Encode disallowed characters around already percent-encoded triples

Reserved ("+") and fragment ("#") expansion must preserve already valid
percent-encoded triples (RFC 6570 Section 3.2.3). The current implementation
does this by returning the value untouched as soon as `urllib.parse.unquote`
detects any triple, which also skips encoding every other disallowed
character in the value.

As a result `URITemplate("{+v}").expand(v="a b%20c")` produced "a b%20c"
(space left raw) instead of "a%20b%20c". This is a regression from the fix
for issue #99, which addressed the opposite problem of double-encoding.

Quote the value segment by segment instead: pass valid percent-encoded
triples through verbatim and percent-encode everything between them. Output
is unchanged for values with no triples or with only triples, so existing
behavior (and all current tests) is preserved.

Author: gaoflow

HISTORY.rst

@@ -1,6 +1,15 @@
 Changelog - uritemplate
 =======================
 
+Unreleased
+----------
+
+- Fix bug where reserved (``+``) and fragment (``#``) expansion left
+  disallowed characters (such as spaces) unencoded whenever the value also
+  contained an already percent-encoded triple. This was a regression from the
+  fix for
+  `issue #99 <https://github.com/python-hyper/uritemplate/issues/99>`_.
+
 4.2.0 - 2025-06-01
 ------------------
 

tests/test_uritemplate.py

@@ -589,6 +589,20 @@ def test_no_mutate(self) -> None:
         t.expand(args, key=1)
         self.assertEqual(args, {})
 
+    def test_reserved_expansion_encodes_around_pct_triples(self) -> None:
+        # A value mixing disallowed characters with an already valid
+        # percent-encoded triple must still encode the disallowed characters
+        # (the triple is preserved, the rest is quoted) -- RFC 6570 3.2.3.
+        self.assertEqual(URITemplate("{+v}").expand(v="a b%20c"), "a%20b%20c")
+        self.assertEqual(
+            URITemplate("{#v}").expand(v="x y%20z"), "#x%20y%20z"
+        )
+        # Uppercase triple preserved; reserved characters left untouched.
+        self.assertEqual(URITemplate("{+v}").expand(v="a%2Fb c"), "a%2Fb%20c")
+        # No triple present -> ordinary quoting; all triples -> left as-is.
+        self.assertEqual(URITemplate("{+v}").expand(v="a b c"), "a%20b%20c")
+        self.assertEqual(URITemplate("{+v}").expand(v="%20"), "%20")
+
 
 class TestVariableModule(unittest.TestCase):
     def test_is_list_of_tuples(self) -> None:

uritemplate/variable.py

@@ -17,6 +17,7 @@
 
 import collections.abc
 import enum
+import re
 import string
 import typing as t
 import urllib.parse
@@ -38,6 +39,7 @@
 _GEN_DELIMS: t.Final[str] = ":/?#[]@"
 _SUB_DELIMS: t.Final[str] = "!$&'()*+,;="
 _RESERVED_CHARACTERS: t.Final[str] = f"{_GEN_DELIMS}{_SUB_DELIMS}"
+_PERCENT_ENCODED: t.Final["re.Pattern[str]"] = re.compile("%[0-9A-Fa-f]{2}")
 
 
 class Operator(enum.Enum):
@@ -150,9 +152,21 @@ def _always_quote(self, value: str) -> str:
         return quote(value, "")
 
     def _only_quote_unquoted_characters(self, value: str) -> str:
-        if urllib.parse.unquote(value) == value:
-            return quote(value, _RESERVED_CHARACTERS)
-        return value
+        # For reserved ("+") and fragment ("#") expansion, already
+        # percent-encoded triples must be preserved (RFC 6570 Section 3.2.3).
+        # Quote every other disallowed character while leaving valid triples
+        # untouched, rather than passing the whole value through unquoted as
+        # soon as a single triple is present.
+        result = []
+        last = 0
+        for match in _PERCENT_ENCODED.finditer(value):
+            result.append(
+                quote(value[last : match.start()], _RESERVED_CHARACTERS)
+            )
+            result.append(match.group())
+            last = match.end()
+        result.append(quote(value[last:], _RESERVED_CHARACTERS))
+        return "".join(result)
 
     def quote(self, value: t.Any) -> str:
         if not isinstance(value, (str, bytes)):