diff --git a/pycodestyle.py b/pycodestyle.py
index 9df2ae4c..b86f9875 100755
--- a/pycodestyle.py
+++ b/pycodestyle.py
@@ -1627,6 +1627,29 @@ def ambiguous_identifier(logical_line, tokens):
         prev_start = start
 
 
+# https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
+_PYTHON_3000_VALID_ESC = frozenset([
+    '\n',
+    '\\',
+    '\'',
+    '"',
+    'a',
+    'b',
+    'f',
+    'n',
+    'r',
+    't',
+    'v',
+    '0', '1', '2', '3', '4', '5', '6', '7',
+    'x',
+
+    # Escape sequences only recognized in string literals
+    'N',
+    'u',
+    'U',
+])
+
+
 @register_check
 def python_3000_invalid_escape_sequence(logical_line, tokens, noqa):
     r"""Invalid escape sequences are deprecated in Python 3.6.
@@ -1637,41 +1660,27 @@ def python_3000_invalid_escape_sequence(logical_line, tokens, noqa):
     if noqa:
         return
 
-    # https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals
-    valid = [
-        '\n',
-        '\\',
-        '\'',
-        '"',
-        'a',
-        'b',
-        'f',
-        'n',
-        'r',
-        't',
-        'v',
-        '0', '1', '2', '3', '4', '5', '6', '7',
-        'x',
-
-        # Escape sequences only recognized in string literals
-        'N',
-        'u',
-        'U',
-    ]
-
     prefixes = []
     for token_type, text, start, _, _ in tokens:
-        if token_type in {tokenize.STRING, FSTRING_START, TSTRING_START}:
+        if (
+                token_type == tokenize.STRING or
+                token_type == FSTRING_START or
+                token_type == TSTRING_START
+        ):
             # Extract string modifiers (e.g. u or r)
             prefixes.append(text[:text.index(text[-1])].lower())
 
-        if token_type in {tokenize.STRING, FSTRING_MIDDLE, TSTRING_MIDDLE}:
+        if (
+                token_type == tokenize.STRING or
+                token_type == FSTRING_MIDDLE or
+                token_type == TSTRING_MIDDLE
+        ):
             if 'r' not in prefixes[-1]:
                 start_line, start_col = start
                 pos = text.find('\\')
                 while pos >= 0:
                     pos += 1
-                    if text[pos] not in valid:
+                    if text[pos] not in _PYTHON_3000_VALID_ESC:
                         line = start_line + text.count('\n', 0, pos)
                         if line == start_line:
                             col = start_col + pos
@@ -1683,7 +1692,11 @@ def python_3000_invalid_escape_sequence(logical_line, tokens, noqa):
                         )
                     pos = text.find('\\', pos + 1)
 
-        if token_type in {tokenize.STRING, FSTRING_END, TSTRING_END}:
+        if (
+                token_type == tokenize.STRING or
+                token_type == FSTRING_END or
+                token_type == TSTRING_END
+        ):
             prefixes.pop()