Introduction to Python Dunder Methods

This will be a short article about Python dunder methods, also known as magic methods.

What are Dunder Methods?

Dunder methods are special methods in Python that start and end with double underscores (__). They allow you to define the behavior of your objects for built-in operations, such as addition, subtraction, string representation, and more.

Some common dunder methods include:

• __init__(self, ...): Called when an object is created.
  • Like our %OnNew method in ObjectScript.
• __str__(self): Called by the str() built-in function and print to represent the object as a string.
• __repr__(self): Called by the repr() built-in function to represent the object for debugging.
• __add__(self, other): Called when the + operator is used.
• __len__(self): Called by the len() built-in function to return the length of the object.
• __getitem__(self, key): Called to retrieve an item from a collection using the indexing syntax.
• __setitem__(self, key, value): Called to set an item in a collection using the indexing syntax.
• ... and many more.

Why are Dunder Methods Important and Relevant in an IRIS Context?

In ObjectScript, we don't have the sugar syntax like in Python, but we can achieve similar behavior using dunder methods.

Example, we have imported a Python module, it has a function that returns a python list, and we want to use it in ObjectScript. We must use the __getitem__ dunder method to access the items in the list.

# src/python/article/dunder_example.py
def get_list():
    return [1, 2, 3, 4, 5]

Class Article.DunderExample Extends %RegisteredObject
{

ClassMethod Run()
{
    Set sys = ##class(%SYS.Python).Import("sys")
    do sys.path.append("/irisdev/app/src/python/article")
    set dunderExample = ##class(%SYS.Python).Import("dunder_example")
    set myList = dunderExample."get_list"()
    for i=0:1:myList."__len__"()-1 {
        write myList."__getitem__"(i), !
    }
}

}

Let's run it:

iris session iris -U IRISAPP '##class(Article.DunderExample).Run()'

This will output:

1
2
3
4
5

This demonstrates how to use dunder methods to interact with Python objects in an IRIS context, allowing you to leverage Python's capabilities while working within the ObjectScript environment.

Bonus

A good use of dunder is to put at the end of your python script a if __name__ == "__main__": block to prevent the code from being executed when the script is imported as a module.

Remember, the first article explained that when you import a script, the code is executed. This block allows you to define code that should only run when the script is executed directly, not when it's imported.

Example:

# src/python/article/dunder_example.py
def get_list():
    return [1, 2, 3, 4, 5]

if __name__ == "__main__":
    print(get_list())

Conclusion

What you can do in python even with it's sugar syntax, you can do it in ObjectScript with dunder methods.