+ current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+
+ def append_to_line(leaf: Leaf) -> Iterator[Line]:
+ """Append `leaf` to current line or to new line if appending impossible."""
+ nonlocal current_line
+ try:
+ current_line.append_safe(leaf, preformatted=True)
+ except ValueError as ve:
+ yield current_line
+
+ current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+ current_line.append(leaf)
+
+ for index, leaf in enumerate(line.leaves):
+ yield from append_to_line(leaf)
+
+ for comment_after in line.comments_after(leaf, index):
+ yield from append_to_line(comment_after)
+
+ if current_line:
+ yield current_line
+
+
+def is_import(leaf: Leaf) -> bool:
+ """Return True if the given leaf starts an import statement."""
+ p = leaf.parent
+ t = leaf.type
+ v = leaf.value
+ return bool(
+ t == token.NAME
+ and (
+ (v == "import" and p and p.type == syms.import_name)
+ or (v == "from" and p and p.type == syms.import_from)
+ )
+ )
+
+
+def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
+ """Leave existing extra newlines if not `inside_brackets`. Remove everything
+ else.
+
+ Note: don't use backslashes for formatting or you'll lose your voting rights.
+ """
+ if not inside_brackets:
+ spl = leaf.prefix.split("#")
+ if "\\" not in spl[0]:
+ nl_count = spl[-1].count("\n")
+ if len(spl) > 1:
+ nl_count -= 1
+ leaf.prefix = "\n" * nl_count
+ return
+
+ leaf.prefix = ""
+
+
+def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
+ """Make all string prefixes lowercase.
+
+ If remove_u_prefix is given, also removes any u prefix from the string.
+
+ Note: Mutates its argument.
+ """
+ match = re.match(r"^([furbFURB]*)(.*)$", leaf.value, re.DOTALL)
+ assert match is not None, f"failed to match string {leaf.value!r}"
+ orig_prefix = match.group(1)
+ new_prefix = orig_prefix.lower()
+ if remove_u_prefix:
+ new_prefix = new_prefix.replace("u", "")
+ leaf.value = f"{new_prefix}{match.group(2)}"
+
+
+def normalize_string_quotes(leaf: Leaf) -> None:
+ """Prefer double quotes but only if it doesn't cause more escaping.
+
+ Adds or removes backslashes as appropriate. Doesn't parse and fix
+ strings nested in f-strings (yet).
+
+ Note: Mutates its argument.
+ """
+ value = leaf.value.lstrip("furbFURB")
+ if value[:3] == '"""':
+ return
+
+ elif value[:3] == "'''":
+ orig_quote = "'''"
+ new_quote = '"""'
+ elif value[0] == '"':
+ orig_quote = '"'
+ new_quote = "'"
+ else:
+ orig_quote = "'"
+ new_quote = '"'
+ first_quote_pos = leaf.value.find(orig_quote)
+ if first_quote_pos == -1:
+ return # There's an internal error
+
+ prefix = leaf.value[:first_quote_pos]
+ unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
+ escaped_new_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{new_quote}")
+ escaped_orig_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{orig_quote}")
+ body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]
+ if "r" in prefix.casefold():
+ if unescaped_new_quote.search(body):
+ # There's at least one unescaped new_quote in this raw string
+ # so converting is impossible
+ return
+
+ # Do not introduce or remove backslashes in raw strings
+ new_body = body
+ else:
+ # remove unnecessary quotes
+ new_body = sub_twice(escaped_new_quote, rf"\1\2{new_quote}", body)
+ if body != new_body:
+ # Consider the string without unnecessary quotes as the original
+ body = new_body
+ leaf.value = f"{prefix}{orig_quote}{body}{orig_quote}"
+ new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
+ new_body = sub_twice(unescaped_new_quote, rf"\1\\{new_quote}", new_body)
+ if "f" in prefix.casefold():
+ matches = re.findall(r"[^{]\{(.*?)\}[^}]", new_body)
+ for m in matches:
+ if "\\" in str(m):
+ # Do not introduce backslashes in interpolated expressions
+ return
+ if new_quote == '"""' and new_body[-1:] == '"':
+ # edge case:
+ new_body = new_body[:-1] + '\\"'
+ orig_escape_count = body.count("\\")
+ new_escape_count = new_body.count("\\")
+ if new_escape_count > orig_escape_count:
+ return # Do not introduce more escaping
+
+ if new_escape_count == orig_escape_count and orig_quote == '"':
+ return # Prefer double quotes
+
+ leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"
+
+
+def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
+ """Make existing optional parentheses invisible or create new ones.
+
+ `parens_after` is a set of string leaf values immeditely after which parens
+ should be put.
+
+ Standardizes on visible parentheses for single-element tuples, and keeps
+ existing visible parentheses for other tuples and generator expressions.
+ """
+ try:
+ list(generate_comments(node))
+ except FormatOff:
+ return # This `node` has a prefix with `# fmt: off`, don't mess with parens.
+
+ check_lpar = False
+ for index, child in enumerate(list(node.children)):
+ if check_lpar:
+ if child.type == syms.atom:
+ maybe_make_parens_invisible_in_atom(child)
+ elif is_one_tuple(child):
+ # wrap child in visible parentheses
+ lpar = Leaf(token.LPAR, "(")
+ rpar = Leaf(token.RPAR, ")")
+ child.remove()
+ node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+ elif node.type == syms.import_from:
+ # "import from" nodes store parentheses directly as part of
+ # the statement
+ if child.type == token.LPAR:
+ # make parentheses invisible
+ child.value = "" # type: ignore
+ node.children[-1].value = "" # type: ignore
+ elif child.type != token.STAR:
+ # insert invisible parentheses
+ node.insert_child(index, Leaf(token.LPAR, ""))
+ node.append_child(Leaf(token.RPAR, ""))
+ break
+
+ elif not (isinstance(child, Leaf) and is_multiline_string(child)):
+ # wrap child in invisible parentheses
+ lpar = Leaf(token.LPAR, "")
+ rpar = Leaf(token.RPAR, "")
+ index = child.remove() or 0
+ node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+
+ check_lpar = isinstance(child, Leaf) and child.value in parens_after
+
+
+def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
+ """If it's safe, make the parens in the atom `node` invisible, recursively."""
+ if (
+ node.type != syms.atom
+ or is_empty_tuple(node)
+ or is_one_tuple(node)
+ or is_yield(node)
+ or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
+ ):
+ return False
+
+ first = node.children[0]
+ last = node.children[-1]
+ if first.type == token.LPAR and last.type == token.RPAR:
+ # make parentheses invisible
+ first.value = "" # type: ignore
+ last.value = "" # type: ignore
+ if len(node.children) > 1:
+ maybe_make_parens_invisible_in_atom(node.children[1])
+ return True
+
+ return False
+
+
+def is_empty_tuple(node: LN) -> bool:
+ """Return True if `node` holds an empty tuple."""
+ return (
+ node.type == syms.atom
+ and len(node.children) == 2
+ and node.children[0].type == token.LPAR
+ and node.children[1].type == token.RPAR
+ )
+
+
+def is_one_tuple(node: LN) -> bool:
+ """Return True if `node` holds a tuple with one element, with or without parens."""
+ if node.type == syms.atom:
+ if len(node.children) != 3:
+ return False
+
+ lpar, gexp, rpar = node.children
+ if not (
+ lpar.type == token.LPAR
+ and gexp.type == syms.testlist_gexp
+ and rpar.type == token.RPAR
+ ):
+ return False
+
+ return len(gexp.children) == 2 and gexp.children[1].type == token.COMMA
+
+ return (
+ node.type in IMPLICIT_TUPLE
+ and len(node.children) == 2
+ and node.children[1].type == token.COMMA
+ )
+
+
+def is_yield(node: LN) -> bool:
+ """Return True if `node` holds a `yield` or `yield from` expression."""
+ if node.type == syms.yield_expr:
+ return True
+
+ if node.type == token.NAME and node.value == "yield": # type: ignore
+ return True
+
+ if node.type != syms.atom:
+ return False
+
+ if len(node.children) != 3:
+ return False
+
+ lpar, expr, rpar = node.children
+ if lpar.type == token.LPAR and rpar.type == token.RPAR:
+ return is_yield(expr)
+
+ return False