Cached values may get changed for Memory backend

Migrated issue, originally created by Yi (qinsoon)

For example,

from dogpile.cache import make_region

region = make_region().configure('dogpile.cache.memory')

@region.cache_on_arguments()
def foo():
	return [1, 2, 3]

def bar():
	a = foo()
	a += [4]
	return a

print bar()
print bar()

The first call to bar() returns [1, 2, 3, 4] as expected. However, the [4] gets appended to the cached value (of type list) in the dictionary for the cache. And the next call to bar() returns [1, 2, 3, 4, 4], which seems unexpected.

This would happen if the cached value is an object, so that the assignment in bar() is assignment by reference.

Though this can be fixed by the user to clone the value, I think the memory backend may return a clone of the cached value instead of directly returning the value, which gives a chance that the cached value may get modified by the user. Also, from the perspective of abstraction, the user should not see a cached value being changed.

[sqlalchemy-bot]

Michael Bayer (zzzeek) wrote:

however, adding a “clone” argument to the backend where the user can just provide a cloning function for the kinds of objects they want to store, that would allow for a memory backend that’s a lot faster than the pickle version. can make it emit a warning if no clone function is given. folks who don’t want to clone can specify it as a no-op so at least they know what they’re getting into.

[zzzeek]

this option can be handled more effectively than the “pickle” version using the new custom serializer argument coming in #191