Python’s magic method
__setitem__(self, key, value) implements the assignment operation to
self[key]. So, if you call
self[key] = value, Python will call
We call this a “Dunder Method” for “Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.
Syntax and Minimal Example
object.__setitem__(self, key, value)
Let’s have a look at an example where you override the
__setitem__ magic method of a custom class
class HelloWorld: def __setitem__(self, key, value): print('hello world', key, value) hi = HelloWorld() hi = '!' # hello world 42 ! hi['wide'] = 'web' # hello world wide web
This code shows several interesting things:
- You use
obj[key] = valueto call
- You can pass arbitrary keys into it.
However, this is just a dummy example, let’s have a look at a full example implementing all methods
__delitem__ that together play together beautifully to implement a collection-like custom data type.
In the following example, you create a custom list-like type that overrides the
class MyList: def __init__(self, lst): self.lst = lst def __getitem__(self, key): print('__getitem__', key) return self.lst[key] def __setitem__(self, key, value): print('__setitem__', key, value) self.lst[key] = value def __delitem__(self, key): print('__delitem__', key) del self.lst[key]
Each time you add a new element, get an element, or delete an element from your custom list, it prints the respective information so you see what happens.
my = MyList(['alice', 'bob', 'carl']) print(my) # __getitem__ 1 # bob
Note how Python first calls the print statement within the
__getitem__ output and key
1 and then returns the element
my itself, puts it into the
print(...) statement which results in the output
Next, you overwrite the value of the second list element with index 1:
my = '42' print(my) # __setitem__ 1 42 # __getitem__ 1 # 42
You can see from the print output
__setitem__ 1 42 that
__setitem__ was called with key
1 and value
Next, you delete an element with the expression
del my print(my) # carl
Where to Go From Here?
Enough theory. Let’s get some practice!
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To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.