[BUG]: Mojo variadics don't work with non-trivial register-only types. about mojo

[JoeLoser]

Your second example works fine on top-of-tree and will work in 24.2:

from python import Python

fn main() raises:
    var json = Python.import_module("json")
    var a = json.loads('{"a": 1, "b": 2}')
    print(a.values())
    for v in a.values():
        print(v)

It prints:

dict_values([1, 2])
1
2

Your first example:

from python import Python 
        
def main():
    var json = Python.import_module("json")
    a = json.loads('{"a": 1}')
    print(a.values())
    print(a.values()[0]) 

crashes on the subscript operator as you mentioned. We should fix that to give a better error message. The print(a.values()) prints:

dict_values([1])

as expected

[lattner]

This is fixed in the next update, thank you for reporting this!

[elhigu]

Tried also iterating over that, which fails to iterator not being mutable or something like that.

(venv) mlepisto@stevetop:~/projects/learn-machine$ cat test.mojo 
from python import Python

fn main() raises:
    var json = Python.import_module("json")
    var a = json.loads('{"a": 1, "b": 2}')
    print(a.values())
    for v in a.values():
        print(v)
(venv) mlepisto@stevetop:~/projects/learn-machine$ mojo run test.mojo 
/home/mlepisto/projects/learn-machine/test.mojo:8:22: error: invalid call to '__iter__': argument #0 must be mutable in order to pass as a by-ref argument
    for v in a.values():
             ~~~~~~~~^~
/home/mlepisto/projects/learn-machine/test.mojo:1:1: note: function declared here
from python import Python
^
mojo: error: failed to parse the provided Mojo

[lattner]

@akirchhoff-modular helped track this down - this is caused by register-only types being handled by VariadicList instead of VariadicListMem, so their copy constructors don't get run correctly. There is a big migration to move the variadic list representation over to this new form, which can hopefully help, but I'm not exactly sure when it will converge