Python: How to set a class variable based on a parameter to init()?

Question

I have a class C. I would like to instantiate this class with one argument. Let's call that argument d. So I would like to do myC = C(d=5). C should also have another variable called e. e's value should be set to d on instantiation of the class.

So I wrote this:

>>> class C:
...     def __init__(self,d):
...         self.d = d
...     e = d
...

But that gives me the following error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in C
NameError: name 'd' is not defined

Ok fine, So I will try something slightly different:

>>> class C:
...     def __init__(self,d):
...         self.d = d
...     e = self.d
...

But that gives me this error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in C
NameError: name 'self' is not defined

So how should I do what I'm trying to do?? This should be very simple.

Martijn Pieters · Accepted Answer · 2013-06-01 10:05:34Z

You need to make e part of the __init__ method as well. d is not defined until that function is called:

class C:
    def __init__(self,d):
        self.d = d
        self.e = d

You can put these on the same line if you want to:

class C:
    def __init__(self,d):
        self.d = self.e = d

If you need e to be a class attribute instead of an instance attribute, refer directly to the class:

class C:
    def __init__(self,d):
        self.d = d
        C.e = d

or use type(self):

class C:
    def __init__(self,d):
        self.d = d
        type(self).e = d

The latter means that if you subclass C, the class attribute will be set on the appropriate subclass instead.

jamylak · Accepted Answer · 2013-06-01 10:05:18Z

0

class C:
    def __init__(self,d):
        self.d = self.e = d

answered Jun 1, 2013 at 10:05

jamylak

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