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Ralsina.Me — Roberto Alsina's website

Garbage Collection Has Side Effects

Just a quick fol­lowup to The prob­lem is is, is it not? This is not mine, I got it from red­dit

This should re­al­ly not sur­prise you:

>>> a = [1,2]
>>> b = [3,4]
>>> a is b
>>> a == b
>>> id(a) == id(b)

Af­ter al­l, a and b are com­plete­ly dif­fer­ent things. How­ev­er:

>>> [1,2] is [3,4]
>>> [1,2] == [3,4]
>>> id([1,2]) == id([3,4])

Turns out that us­ing lit­er­al­s, one of those things is not like the oth­er­s.

First, the ex­pla­na­tion so you un­der­stand why this hap­pen­s. When you don't have any more ref­er­ences to a piece of data, it will get garbage col­lect­ed, the mem­o­ry will be freed, so it can be reused for oth­er things.

In the first case, I am keeping references to both lists in the variables a and b. That means the lists have to exist at all times, since I can always say print a and python has to know what's in it.

In the second case, I am using literals, which means there is no reference to the lists after they are used. When python evaluates id([1,2]) == id([3,4]) it first evaluates the left side of the ==. After that is done, there is no need to keep [1,2] available, so it's deleted. Then, when evaluating the right side, it creates [3,4].

By pure chance, it will use the exact same place for it as it was using for [1,2]. So id will return the same value. This is just to remind you of a couple of things:

  1. a is b is usu­al­ly (but not al­ways) the same as id(a) == id(b)

  2. garbage col­lec­­tion can cause side ef­­fects you may not be ex­pec­t­ing


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