DEV: Add zipline.utils.memoize.

- Moved zipline.utils.lazyval.
- Added `remember_last` which is just `lru_cache(1)` with simpler logic.

tests/modelling/test_engine.py

@@ -45,7 +45,7 @@ from zipline.modelling.factor.technical import (
     MaxDrawdown,
     SimpleMovingAverage,
 )
-from zipline.utils.lazyval import lazyval
+from zipline.utils.memoize import lazyval
 from zipline.utils.test_utils import (
     make_rotating_asset_info,
     make_simple_asset_info,

tests/test_doctests.py

@@ -9,7 +9,7 @@ from zipline.modelling import (
     expression,
 )
 from zipline.utils import (
-    lazyval,
+    memoize,
     test_utils,
 )
 
@@ -47,8 +47,8 @@ class DoctestTestCase(TestCase):
     def test_engine_docs(self):
         self._check_docs(engine)
 
-    def test_lazyval_docs(self):
-        self._check_docs(lazyval)
+    def test_memoize_docs(self):
+        self._check_docs(memoize)
 
     def test_test_utils_docs(self):
         self._check_docs(test_utils)

tests/test_memoize.py

@@ -0,0 +1,34 @@
+"""
+Tests for zipline.utils.memoize.
+"""
+from unittest import TestCase
+
+from zipline.utils.memoize import remember_last
+
+
+class TestRememberLast(TestCase):
+
+    def test_remember_last(self):
+
+        # Store the count in a list so we can mutate it from inside `func`.
+        call_count = [0]
+
+        @remember_last
+        def func(x):
+            call_count[0] += 1
+            return x
+
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+
+        # Calling again with the same argument should just re-use the old
+        # value, which means func shouldn't get called again.
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+        self.assertEqual((func(1), call_count[0]), (1, 1))
+
+        # Calling with a new value should increment the counter.
+        self.assertEqual((func(2), call_count[0]), (2, 2))
+        self.assertEqual((func(2), call_count[0]), (2, 2))
+
+        # Calling the old value should still increment the counter.
+        self.assertEqual((func(1), call_count[0]), (1, 3))
+        self.assertEqual((func(1), call_count[0]), (1, 3))

zipline/modelling/term.py

@@ -15,7 +15,7 @@ from zipline.errors import (
     WindowLengthNotPositive,
     WindowLengthNotSpecified,
 )
-from zipline.utils.lazyval import lazyval
+from zipline.utils.memoize import lazyval
 
 
 NotSpecified = (object(),)

zipline/utils/memoize.py

@@ -0,0 +1,101 @@
+"""
+Tools for memoization of function results.
+"""
+from functools import wraps
+from six import iteritems
+from weakref import WeakKeyDictionary
+
+
+class lazyval(object):
+    """
+    Decorator that marks that an attribute should not be computed until
+    needed, and that the value should be memoized.
+
+    Example
+    -------
+
+    >>> from zipline.utils.memoize import lazyval
+    >>> class C(object):
+    ...     def __init__(self):
+    ...         self.count = 0
+    ...     @lazyval
+    ...     def val(self):
+    ...         self.count += 1
+    ...         return "val"
+    ...
+    >>> c = C()
+    >>> c.count
+    0
+    >>> c.val, c.count
+    ('val', 1)
+    >>> c.val, c.count
+    ('val', 1)
+    """
+    def __init__(self, get):
+        self._get = get
+        self._cache = WeakKeyDictionary()
+
+    def __get__(self, instance, owner):
+        if instance is None:
+            return self
+        try:
+            return self._cache[instance]
+        except KeyError:
+            self._cache[instance] = val = self._get(instance)
+            return val
+
+
+def remember_last(f):
+    """
+    Decorator that remembers the last computed value of a function and doesn't
+    recompute it when called with the same inputs multiple times.
+
+    Parameters
+    ----------
+    f : The function to be memoized. All arguments to f should be hashable.
+
+    Example
+    -------
+    >>> counter = 0
+    >>> @remember_last
+    ... def foo(x):
+    ...     global counter
+    ...     counter += 1
+    ...     return x, counter
+    >>> foo(1)
+    (1, 1)
+    >>> foo(1)
+    (1, 1)
+    >>> foo(0)
+    (0, 2)
+    >>> foo(1)
+    (1, 3)
+
+    Notes
+    -----
+    This decorator is equivalent to `lru_cache(1)` in Python 3, but with less
+    bells and whistles for handling things like threadsafety.  If we ever
+    decide we need such bells and whistles, we should just make functools32 a
+    dependency.
+    """
+    # This needs to be a mutable data structure so we can change it from inside
+    # the function.  In pure Python 3, we'd use the nonlocal keyword for this.
+    _previous = [None, None]
+    KEY, VALUE = 0, 1
+
+    _kwd_mark = object()
+
+    @wraps(f)
+    def memoized_f(*args, **kwds):
+        # Hashing logic taken from functools32.lru_cache.
+        key = args
+        if kwds:
+            key += _kwd_mark + tuple(sorted(iteritems(kwds)))
+
+        key_hash = hash(key)
+        if key_hash != _previous[KEY]:
+            _previous[VALUE] = f(*args, **kwds)
+            _previous[KEY] = key_hash
+        return _previous[VALUE]
+
+    return memoized_f