Add functools.cached_property backport (#5031)

The [`@functools.cached_property`](https://docs.python.org/3/library/functools.html#functools.cached_property) decorator is really useful but not currently available in cirq since it was introduced in python 3.8 but we still support python 3.7. This adds `backports.cached_property` to make cached properties available in python 3.7, using import logic in `cirq._compat` to get it from the standard library or backport module, as needed.

cirq-core/cirq/_compat.py

@@ -31,6 +31,12 @@
 import sympy.printing.repr
 
 
+try:
+    from functools import cached_property  # pylint: disable=unused-import
+except ImportError:
+    from backports.cached_property import cached_property  # type: ignore[no-redef]
+
+
 def proper_repr(value: Any) -> str:
     """Overrides sympy and numpy returning repr strings that don't parse."""
 

cirq-core/cirq/_compat_test.py

@@ -35,6 +35,7 @@
 import cirq.testing
 from cirq._compat import (
     block_overlapping_deprecation,
+    cached_property,
     proper_repr,
     dataclass_repr,
     deprecated,
@@ -967,3 +968,20 @@ def f(x):
 
     with cirq.testing.assert_deprecated('f', deadline='v1000.0', count=1):
         f(5)
+
+
+def test_cached_property():
+    class Foo:
+        def __init__(self):
+            self.bar_calls = 0
+
+        @cached_property
+        def bar(self):
+            self.bar_calls += 1
+            return []
+
+    foo = Foo()
+    bar = foo.bar
+    bar2 = foo.bar
+    assert bar2 is bar
+    assert foo.bar_calls == 1

cirq-core/requirements.txt

@@ -3,6 +3,9 @@
 # for python 3.6 and below dataclasses needs to be installed
 dataclasses; python_version < '3.7'
 
+# functools.cached_property was introduced in python 3.8
+backports.cached_property~=1.0.1; python_version < '3.8'
+
 duet~=0.2.0
 matplotlib~=3.0
 networkx~=2.4