Static memory in python: do loops create new instances of variables in memory?

Question

I've been running Python scripts that make several calls to some functions, say F1(x) and F2(x), that look a bit like this:

x = LoadData()

for j in range(N):
    y = F1(x[j])
    z[j] = F2(y)

    del y

SaveData(z)

Performance is a lot faster if I keep the "del y" line. But I don't understand why this is true. If I don't use "del y", then I quickly run out of RAM and have to resort to virtual memory, and everything slows to a crawl. Buy if I use "del y", then I am repeatedly flushing and re-allocating the memory for y. What I would like to do is have y sit as static memory, and reuse the memory on every F1(x) call. But from what I can tell, that isn't what's happening.

Also, not sure if it's relevant, but my data consists of numpy arrays.

Answer 1

Without the del y you might need twice as much memory. This is because for each pass through the loop, y is bound to the previous value of F1 while the next one is calculated.

once F1 returns y is rebound to that new value and the old F1 result can be released.

This would mean that the object returned by F1 occupies quite a lot of memory

Unrolling the loop for the first couple of iterations would look like this

y = F1(x[0])   # F1(x[0]) is calculated, then y is bound to it
z[j] = F2(y)
y = F1(x[1])   # y is still bound to F1(x[0]) while F1(x[1]) is computed
               # The memory for F1(X[0]) is finally freed when y is rebound
z[j] = F2(y)

using del y is a good solution if this is what is happening in your case.

Answer 2

what you actually want is something that's weird to do in python -- you want to allocate a region of memory for y and pass the pointer to that region to F1() so it can use that region to build up the next value of y. this avoid having F1() do it's own allocation for the new value of y, the reference to which is then written into your own variable y (which is actually not the value of whatever F1() calculated but a reference to it)

There's already an SO question about passing by reference in python: How do I pass a variable by reference?