping:"
-    f" {desc['db_id']} {foo('bar',x=123)} {'foo' != 'bar'} {(x := 'abc=')} {pos_share=} {desc['status']} {desc['exposure_max']}"
-)
-
 log.info(
     "Skipping:"
     f' {desc["db_id"]} {desc["ms_name"]} {money=} {(x := "abc=")=} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
@@ -900,11 +891,6 @@ def foo():
     f' {foo("asdf")=} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}'
 )
 
-log.info(
-    "Skipping:"
-    f" {'a' == 'b' == 'c' == 'd'} {desc['ms_name']} {money=} {dte=} {pos_share=} {desc['status']} {desc['exposure_max']}"
-)
-
 log.info(
     "Skipping:"
     f' {"a" == "b" == "c" == "d"=} {desc["ms_name"]} {money=} {dte=} {pos_share=} {desc["status"]} {desc["exposure_max"]}'

tests/data/cases/preview_long_strings__regression.py

@@ -551,16 +551,6 @@ async def foo(self):
         ("item1", "item2", "item3"),
 }
 
-# Regression test for https://github.com/psf/black/issues/3506.
-s = (
-    "With single quote: ' "
-    f" {my_dict['foo']}"
-    ' With double quote: " '
-    f' {my_dict["bar"]}'
-)
-
-s = f'Lorem Ipsum is simply dummy text of the printing and typesetting industry:\'{my_dict["foo"]}\''
-
 
 # output
 
@@ -1237,11 +1227,3 @@ async def foo(self):
     # And there is a comment before the value
     ("item1", "item2", "item3"),
 }
-
-# Regression test for https://github.com/psf/black/issues/3506.
-s = f"With single quote: '  {my_dict['foo']} With double quote: \"  {my_dict['bar']}"
-
-s = (
-    "Lorem Ipsum is simply dummy text of the printing and typesetting"
-    f" industry:'{my_dict['foo']}'"
-)