Add (very debug) selector hijacking API

fireplace/dsl/__init__.py

@@ -1,5 +1,6 @@
 from .copy import *
 from .evaluator import *
+from .hijack import *
 from .lazynum import *
 from .random_picker import *
 from .selector import *

fireplace/dsl/hijack.py

@@ -0,0 +1,36 @@
+from contextlib import contextmanager
+from .selector import Selector
+
+
+class HijackedSelector(Selector):
+	def __init__(self, *a, **kw):
+		raise NotImplementedError
+
+	def eval(self, entities, source):
+		return self._hijack_.eval(entities, source)
+
+
+def hijack(victim, replace):
+	if victim.__class__ is not HijackedSelector:
+		victim._truth_ = victim.__class__
+	victim.__class__ = HijackedSelector
+	victim._hijack_ = replace
+
+
+def unhijack(victim):
+	try:
+		victim.__class__ = victim._truth_
+	except AttributeError as e:
+		raise ValueError("not a hijacked selector") from e
+
+
+@contextmanager
+def hijacked(victim, replace):
+	if not isinstance(victim, Selector):
+		raise TypeError("not a selector: %r" % victim)
+	prev = victim.__class__
+	try:
+		hijack(victim, replace)
+		yield
+	finally:
+		victim.__class__ = prev

tests/test_dsl.py

@@ -1,6 +1,8 @@
 #!/usr/bin/env python
+import pytest
 from utils import *
 from fireplace.dsl import *
+from fireplace.exceptions import *
 from fireplace.card import Card
 
 
@@ -189,3 +191,11 @@ def test_positional_selectors():
 	assert len(adjacent) == 2
 	assert adjacent[0] is wisp2
 	assert adjacent[1] is wisp3
+
+
+def test_hijack():
+	game = prepare_game()
+	vial = game.player1.give("LOEA16_8")
+	with hijacked(RANDOM_ENEMY_MINION, FRIENDLY_HERO):
+		with pytest.raises(GameOver):
+			vial.play()