Creating an Instance Without Invoking init

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

You need to create an instance but want to bypass the execution of the __init__() method for some reason.

Solution

A bare uninitialized instance can be created by directly calling the __new__() method of a class. For example, consider this class:

class Date(object):
    def __init__(self, year, month, day):
        self.year = year
        self.month = month
        self.day = day

Here’s how you can create a Date instance without invoking __init__():

>>> d = Date.__new__(Date)
>>> d
<__main__.Date object at 0x1006716d0>
>>> d.year
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'Date' object has no attribute 'year'
>>>

As you can see, the resulting instance is uninitialized. Thus, it is now your responsibility to set the appropriate instance variables. For example:

>>> data = {'year':2012, 'month':8, 'day':29}
>>> for key, value in data.items():
...     setattr(d, key, value)
...
>>> d.year
2012
>>> d.month
8
>>>

Discussion

The problem of bypassing __init__() sometimes arises when instances are being created in a nonstandard way, such as when deserializing data or in the implementation of a class method that’s been defined as an alternate constructor. For example, on the Date class shown, someone might define an alternate constructor ...