Python: from a dict, how retrieve object as key
If I have a dictionary of several Object:value,, How can I retrieve certain Object using it as [key]?
For example
class Obj():
def __init__(self, value):
self.value = value
dct = {Obj(foo):foo_value, Obj(bar):bar_value}
#How to do something like
#>>> dct[foo]
#foo_value
Suppose that foo_value can't be aasigned as property of Obj.
So far, this is what I get (abstracted)
class Obj():
def __init__(self, name):
self.name = name
def __hash__(self):
return hash(tuple(sorted(self.__dict__.items())))
def __eq__(self, other):
if isinstance(other, self.__class__):
return self.__dict__ == other.__dict__
else:
return False
def __repr__(self):
return str(self.name)
dct = {Obj('item1'):1, Obj('item2'):2}
print(dct.keys())
dct['item1']
And the output
dict_keys([item1, item2])
Traceback (most recent call last):
File "C:\Users\ivan\Desktop\multi_e.py", line 197, in <module>
dct['item1']
KeyError: 'item1'
Asked by: Clark650 | Posted: 27-01-2022
Answer 1
What about using a custom implementation of dict?
class FieldDict(dict):
def __getitem__(self, item):
return dict.__getitem__(self, Obj(item))
dct = FieldDict({Obj('item1'):1, Obj('item2'):2})
print(dct.keys())
print(dct['item1']) # prints 1
Answer 2
That will not work as the keys are not the strings, but they are objects of type Obj. Even though all the objects hold that string
You could do this. Store variables as references to objects as keys
x = Obj("item1")
y = Obj("item2")
dct= {x:1, y:2}
And to retrieve you need to do:
>>> dct[x]
1
Answer 3
You could roll-out your own dict subclass with custom __getitem__ and __setitem__ and you really don't need any complicated __hash__ method on your Obj class after this, just __init__ will suffice.
class Obj:
def __init__(self, value):
self.name = value
class MyDict(dict):
def __setitem__(self, key, value):
if isinstance(key, Obj):
dict.__setitem__(self, key.name, value)
else:
dict.__setitem__(self, key, value)
def __getitem__(self, key):
if isinstance(key, Obj):
return dict.__getitem__(self, key.name)
return dict.__getitem__(self, key)
Demo:
>>> dct = MyDict()
>>> dct[Obj('item1')] = 1
>>> dct[Obj('item2')] = 2
>>> dct
{'item1': 1, 'item2': 2}
>>> dct['item1']
1
>>> dct[Obj('item1')]
1
Answer 4
I was a trying for a little bit time, but I think I got what you want, look:
edit thanks to @user2357112
class Obj():
def __init__(self, value):
self.value = value
def __eq__(self, other):
"""Override the default Equals behavior"""
if isinstance(other, self.__class__):
return self.value == other.value
return False
def __ne__(self, other):
"""Define a non-equality test"""
return not self.__eq__(other)
def __hash__(self):
return id(self.value)
class Custom_dict(dict):
def __getitem__(self, item):
return dict.__getitem__(self, Obj(item))
x = Custom_dict()
x[Obj('asdf')] = 5
print(x['asdf'])
dct = Custom_dict({Obj('item1'):1, Obj('item2'):2})
print([key.value for key in dct.keys()])
print(dct['item1'])
Answered by: John444 | Posted: 28-02-20225 ['item1', 'item2']
1
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