* Cache condition in helpers.Script
The caching is a simple in-memory, per-instance dictionary.
We use __str__ to format cache keys.
This naive implementation has some disadvantages (e.g., we won't be able
to cache two conditions that contain references to
distinct-but-equivalent object instances and we don't have any control
over the size of the condition cache), but for most simple use-cases the
approach should be good enough.
Resolves#3629
* Fix docstring style
711526e574