From d191ef25c11f2e4d6a8c27f246ca18c6ff05997e Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Sat, 7 Jan 2017 20:44:48 -0800 Subject: [PATCH] Issue #29200: Add test for lru cache only calling __hash__ once --- Lib/test/test_functools.py | 36 ++++++++++++++++++++++++++++++++++++ 1 file changed, 36 insertions(+) diff --git a/Lib/test/test_functools.py b/Lib/test/test_functools.py index 3a40861594..535b35380b 100644 --- a/Lib/test/test_functools.py +++ b/Lib/test/test_functools.py @@ -8,6 +8,7 @@ from random import choice import sys from test import support import unittest +import unittest.mock from weakref import proxy import contextlib try: @@ -1190,6 +1191,41 @@ class TestLRU: self.assertEqual(misses, 4) self.assertEqual(currsize, 2) + def test_lru_hash_only_once(self): + # To protect against weird reentrancy bugs and to improve + # efficiency when faced with slow __hash__ methods, the + # LRU cache guarantees that it will only call __hash__ + # only once per use as an argument to the cached function. + + @self.module.lru_cache(maxsize=1) + def f(x, y): + return x * 3 + y + + # Simulate the integer 5 + mock_int = unittest.mock.Mock() + mock_int.__mul__ = unittest.mock.Mock(return_value=15) + mock_int.__hash__ = unittest.mock.Mock(return_value=999) + + # Add to cache: One use as an argument gives one call + assert f(mock_int, 1) == 16 + assert mock_int.__hash__.call_count == 1 + assert f.cache_info() == (0, 1, 1, 1) + + # Cache hit: One use as an argument gives one additional call + assert f(mock_int, 1) == 16 + assert mock_int.__hash__.call_count == 2 + assert f.cache_info() == (1, 1, 1, 1) + + # Cache eviction: No use as an argument gives no additonal call + assert f(6, 2) == 20 + assert mock_int.__hash__.call_count == 2 + assert f.cache_info() == (1, 2, 1, 1) + + # Cache miss: One use as an argument gives one additional call + assert f(mock_int, 1) == 16 + assert mock_int.__hash__.call_count == 3 + assert f.cache_info() == (1, 3, 1, 1) + def test_lru_reentrancy_with_len(self): # Test to make sure the LRU cache code isn't thrown-off by # caching the built-in len() function. Since len() can be -- 2.40.0