--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/planemo/lib/python3.7/site-packages/galaxy/util/expressions.py	Fri Jul 31 00:32:28 2020 -0400
@@ -0,0 +1,57 @@
+"""
+Expression evaluation support.
+
+For the moment this depends on python's eval. In the future it should be
+replaced with a "safe" parser.
+"""
+
+from collections import MutableMapping
+from itertools import chain
+
+
+class ExpressionContext(MutableMapping):
+    def __init__(self, dict, parent=None):
+        """
+        Create a new expression context that looks for values in the
+        container object 'dict', and falls back to 'parent'
+        """
+        self.dict = dict
+        self.parent = parent
+
+    def __delitem__(self, key):
+        if key in self.dict:
+            del self.dict[key]
+        elif self.parent is not None and key in self.parent:
+            del self.parent[key]
+
+    def __iter__(self):
+        return chain(iter(self.dict), iter(self.parent or []))
+
+    def __len__(self):
+        return len(self.dict) + len(self.parent or [])
+
+    def __getitem__(self, key):
+        if key in self.dict:
+            return self.dict[key]
+        if self.parent is not None and key in self.parent:
+            return self.parent[key]
+        raise KeyError(key)
+
+    def __setitem__(self, key, value):
+        self.dict[key] = value
+
+    def __contains__(self, key):
+        if key in self.dict:
+            return True
+        if self.parent is not None and key in self.parent:
+            return True
+        return False
+
+    def __str__(self):
+        return str(self.dict)
+
+    def __bool__(self):
+        if not self.dict and not self.parent:
+            return False
+        return True
+    __nonzero__ = __bool__