Introduction to Python Dunder Methods

[InterSystems Developer]

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.
• [B]str[/B](self): Called by the str() built-in function and print to represent the object as a string.
• [B]repr[/B](self): Called by the repr() built-in function to represent the object for debugging.
• [B]add[/B](self, other): Called when the + operator is used.
• [B]len[/B](self): Called by the len() built-in function to return the length of the object.
• [B]getitem[/B](self, key): Called to retrieve an item from a collection using the indexing syntax.
• [B]setitem[/B](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.

Continue reading...

 

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