Choose the shorter quote style for string literals

src/python_minifier/token_printer.py

@@ -4,6 +4,31 @@
 import sys
 
 
+def _shortest_repr(value):
+    """
+    The shortest repr of a str, preferring single quotes on a tie.
+
+    repr() only switches to double quotes when the string contains a single
+    quote and no double quote, so a string containing more single quotes than
+    double quotes is rendered with every single quote escaped. Try the double
+    quoted form too and keep whichever is shorter.
+    """
+
+    s = repr(value)
+
+    if "'" not in value or '"' not in value:
+        return s
+
+    prefix = s[:s.index("'")]
+    body = s[len(prefix) + 1:-1]
+    double_quoted = prefix + '"' + body.replace("\\'", "'").replace('"', '\\"') + '"'
+
+    if len(double_quoted) < len(s):
+        return double_quoted
+
+    return s
+
+
 class TokenTypes(object):
     NoToken = 0
     Identifier = 1
@@ -145,7 +170,7 @@ def keyword(self, kw):
 
     def stringliteral(self, value):
         """Add a string literal to the output code."""
-        s = repr(value)
+        s = _shortest_repr(value)
 
         if sys.version_info < (3, 0) and self.unicode_literals:
             if s[0] == 'u':

test/test_string_quotes.py

@@ -0,0 +1,22 @@
+"""Tests for the quote style chosen for string literals."""
+
+from python_minifier import minify
+
+
+def test_no_quotes_uses_single_quotes():
+    assert minify('print("")') == "print('')"
+
+
+def test_single_quote_in_string_uses_double_quotes():
+    assert minify('print(\'\\\'\')') == 'print("\'")'
+
+
+def test_tie_prefers_single_quotes():
+    assert minify('print("\'\\"")') == 'print(\'\\\'"\')'
+
+
+def test_more_single_quotes_than_double_uses_double_quotes():
+    # A string with more single quotes than double quotes must not be
+    # rendered with every single quote escaped, which made the output
+    # longer than the input.
+    assert minify('print("\'\\"\'")') == 'print("\'\\"\'")'