fix handling of empty triple quoted strings (#314)

README.md

@@ -813,6 +813,7 @@ More details can be found in [CONTRIBUTING](CONTRIBUTING.md).
 
 * fixed improper unmodified file caching when `-S` was used
 
+* fixed formatting of empty triple quoted strings (#313)
 
 ### 18.6b1
 

black.py

@@ -2546,7 +2546,7 @@ def normalize_string_quotes(leaf: Leaf) -> None:
             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 new_quote == '"""' and new_body[-1] == '"':
+    if new_quote == '"""' and new_body[-1:] == '"':
         # edge case:
         new_body = new_body[:-1] + '\\"'
     orig_escape_count = body.count("\\")

tests/data/string_quotes.py

@@ -1,3 +1,4 @@
+''''''
 '\''
 '"'
 "'"
@@ -45,6 +46,7 @@
 
 # output
 
+""""""
 "'"
 '"'
 "'"