Stream: helpdesk (published)

Topic: using SomeModule: SomeModule vs. import SomeModule

Patrick Bouffard (Jan 11 2023 at 17:39):

Normally if I don't want all the exported names from a module I use import SomeModule and access certain names with SomeModule.somename, or sometimes to avoid verbosity, using SomeModule: somename.

Just recently I saw a pattern new to me, using SomeModule: SomeModule. Is there any difference between this and simply import SomeModule? And is there any difference between import SomeModule: somename and using SomeModule: somename?

Sukera (Jan 11 2023 at 17:47):

Yes, there is - import allows extension of somename with more methods, while using does not

Sukera (Jan 11 2023 at 17:48):

using SomeModule: SomeModule is like import SomeModule light - it only brings SomeModule into scope, but without making the names exported from SomeModule available for implicit extension without the SomeModule prefix

Mason Protter (Jan 11 2023 at 18:05):

Yeah, IMO using SomeModule: SomeModule should typically be preferred over import.

Patrick Bouffard (Jan 11 2023 at 19:17):

Ok that makes some sense; I guess a good way to remember is using = "I just want to use". @Mason Protter I assume what you're saying applies to using/import of names as well, i.e. usually using SomeModule: somename unless somename is to be extended.

Mason Protter (Jan 11 2023 at 19:19):

I'd even use it if I'm extending somename personally

Mason Protter (Jan 11 2023 at 19:20):

E.g. if I have MyType, I would never write

import Base: show
function show(::IO, ::MyType)
    ...
end

I would always write

function Base.show(::IO, ::MyType)
    ...
end

Mason Protter (Jan 11 2023 at 19:21):

This way it's clear at the function definition site that I'm extending a function that didn't originate here.

Patrick Bouffard (Jan 11 2023 at 19:23):

:100: ... in the second example you'd still need using Base: Base, no? (well, maybe not Base since I guess that's already in scope)

Patrick Bouffard (Jan 11 2023 at 19:24):

.. or does it need to be import SomeModule; SomeModule.methodbeingextended() = ...?

Mason Protter (Jan 11 2023 at 19:26):

Yeah, if it was another package, the package name needs to be in scope.

Mason Protter (Jan 11 2023 at 19:27):

using SomeModule: SomeModule, somefunction, anotherfunction
SomeModule.methodbeingextended() = ...

is my preferred pattern.

Notification Bot (Jan 11 2023 at 20:19):

Patrick Bouffard has marked this topic as resolved.

Notification Bot (Jan 11 2023 at 20:19):

Patrick Bouffard has marked this topic as unresolved.

Patrick Bouffard (Jan 11 2023 at 20:20):

So in what situations is import SomeModule needed? Is it in effect the same as using SomeModule: SomeModule?

Expanding Man (Jan 11 2023 at 20:21):

You can define new methods on functions that have been imported without explicitly declaring the parent module, which is a very dangerous foot-gun.

Patrick Bouffard (Jan 11 2023 at 20:23):

Agreed, and also a lot less clear.

Sukera (Jan 11 2023 at 20:24):

see also https://github.com/JuliaLang/julia/issues/39235

Brian Chen (Jan 11 2023 at 21:36):

Expanding Man said:

You can define new methods on functions that have been imported without explicitly declaring the parent module, which is a very dangerous foot-gun.

I don't believe exports are put in scope when you use import SomeModule sans colon, only using?

Expanding Man (Jan 11 2023 at 22:18):

You do have to explicitly import the specific functions, but the problem is the same.

Expanding Man (Jan 11 2023 at 22:19):

I guess some might argue that because the functions are imported explicitly it should not be confusing, but I used to do this and have spent a day debugging utterly bewildering bug I caused this way so now I never, ever do it.

Brian Chen (Jan 12 2023 at 00:21):

I'm inclined to agree. Maybe a good custom linter rule (once such a linter exists)

Expanding Man (Jan 12 2023 at 00:28):

To me the issue is not so much the risk that you might do this accidentally, but that if you do the failure mode will potentially be very weird.

Last updated: Oct 02 2023 at 04:34 UTC