getattr-dynamic-and-static-struct-members.md

🏞️ getattr: dynamic and static struct members

with v0.5.0

__getattr__() is a feature that makes it possible to design flexible and versatile structs in mojo (like other features too).

It feels like merging the static and the dynamic world.

Theses features are just there when needed, no obligation to use them all the time!

Here is an example, note that try: and except: blocks are available in mojo and python!

my_package.py

def method_from_package():
    return "Called on package"

def package_double_the_value(arg):
    return arg*2

package_value = 123

The struct (in a .mojo or .🔥 file)

Let implement one that can refer to both static and dynamic struct members.

That way, we can do both "mojo things" and "python things" in a seamless manner.

from python import Python

@value
struct PackageHelper:

    var my_package: PythonObject
    var struct_value: Int
    
    def __init__(inout self):
        Python.add_to_path(".")
        self.my_package = Python.import_module("my_package")
        
        self.struct_value = 456

    def method_from_struct(self) -> String:
        return "called on struct"

    def __getattr__(inout self,name:StringLiteral) ->PythonObject:
        return self.my_package.__getattr__(name) 

The __getattr__() implementation is quite small, but is powerfull:

If a struct do not define a requested attribute, it will call that function and pass the name of it.

In this implementation, if the attribute is not in the struct, get it from the PythonObject.

That way, it is possible to do the_struct_ìnstance.anything in a dynamic manner.

Consider this example for more clarity:

• struct_instance.method_defined_in_the_struct()
  • It works as usual.
• struct_instance.method_not_defined_in_the_struct()
  • Let the__getattr__(self, the_name) handle that!

The main function

(To place just below the mojo struct)

def main():
    #create an instance
    w = PackageHelper()
    
    #that attribute is defined in the struct
    print(w.method_from_struct())

    #that attribute comes from the PythonObject
    print(w.method_from_package())

    #that attribute is defined in the struct
    print(w.struct_value)

    #that attribute is defined in the PythonObject
    print(w.package_value)

    temp_value = PythonObject(1.0)
    print(w.package_double_the_value(temp_value))
    temp_value = 1.5
    print(w.package_double_the_value(temp_value))

Notes

• inside def() function, let and var are optional.
• struct can store PythonObject type instances.
• implicit type
  • temp_value = PythonObject(1.0)
    • temp_value is of PythonObject type
  • temp_value = 1.5
    • instanciated by PythonObject again

try: and except: blocks can be placed in both def() and fn() functions

__getattr__() and __setattr()__ were introduced in Changelog: Week of 2023-04-03