How to set a python property in __init__

Question

Hi,

I have a class with an attribute I wish to turn into a property, but this attribute is set within __init__. Not sure how this should be done. Without setting the propert in __init__ this is easy and works well

import datetime

class STransaction(object):
    """A statement transaction"""
    def __init__(self):
        self._date = None

    @property
    def date(self):
        return self._date

    @date.setter
    def date(self, value):
        d = datetime.datetime.strptime(value, "%d-%b-%y")
        self._date = d

st = STransaction()
st.date = "20-Jan-10"

But once initialisation needs to occur in __init__ it gets more complicated and I am not sure of the correct course of action.

class STransaction(object):
    """A statement transaction"""
    def __init__(self, date):
        self._date = None

Strangely to me, the following seems to work but smells very bad.

class STransaction(object):
    """A statement transaction"""
    def __init__(self, date):
        self._date = None
        self.date = date

    @property
    def date(self):
        return self._date

    @date.setter
    def date(self, value):
        d = datetime.datetime.strptime(value, "%d-%b-%y")
        self._date = d

What is the correct way to go about setting properties in __init__?

Thanks, aaron.

Answer 1

I do not see any real problem with your code. In __init__, the class is fully created and thus the properties accessible.

Answer 2

class STransaction(object):
"""A statement transaction"""
def __init__(self, date):
    self._date = None #1
    self.date = date  #2

If you want to set the proxy self._date object without executing your setter use the #1 line. If you would like to execute the setter at startup too use the #2. Both ways are correct, it's just a matter of what do you need.