bpo-35300: Add usage note to the lru_cache() docs (GH-10707)

https://bugs.python.org/issue35300

diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
index 40abdc24883c978c9dfc928e0bd368961f3514cd..cd59e5bebfd565ac034f1d300cdd3a144db1194e 100644 (file)
--- a/Doc/library/functools.rst
+++ b/Doc/library/functools.rst
@@ -118,6 +118,11 @@ The :mod:`functools` module defines the following functions:
    The cache's size limit assures that the cache does not grow without bound on
    long-running processes such as web servers.
 
+   In general, the LRU cache should only be used when you want to reuse
+   previously computed values.  Accordingly, it doesn't make sense to cache
+   functions with side-effects, functions that need to create distinct mutable
+   objects on each call, or impure functions such as time() or random().
+
    Example of an LRU cache for static web content::
 
         @lru_cache(maxsize=32)