CODEWITHSUNDEEP
python
yari
yari

Reputation: 165

Variables as hash values in Python

How can I link a variable to a dictionary value in Python? Consider the following code:

a_var = 10
a_dict = {'varfield':a_var, 'first':25, 'second':57} 

# a_dict['varfield'] == 10 now

a_var = 700   
# a_dict['varfield'] == 10 anyway

So is there a way to link the value of a variable to a field in a dictictionary without looking up for that field an updating it's value manually?

Upvotes: 2

Views: 121

Answers (2)

eyquem
eyquem

Reputation: 27575

w = [410]

myDict = {'myvar': w}

print myDict 
#{'myvar': [410]}

w[0] = 520

print myDict
#{'myvar': [520]}

That's the version of the code of M4rtini with a list instead of an instance of a class.
He is obliged to modify v1 (in fact its attribute value) with the instruction v1.value = ...,
I am obliged to modify the value in the list with w[0] = ...

The reason to act like this is that what you erroneously called a variable, and that is in fact an identifier, doesn't designates a variable in the sense of a "chunk of memory whose content can change" but references an object to which the identifier is binded, object whose value cannot change because it is an immutable object.

Please read the explanations of the documentation on the data model and the execution model of Python which is quite different from the ones of languages such as Java, PHP, etc.

Upvotes: 1

M4rtini
M4rtini

Reputation: 13539

You would need to set the value of the dictionary key, to an object that you can change the value of.

For example like this:

class valueContainer(object):
    def __init__(self, value):
        self.value = value
    def __repr__(self):
        return self.value.__repr__()


v1 = valueContainer(1)

myDict = {'myvar': v1}
print myDict 
#{'myvar': 1}

v1.value = 2 
print myDict
#{'myvar': 2}

Upvotes: 1

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