What's the recommended way to iterate over PyMapping object?

[tushushu]

I thought I could iterate over PyMapping object like this:

for (key, value) in mapping.iter()

But actually I cannot, and the below codes explain what is inside the PyMapping object:

fn foo(d: &PyMapping){
    println!("d.iter() is PyResult<&PyIterator> type.");
    if let Ok(it) = d.iter(){
        println!("x is &PyIterator type.");
        for x in it{
            println!("x is Result<&PyAny, PyErr> type.");
            if let Ok(y) = x{
                println!("y is &PyAny type.");
            }
        }
    }
}

So it seems that the iter method isn't something I am finding. Is there any suggestion?
Thanks very much!

[samuelcolvin]

I think you need d.items().iter(), see here

[tushushu]

Thanks for the answer. It seems that the items method is eager executed, and sometimes the for loop may need break in the middle, which does not require iterating over all the elements in the PyDict object. There might be some extra performance overhead?

[tushushu]

By the way, I tried items().iter(), and still need to use multiple layers of if let to "extract" the element of the PyMapping object. So the further question could be:

• In what kind of case, the iterating over PyMapping should return an PyErr? Shouldn't this data structure be always iterable?

[davidhewitt]

The PyMapping_Items C API this uses is indeed eager, it returns a list: https://docs.python.org/3/c-api/mapping.html#c.PyMapping_Items

In general we can't offer anything concrete methods in PyO3 which differ from this. However you could call the Python "items" method which might be support lazy iteration (e.g. for dict):

fn foo(d: &PyMapping) -> PyResult<()> {
    for item in d.call_method0("items")?.iter()? {
        let (key, value) = item.extract()?;
        // do your thing
    }
    Ok(())
}

[tushushu]

Thanks @davidhewitt , let me also try the way you suggested.