Python’s magic method __delitem__(self, key) implements the deletion of self[key]. So, if you call del self[key], Python will call self.__delitem__(key).
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.__delitem__(self, key)Let’s have a look at an example where you override the __delitem__ magic method of a custom class HelloWorld.
class HelloWorld:
def __delitem__(self, key):
print('hello world', key)
hi = HelloWorld()
del hi[42]
# hello world 42
del hi['wide web']
# hello world wide web
This code shows several interesting things:
- You use the
delkeyword to initiate the__delitem__call. - You use
del obj[key]to callobj.__delitem__(key). - 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 __getitem__, __setitem__, and __delitem__ that together play together beautifully to implement a collection-like custom data type.
Full Example
In the following example, you create a custom list-like type that overrides the __getitem__, __setitem__, and __delitem__ methods.
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.
Now, let’s use this to create a simple list wrapper and print the second element with index 1 (zero-based indexing).
my = MyList(['alice', 'bob', 'carl']) print(my[1]) # __getitem__ 1 # bob
Note how Python first calls the print statement within the __getitem__ output and key 1 and then returns the element my[1] itself, puts it into the print(...) statement which results in the output bob.
Next, you overwrite the value of the second list element with index 1:
my[1] = '42' print(my[1]) # __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 42.
Next, you delete an element with the expression del my[1]:
del my[1] print(my[1]) # carl
References:
Where to Go From Here?
Enough theory. Let’s get some practice!
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