Check f-string arguments for quotes/backslashes instead of _simple_arg

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

pyupgrade/_main.py

@@ -510,18 +510,6 @@ def _fix_tokens(contents_text: str, min_version: Version) -> str:
     return tokens_to_src(tokens).lstrip()
 
 
-def _simple_arg(arg: ast.expr) -> bool:
-    return (
-        isinstance(arg, ast.Name) or
-        (isinstance(arg, ast.Attribute) and _simple_arg(arg.value)) or
-        (
-            isinstance(arg, ast.Call) and
-            _simple_arg(arg.func) and
-            not arg.args and not arg.keywords
-        )
-    )
-
-
 def _format_params(call: ast.Call) -> Set[str]:
     params = {str(i) for i, arg in enumerate(call.args)}
     for kwd in call.keywords:
@@ -566,8 +554,6 @@ def _parse(self, node: ast.Call) -> Optional[Tuple[DotFormatPart, ...]]:
                 isinstance(node.func, ast.Attribute) and
                 isinstance(node.func.value, ast.Str) and
                 node.func.attr == 'format' and
-                all(_simple_arg(arg) for arg in node.args) and
-                all(_simple_arg(k.value) for k in node.keywords) and
                 not has_starargs(node)
         ):
             return None
@@ -794,6 +780,10 @@ def _fix_py36_plus(contents_text: str) -> str:
             if tokens[end - 1].line != token.line:
                 continue
 
+            args_src = tokens_to_src(tokens[paren:end])
+            if '\\' in args_src or '"' in args_src or "'" in args_src:
+                continue
+
             tokens[i] = token._replace(
                 src=_to_fstring(token.src, tokens, args),
             )

tests/features/fstrings_test.py

@@ -30,6 +30,10 @@
         r'"\N{snowman} {}".format(a)',
         # not enough placeholders / placeholders missing
         '"{}{}".format(a)', '"{a}{b}".format(a=a)',
+        # backslashes and quotes cannot nest
+        r'''"{}".format(a['\\'])''',
+        '"{}".format(a["b"])',
+        "'{}'.format(a['b'])",
     ),
 )
 def test_fix_fstrings_noop(s):
@@ -50,6 +54,7 @@ def test_fix_fstrings_noop(s):
         ('"hello {}!".format(name)', 'f"hello {name}!"'),
         ('"{}{{}}{}".format(escaped, y)', 'f"{escaped}{{}}{y}"'),
         ('"{}{b}{}".format(a, c, b=b)', 'f"{a}{b}{c}"'),
+        ('"{}".format(0x0)', 'f"{0x0}"'),
         # TODO: poor man's f-strings?
         # '"{foo}".format(**locals())'
     ),