Is it possible to use self.value as default argument in method? [duplicate]

Question

I would like to use element defined in class __init__ as default argument of some other method. Here is my code:

class Main():
    def __init__(self):
        self.value = 3

    def print_element(self, element=self.value):
        print(element)

main = Main()
main.print_element()

It generates error:

Traceback (most recent call last):
  File "so.py", line 1, in <module>
    class Main():
  File "so.py", line 5, in Main
    def print_element(self, element=self.value):
NameError: name 'self' is not defined

John Gordon · Accepted Answer · 2019-09-26 22:52:18Z

2

I don't believe you can use an instance attribute value as a default. You can work around it by defaulting to None, then doing the rest inside the function:

def print_element(self, element=None):
    if element is None:
        element = self.value

answered Sep 26, 2019 at 22:52

John Gordon

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Prune · Accepted Answer · 2019-09-26 22:57:18Z

1

You cannot do this: the default value is evaluated when you define the function.

Your run-time logic appears to be

if element is passed in, print that
else, print the current self.value

You can get this with a conditional inside the method.

def print_element(self, element=None):

    print(element if element else self.value)

Is that brief enough to suit your purposes?

answered Sep 26, 2019 at 22:57

Prune

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