Add a bstring Parser to match bytestrings.

src/parsy/__init__.py

@@ -545,6 +545,27 @@ def string_parser(stream, index):
  return string_parser
 
 
+def bstring(expected_bytes: bytes, transform: Callable[[bytes], bytes] = noop) -> Parser:
+ """Returns a parser that expects the ``expected_bytes`` and produces
+ that bytestring value.
+
+ Optionally, a transform function can be passed, which will be used on both
+ the expected bytestring and tested bytestring.
+ """
+
+ blen = len(expected_bytes)
+ transformed_b = transform(expected_bytes)
+
+ @Parser
+ def bytes_parser(stream, index):
+ if transform(stream[index : index + blen]) == transformed_b:
+ return Result.success(index + blen, transformed_b)
+ else:
+ return Result.failure(index, expected_bytes)
+
+ return bytes_parser
+
+
 def regex(exp: str, flags=0, group: int | str | tuple = 0) -> Parser:
  """
  Returns a parser that expects the given ``exp``, and produces the

tests/test_parsy.py

@@ -9,6 +9,7 @@
  ParseError,
  alt,
  any_char,
+ bstring,
  char_from,
  decimal_digit,
  digit,
@@ -53,6 +54,19 @@ def test_string_transform_2(self):
 
  self.assertRaises(ParseError, parser.parse, "dog")
 
+ def test_bstring(self):
+ parser = bstring(b"x")
+ self.assertEqual(parser.parse(b"x"), b"x")
+
+ self.assertRaises(ParseError, parser.parse, b"y")
+
+ def test_bstring_transform(self):
+ parser = bstring(b"x", transform=lambda s: s.lower())
+ self.assertEqual(parser.parse(b"x"), b"x")
+ self.assertEqual(parser.parse(b"X"), b"x")
+
+ self.assertRaises(ParseError, parser.parse, b"y")
+
  def test_regex_str(self):
  parser = regex(r"[0-9]")