[fix] Fix bug revealed by psf#1243

black.py

@@ -3924,7 +3924,7 @@ def _assign_match(LL: List[Leaf]) -> Optional[int]:
         # If this line is apart of an expression statement or is a function
         # argument AND the first leaf contains a variable name...
         if (
-            parent_type(LL[0]) in [syms.expr_stmt, syms.argument]
+            parent_type(LL[0]) in [syms.expr_stmt, syms.argument, syms.power]
             and LL[0].type == token.NAME
         ):
             is_valid_index = is_valid_index_factory(LL)

tests/data/long_strings__regression.py

@@ -300,6 +300,15 @@ def foo():
     "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 )
 
+class Step(StepBase):
+    def who(self):
+        self.cmd = 'SR AAAA-CORRECT NAME IS {last_name} {first_name}{middle_name} {title}/P{passenger_association}'.format(
+            last_name=last_name,
+            first_name=first_name,
+            middle_name=middle_name,
+            title=title,
+            passenger_association=passenger_association,
+        )
 
 # output
 
@@ -673,3 +682,17 @@ def foo():
     "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
     "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 )
+
+
+class Step(StepBase):
+    def who(self):
+        self.cmd = (
+            "SR AAAA-CORRECT NAME IS {last_name} {first_name}{middle_name}"
+            " {title}/P{passenger_association}".format(
+                last_name=last_name,
+                first_name=first_name,
+                middle_name=middle_name,
+                title=title,
+                passenger_association=passenger_association,
+            )
+        )