Give UNSET its own type with a nice repr to make help() output nicer to read

Author: michaelosthege

pymc3/tests/test_util.py

@@ -20,7 +20,7 @@
 import pymc3 as pm
 
 from pymc3.distributions.transforms import Transform
-from pymc3.util import hash_key, hashable, locally_cachedmethod
+from pymc3.util import UNSET, hash_key, hashable, locally_cachedmethod
 
 
 class TestTransformName:
@@ -127,3 +127,12 @@ def some_method(self, x):
 
     tc = TestClass()
     assert tc.some_method(b1) != tc.some_method(b2)
+
+
+def test_unset_repr(capsys):
+    def fn(a=UNSET):
+        return
+
+    help(fn)
+    captured = capsys.readouterr()
+    assert "a=UNSET" in captured.out

pymc3/util.py

@@ -24,7 +24,18 @@
 
 from cachetools import LRUCache, cachedmethod
 
-UNSET = object()
+
+class _UnsetType:
+    """Type for the `UNSET` object to make it look nice in `help(...)` outputs."""
+
+    def __str__(self):
+        return "UNSET"
+
+    def __repr__(self):
+        return str(self)
+
+
+UNSET = _UnsetType()
 
 
 def withparent(meth):