Copying R Environments

I’ve been working on a codebase that relied on storing a lot of objects in R environments, mainly because of the potential speed improvements with large numbers of objects.

See this article for a pretty good explanation.

After a recent spec change, I needed to start looping around a block of code that was previously using a single environment to store objects. The easiest approach was to create an initial base environment to use at the start of each iteration of the loop, and then create a copy of that environment that would be specific to the loop iteration.

But I got a surprise!

All of the result environments looked like the last iteration.

First, let’s look at how this works when you start with a base list, make a copy,  and modify the copy:

> original <- list()
> copy <- original

Because copy hasn’t been modified, it shares the same memory
address as​ original, for efficiency:

tracemem(original)
[1] "<0x4c5d738>"
> tracemem(copy)
[1] "<0x4c5d738>"

So let’s modify copy, and notice that it’s now copied to its own memory address:

> copy$item <- 1L
tracemem[0x4c5d738 -> 0x5a4c1b8]:

And if we now compare original and copy, we get completely expected differences:

> original
list()
> copy
$item
[1] 1

> original$item
NULL
> identical(original, copy)
[1] FALSE

 

Now let’s repeat with environments:

> original <- new.env()
> copy <- original

And if we use tracemem as before, we get a hint that something is different:

> tracemem(original)
Error in tracemem(original) : 
  'tracemem' is not useful for promise and environment objects

So if we simply print the environments, we’ll see the memory addresses:

> original
<environment: 0x69d6cc8>
> copy
<environment: 0x69d6cc8>
> identical(original, copy)
[1] TRUE

Now let’s modify copy, and notice that the memory address doesn’t change:

> copy$item <- 1L
> original
<environment: 0x69d6cc8>
> copy
<environment: 0x69d6cc8>

Presumably that means that original also now has an element named item, right?

> copy$item
[1] 1
> original$item
[1] 1
> identical(original, copy)
[1] TRUE

Right!

So if we remove the element item from original, it should also disappear from copy:

> rm('item', pos = as.environment(original))
> original$item
NULL
> names(original)
character(0)
> copy$item
NULL
> identical(original, copy)
[1] TRUE

So, if you’ve ever copied an existing environment thinking you could modify it independently of the original, you should revisit that code to make sure it’s not causing problems!

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