Unclarity on variable cloning behavior

Question

Along with a book I was provided with a Python program, into which I am digging deep now.

The program uses a global data structure named globdat, in a specific routine a numpy array inside globdat is assigned to a local variable:

a = globdat.array

Then in a following while loop the variable a is updated every iteration according to:

a[:] += da[:]

The result of this operation is that globdat.array is updated, which is used in subsequent operations.

Is the usage of [:] required here, or is it merely used to indicate that it also clones into globdat.array? Can anyone clarify this coding style?

Answer 1

The second [:], in the right-hand-side, is redundant. It just copies da before using it in the concatenation, which is pointless.

We're left with:

a[:] += da

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First, let's understand what a += da does. It maps to:

a = a.__iadd__(da)

The call to __iadd__ extends the original list a, and returns self, i.e. a reference to the list. The assignment which happens after, then, has no effect in this case (same as a=a).

This achieves the original goal, i.e. to extend the global array.

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Now, What does a[:] += da do? It maps to:

a[:] = a[:].__iadd__(da)

Or more tediously to:

a.__setitem__(slice(None), a.__getitem__(slice(None)).__iadd__(da))

For readability, let's write it as (not a valid python syntax):

a.__setitem__(:, a.__getitem__(:).__iadd__(da))

So a[:].__iadd__(da):

1. creates a copy of a (call is a2)
2. concatenate da to a2 in place
3. return a2

Then the assignment a[:] = ...:

4. replaces all values in a with all values in a2 in place.

So that too, achieves the original goal, but is wayyyyyy less efficient.

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There are some interesting details about this stuff in the answers to this question.