Prevent random filter from being folded

CHANGES

@@ -22,6 +22,10 @@ Version 2.10
 - Added a `trimmed` modifier to `{% trans %}` to strip linebreaks and
   surrounding whitespace. Also added a new policy to enable this for all
   `trans` blocks.
+- The ``random`` filter is no longer incorrectly constant folded and will
+  produce a new random choice each time the template is rendered. (`#478`_)
+
+.. _#478: https://github.com/pallets/jinja/pull/478
 
 Version 2.9.6
 -------------

jinja2/filters.py

@@ -10,8 +10,8 @@
 """
 import re
 import math
+import random
 
-from random import choice
 from itertools import groupby
 from collections import namedtuple
 from jinja2.utils import Markup, escape, pformat, urlize, soft_unicode, \
@@ -359,13 +359,13 @@ def do_last(environment, seq):
         return environment.undefined('No last item, sequence was empty.')
 
 
-@environmentfilter
-def do_random(environment, seq):
+@contextfilter
+def do_random(context, seq):
     """Return a random item from the sequence."""
     try:
-        return choice(seq)
+        return random.choice(seq)
     except IndexError:
-        return environment.undefined('No random item, sequence was empty.')
+        return context.environment.undefined('No random item, sequence was empty.')
 
 
 def do_filesizeformat(value, binary=False):

tests/test_filters.py

@@ -8,6 +8,7 @@
     :copyright: (c) 2017 by the Jinja Team.
     :license: BSD, see LICENSE for more details.
 """
+import random
 import pytest
 from jinja2 import Markup, Environment
 from jinja2._compat import text_type, implements_to_string
@@ -182,11 +183,21 @@ def test_pprint(self, env):
         data = list(range(1000))
         assert tmpl.render(data=data) == pformat(data)
 
-    def test_random(self, env):
-        tmpl = env.from_string('''{{ seq|random }}''')
-        seq = list(range(100))
-        for _ in range(10):
-            assert int(tmpl.render(seq=seq)) in seq
+    def test_random(self, env, request):
+        # restore the random state when the test ends
+        state = random.getstate()
+        request.addfinalizer(lambda: random.setstate(state))
+        # generate the random values from a known seed
+        random.seed('jinja')
+        expected = [random.choice('1234567890') for _ in range(10)]
+
+        # check that the random sequence is generated again by a template
+        # ensures that filter result is not constant folded
+        random.seed('jinja')
+        t = env.from_string('{{ "1234567890"|random }}')
+
+        for value in expected:
+            assert t.render() == value
 
     def test_reverse(self, env):
         tmpl = env.from_string('{{ "foobar"|reverse|join }}|'