How are function definitions distributed to parallel kernels?

Question

For example, I define a function f.

f[x_] := x^2;

And when I execute

ParallelTable[f[i], {i, 1, 10}]

I get

{1, 4, 9, 16, 25, 36, 49, 64, 81, 100}

as expected. (my function definition has propagated to the parallel kernels)

Now, if I do

Unprotect[Print];
Print = Null;

and

ParallelDo[Print[i], {i, 1, 10}]

I expect nothing to be printed out because I have changed the Print function. But instead, I get the printout.

1
3
5
7
9
10
2
4
6
8

Why did my function definition propagate to the parallel kernels in the first example, but not in the second example?

Answer 1

Print didn't get distributed because it was in the System` context. By default only Global` (or rather $Context) symbols are distributed.

Definitions are sent to subkernels using DistributeDefinitions. Both this function and those that call is have the DistributedContexts options which usually defaults to $DistributedContexts, which in turn is set to

$DistributedContexts := $Context

You can set these to choose which contexts will get distributed.

Auto-distributing everything by default would lead to breakage soon due to many different reasons (loading a package often does more than issue definitions, some contexts hold mutable state such as caches, etc.)

Related:

• Contexts and parallelization

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That said, attempting to distribute (automatically or manually) any System` symbols is a patently bad idea that could very easily lead to breakage. While I haven't tried to find out if it is so, I wouldn't be surprised if the system protected itself against this even beyond the fact that these symbols are Protected on the subkernels as well, which alone would break distribution anyway.