Combine f-strings which would be placed on the same line

CHANGES.md

@@ -10,6 +10,8 @@
 
 <!-- Changes that affect Black's stable style -->
 
+- Combine multiple f-strings into a single f-string when appropriate (#4480)
+
 ### Preview style
 
 <!-- Changes that affect Black's preview style -->

src/black/linegen.py

@@ -506,6 +506,41 @@ def visit_NUMBER(self, leaf: Leaf) -> Iterator[Line]:
         normalize_numeric_literal(leaf)
         yield from self.visit_default(leaf)
 
+    def visit_atom(self, node: Node) -> Iterator[Line]:
+        # Addressing https://github.com/psf/black/issues/4389
+        # turn an atom with only f-strings into a single f-string
+        # but only if doing so won't go over the line limit
+        is_all_fstring = True
+        for c in node.children:
+            if c.type != syms.fstring:
+                is_all_fstring = False
+                break
+        total_len = len("".join([c.value for c in node.leaves()]))
+        if (
+            len(node.children) > 0
+            and is_all_fstring
+            and total_len < self.mode.line_length
+        ):
+            new_node = Node(syms.fstring, [])
+            # remove all FSTRING_START but first and all FSTRING_END but last
+            new_children = [
+                gc
+                for i, c in enumerate(node.children)
+                for gc in c.children
+                if (
+                    not (gc.type == token.FSTRING_START and i != 0)
+                    and not (
+                        gc.type == token.FSTRING_END and i != len(node.children) - 1
+                    )
+                )
+            ]
+            for c in new_children:
+                new_node.append_child(c)
+            node.replace(new_node)
+            yield from self.visit_fstring(new_node)
+        else:
+            yield from self.visit_default(node)
+
     def visit_fstring(self, node: Node) -> Iterator[Line]:
         # currently we don't want to format and split f-strings at all.
         string_leaf = fstring_to_string(node)

tests/data/cases/combine-fstring.py

@@ -0,0 +1,7 @@
+sep= " "
+FSTRING = f"many" f"{sep}" f"tiny" f"{sep}" f"fstrings"
+
+# output
+
+sep = " "
+FSTRING = f"many{sep}tiny{sep}fstrings"

tests/data/cases/combine-multi-line-fstring.py

@@ -0,0 +1,13 @@
+sep= " "
+FSTRING = (
+    f"many"
+    f"{sep}"
+    f"tiny"
+    f"{sep}"
+    f"fstrings"
+)
+
+# output
+
+sep = " "
+FSTRING = f"many{sep}tiny{sep}fstrings"