Default values of arguments with the same id

Default values of arguments with the same id - Printable Version

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Default values of arguments with the same id - DrVictor - Dec-02-2020

class test:
    def __init__(self,val=[]):
        self.val=val

a=test()
b=test()
print(id(a.val)==id(b.val)) #True

I noticed that the default value of the arguments has the same id as other instances.
And by changing that value, it changes for other instances.

a.val+=[12]
print(b.val) # [12]

Is this normal for you?

RE: Default values of arguments with the same id - stranac - Dec-02-2020

Ah, the good old mutable defaults...

https://python-forum.io/Thread-Basic-Various-Python-Gotchas has an explanation of what's going on.
Search for "empty list".

RE: Default values of arguments with the same id - DrVictor - Dec-02-2020

(Dec-02-2020, 02:16 PM)stranac Wrote: Ah, the good old mutable defaults...

https://python-forum.io/Thread-Basic-Various-Python-Gotchas has an explanation of what's going on.
Search for "empty list".

Thank you for your answer.
According to you, I searched the internet.
And I used the following solution

class test:
    def __init__(self,val=None):
        if val==None:val=[]
        self.val=val

Writing the third line sounds a little silly but I have to.This reduces the readability of Python.
I think for other people with a different id is more useful than the same id.

RE: Default values of arguments with the same id - deanhystad - Dec-02-2020

class test:
    def __init__(self,val=None):
        self.val = [] if val is None else val

I think "None" is the only default value that should be used for methods. When subclassing I kept running into problems with default arguments hiding that no value had been provided. In the example below I wanted b to "inherit" a's default value.

class a:
    def __init__(self, value=2):
        self.value = value

    def __repr__(self):
        return str(self.value)

class b(a):
    def __init__(self, value=None):  # Want superclass to provide default
        super().__init__(value)

print(b(), b(5))

Output:
None 5

The inheritance works if I use None in the method signature and set the value in the method body.

class a:
    def __init__(self, value=None):
        self.value = 2 if value is None else value

    def __repr__(self):
        return str(self.value)

class b(a):
    def __init__(self, value=None):  # Want superclass to provide default
        super().__init__(value)

print(b(), b(5))

Output:
2 5