Trailing-based state restoration
Hi, I got a proposal for a new feature in Python in order to perform hypothetical reasoning in a very simple way. Hypothetical reasoning is mainly used in problem solving search algorithms. In such a process, variables are temporally assigned and later restored to their preceding value. With the assumption that only a small fraction of the variables changes at the same time, it is more efficient to store undo information instead of copying the entire state. The basic data structure to handle these moves efficiently is called a trail which is mainly a stack with markers. Potentially this mechanism can be provided to any object type of Python. Only few lines are needed to implement the mechanism using the reflexive power of Python. A deeper implementation could hide the process in the internal data representation in order to offer this new feature in a seamless way. This new feature is activated with the specification of thee primitives: - push() - back() - assign(object, attribute, value) push : declare a new hypothetical state. back : restore the previous state. assign(object, attribute, value) : assign the value to the attribute of the object in the current state. In addition a primitive can be provided to go back to a specific state: backtrack(n): restore the state number n. EXAMPLES class Var: def __init__(self, value): self.value = value def __str__(self): return"{}".format(self.value) def test1(): V1 = Var(10) assert V1.value == 10 push() assign(V1, 'value', 100) assert V1.value == 100 back() assert V1.value == 10 def test2(): V1 = Var(0) for i in range(8): push() for j in range(5): assign(V1, 'value',(j+1)+i*5-1) assert V1.value == 39 assert current() == 8 backtrack(6) assert V1.value == 29 assert current() == 6 backtrack(4) assert V1.value == 19 assert current() == 4 backtrack(0) assert V1.value == 0 assert current() == 0 The assign procedure is used in place of the standard assignment denoted by the "=" operator. In a seamless integration the "=" assignment operator could be overridden when the variable has been previously declared as "recordable" (require a declaration primitive or a type declaration). Is anybody interested by this new feature? Pierre Savéant
On Dec 17, 2015, at 09:12, SAVEANT Pierre <pierre.saveant@thalesgroup.com> wrote:
It seems like all of that can be implemented in Python today (presumably storing the set of trailed Vars as a global or as a class attribute of Var, since there doesn't seem to be any scoping involved). So, why not just build it and put it on PyPI?
In a seamless integration the "=" assignment operator could be overridden when the variable has been previously declared as "recordable" (require a declaration primitive or a type declaration).
That's one thing that couldn't be implemented in Python today, because = can't be overridden (it's a binding-and-possibly-declaring statement, not an assignment operator, in Python). But if you just made all these things attributes on some object, then you could hook its __setattr__. Plus, you could have different sets of trails in parallel, and implement whatever kind of scoping you want instead of everything being sort of global but sort of Python-scoped, and so on.
On Dec 17, 2015, at 09:50, Andrew Barnert via Python-ideas <python-ideas@python.org> wrote:
Actually, reading things a bit more carefully, that's just as doable. Just define Var.__setattr__ to call assign and you're done. Now "v1.value = 10" gets trailed. (I'm a bit confused about why the "value" attribute is constructed at __init__ time from the constructor argument, but assign lets you assign any attribute you want even though you only "value". If you only want "value" to be trailed, skip the __setattr__ and make it a @property.)
On Dec 17, 2015, at 09:12, SAVEANT Pierre <pierre.saveant@thalesgroup.com> wrote:
It seems like all of that can be implemented in Python today (presumably storing the set of trailed Vars as a global or as a class attribute of Var, since there doesn't seem to be any scoping involved). So, why not just build it and put it on PyPI?
In a seamless integration the "=" assignment operator could be overridden when the variable has been previously declared as "recordable" (require a declaration primitive or a type declaration).
That's one thing that couldn't be implemented in Python today, because = can't be overridden (it's a binding-and-possibly-declaring statement, not an assignment operator, in Python). But if you just made all these things attributes on some object, then you could hook its __setattr__. Plus, you could have different sets of trails in parallel, and implement whatever kind of scoping you want instead of everything being sort of global but sort of Python-scoped, and so on.
On Dec 17, 2015, at 09:50, Andrew Barnert via Python-ideas <python-ideas@python.org> wrote:
Actually, reading things a bit more carefully, that's just as doable. Just define Var.__setattr__ to call assign and you're done. Now "v1.value = 10" gets trailed. (I'm a bit confused about why the "value" attribute is constructed at __init__ time from the constructor argument, but assign lets you assign any attribute you want even though you only "value". If you only want "value" to be trailed, skip the __setattr__ and make it a @property.)
participants (2)
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Andrew Barnert -
SAVEANT Pierre