Using instance of class

Question

Is there a "pythonic" way to use an instance of a class for a module. For example:

class MyClass:
   def __init__(self):
       self.my_var = "cool value"

_instance = MyClass()

def getInstance():
    return _instance()

The goal is to share large lookup classes through the application instead of initializing them on every import.

"instead of initializing them on every import" - this part of your question suggests that you may have a bad mental model of how Python's import system works. Python will not redo module initialization on every import; a module is already shared between all other modules that import it. — user2357112
– user2357112, Commented Jul 11, 2017 at 23:27
I'm not following what you mean by "share large lookup classes through the application". Care to give an example of how you envisage this might be used? — Tasos Papastylianou
– Tasos Papastylianou, Commented Jul 11, 2017 at 23:38
What's the point of the getInstance function? Note, it throws an error, since you return the result of calling _instance, but MyClass objects aren't callable... — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Jul 11, 2017 at 23:46
To answer all these questions I wanted to do what is in the answer below. Still not used to import in python. — Steven Bayer
– Steven Bayer, Commented Jul 12, 2017 at 20:59

pythad · Accepted Answer · 2017-07-11 23:42:52Z

3

You can import anything from a module. Class instance is not an exception:

file1.py

class MyClass:
   def __init__(self):
       self.my_var = "cool value"

instance = MyClass()

file2.py

from file1 import instance

answered Jul 11, 2017 at 23:42

pythad

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Comments

darksky · Accepted Answer · 2017-07-12 00:02:36Z

0

If your instances are dynamically created somehow, and you really need to keep track of them, you can override __new__, like this:

class MyClass:
    instances = []

    def __new__(cls, *args, **kwargs):
        my_instance = super(MyClass, cls).__new__(cls, *args, **kwargs)
        MyClass.instances.append(my_instance)
        return my_instance

a = MyClass()

print(MyClass.instances)

answered Jul 12, 2017 at 0:02

darksky

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1 Comment

Steven Bayer Over a year ago

Good idea but I don't think I should mask what is really happening like this. Maybe if I was packaging this for an external project to use.

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