[dTal]

Because I don't want to "pass around" the state - I want to hide it. Yes, of course it's possible to mingle the state in all the rest of the program, just like it's possible to scatter gotos everywhere instead of using structured control flow. But what I really want is to call EnablePowerSupply(), rapidly followed by SetVoltage(30), and have all the messy business of statefully talking over a serial port (and not having commands stomp on each other) neatly abstracted away. EnablePowerSupply and SetVoltage need to share state to do that. That could indeed be done by passing an extra parameter - EnablePowerSupply(blobOfState) and SetVoltage(blobOfState, 30) - but that's basically exactly what objects are syntactic sugar for in Python. Only blobOfState is more usually called "self".

Oh, and of course there's not one, but half a dozen power supplies. You could pass around the blobs of state seperately of course, but now you have to manage their scope independently from the functions that operate on them - a useless decoupling that adds overhead. What I ended up with was something like:

[psu.enable() for psu in psus]

Which you can always do, whenever psus is within scope. Hard to get terser and more idiomatic than that.

[mos_basik]

I know it's a contrived example, but it's worth pointing out that using comprehensions to cause side effects is considered by some to be at least unPythonic and at worst an abuse of the construct.

I know this because I wanted to do the same thing and have looked all over for a justification for it. In this case most people seem to agree that the bog standard for loop is the way to go:

    for psu in psus:
        psu.enable()

Some people (my boss) will even put it on a single line, so you don't lose much terseness.

I think the rule of thumb is don't use a list comprehension unless you're going to use the list afterwards, else you're wasting an allocation.

[dTal]

Thanks for the heads up. I'm not totally convinced about "Pythonic" as a figure of merit for anything (often it seems to merely mean "clunky"), but it's a good point about the wasted allocation [1]. And the one-liner for-loop is almost identical to the comprehension anyway.

I think the reason I instinctively write it this way is because in my mind, I basically think of list comprehensions as sugar for map + lambda. I'm "really" trying to write map(enable, psus). But of course, it's a method, so you need the instance[2] - map(lambda psu: psu.enable(), psus). The reason I prefer a map over a for loop is because it's a habit borne of the principle of least power - map provides a guarantee than no "funny business" (data dependency) is going on between the elements of the list you're iterating over. I scrupulously avoid for loops on principle, unless I need that kind of funny business. Of course in this case the for loop is so short as to make no difference, but like I say - it's a habit. In my code, "for" means "funny business here".

[1] not that it matters in this case - you're not toggling power supplies in a tight loop.

[2] Technically, in Python, map(psus[0].enable, psus) would work if psus was not empty. Or you could spawn a new instance: map(PSU().enable, psus). But ugh, talk about defeating the purpose.