session variables.  What I want to be able to do is have the coders


to get the current value of the session variable and

        setSessionVariable(name, value)

to save the value. The type of the variable should be transparent to the programmer--that is, it could be a number, a string, a dictionary, or a

The two functions you describe are all that you need.

My question restated: Is there a way to write a generic
setSessionVariable()routine that handles the mutated values properly
for arbitrary types.

The simplest implementations are sufficient:

def getSessionVariable(name):
    return context.request.SESSION[name]

def setSessionVariable(name, value):
    context.request.SESSION[name] = value

Where this behaves in an unexpected manner is if someone stores a mutable value in the session, e.g.:

setSessionVariable('a', {})

and *later in the same request* does something like this *without ever again calling setSessionVariable*:

a = getSessionVariable('a')
a['foo'] = 'bar'

The programmer has mutated the dictionary, which is a basic type, and thus doesn't know anything about persistence. Since it's mutated, and its value isn't stored back into the session explicitly, it will eventually disappear when the session data object is "ghosted".

The best (and only) way to prevent this is to instruct your developers to always explicitly re-persist session variables after they mutate them:

a = getSessionVariable('a')
a['foo'] = 'bar'
setSessionVariable('a', a)

There is no sane mechanism to prevent them from needing to do this for mutable basic types.

and mutated from the old value, whatever it may have been.  Or is it
necessary to specifically include an assignment for each mutated
component of the dictionary or list?

It is not necessary to do this for each value in the dictionary or list. Just for the dictionary or list itself.

What happens when the value to be stored is a dictionary of dictionaries
of lists and has been mutated?

The mutations are lost unless you call setSessionVariable on the top- level dictionary.

Is there an advantage to using the persistent Dictionary and List classes
in the session rather than ordinary ones?

They avoid the requirement that you think about this, as they are not basic types and thus know about persistence and can manage this for you.

Thanks again for your insight.

- C

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