Below is a class MyDict
that inherits from dict
. What I want to
do is basically to perform some operation each time a key of a
MyDict
object is assigned a value. So __setitem__
is my friend
here:
class MyDict(dict): def __setitem__(self, key, value): print(f'key {key} set to {value}') super().__setitem__(key, value) m = MyDict() m[0] = 'hello' # prints "0 set to hello" m[1] = 'world!' # prints "1 set to world!"
This is great but now if I do this, __setitem__
is not called anymore.
m = MyDict({0: 'hello', 1: 'world!'})
I could of course overload __init__
to solve my problem but is
there a simpler solution?
Besides, as I understand the dict.__init__
help message:
| dict(iterable) -> new dictionary initialized as if via: | d = {} | for k, v in iterable: | d[k] = v
__setitem__
should be called in my example. Have I misunderstood
something?
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Answer
You should subclass UserDict
:
from collections import UserDict class MyDict(UserDict): def __setitem__(self, key, value): print(f'key {key} set to {value}') m = MyDict({0: 'hello', 1: 'world!'})
Outputs
key 0 set to hello key 1 set to world!
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