Revision on fstring issues (#100)

* f-string expression part cannot include a backslash
 * failing example `f"{'\n'}"` for tests

parso/python/errors.py

@@ -863,6 +863,7 @@ class _TryStmtRule(SyntaxRule):
 @ErrorFinder.register_rule(type='fstring')
 class _FStringRule(SyntaxRule):
     _fstring_grammar = None
+    message_expr = "f-string expression part cannot include a backslash"
     message_nested = "f-string: expressions nested too deeply"
     message_conversion = "f-string: invalid conversion character: expected 's', 'r', or 'a'"
 
@@ -873,6 +874,10 @@ class _FStringRule(SyntaxRule):
         if depth >= 2:
             self.add_issue(fstring_expr, message=self.message_nested)
 
+        expr = fstring_expr.children[1]
+        if '\\' in expr.get_code():
+            self.add_issue(expr, message=self.message_expr)
+
         conversion = fstring_expr.children[2]
         if conversion.type == 'fstring_conversion':
             name = conversion.children[1]

test/failing_examples.py

@@ -281,6 +281,9 @@ if sys.version_info >= (3, 6):
         # Same as above, but for f-strings.
         'f"s" b""',
         'b"s" f""',
+
+        # f-string expression part cannot include a backslash
+        r'''f"{'\n'}"''',
     ]
 FAILING_EXAMPLES.append('[a, 1] += 3')