6 Restrictions on arguments to cacheable components

If you want to call a component with cache=yes (or cache=defer or cache=force), there is one restriction you must obey:

The attributes passed to the cached component must be, as Python says, hashable.

The reason for this restriction is that the SkunkWeb server needs to take all of the attributes you pass and flatten them into a short, unique value. The SkunkWeb server uses this value to look up the cached output it has stored in its disk cache.

So, what does hashable mean? Well, just consider it to mean ``able to be flattened'' until you learn more Python. Here's a rule of thumb:

Numbers, strings, dates, and the None object are hashable
Tuples (read-only sequences) are hashable if all of its items are hashable
Lists (changeable sequences) are not hashable, but SkunkWeb makes an exception and lets you use them as arguments to cacheable components, as long as the list's items are either hashable or are lists or dictionaries. However, you must be very careful that the called component does not try to change the list in any way; if it does, you will find that the list changes only the first time the component is called, becuase on subequent calls, the output is cached and the component will not be executed.
Dictionaries are not hashable, but SkunkWeb makes an exception for them just as it does for lists, with the same cautions and restrictions.
Any combination of acceptable objects is acceptable: a tuple of lists of dates, etc.
DateTime objects are not natively hashable, but SkunkWeb has a provision in the cache key generation code to make it as if they were hashable.
Instances of Python classes which you write yourself are not acceptable, unless you implement a __hash__ or __cachekey__ method in the class. Consult the Skunk mailing list if you want to make instances of your own classes acceptable arguments to cacheable components.