How is the pythonic way to create a reference to a own python class instance and use it?

Question

I would like to store the instance of a class in a container like a list. Other classes/methods should access this instance.

Below is a code snipped which defines a data point.

class dataPoint(object):
    def __init__(self, name, typeUUID, value = None):
        self.name = name
        self.typeUUID = typeUUID
        self.value = value

I like to define a method which gives me the reference (no copy constructor, etc.) to this object. Maybe like this:

def getRef(self):
    return ???

These references I like to use in different list. The reference I like to use to set properties/call functions of the data point. Below is some pseudocode:

# define a conatiner with datapoints    
myContainer = [dataPoint("temperature","double",273.15), dataPoint("power","double",230), dataPoint("errorcode","uint32",666)]

# define interfaces which refers to the datapoints
interface1 = [ref("temperature"), ref("power")]

interface2 = [ref("errorcode"), ]

interface3 = [ref("temperature"), ref("power"), ref("errorcode")]
# set temperature to 300K
ref("temperature") = 300.0

# interfaces
print (interface1[ref("temperature")]) --> 300K
print (interface3[ref("temperature")]) --> 300K

How to do this in Python and how to do this pythonic?

myContainer is the only thing that knows about the specific instances of dataPoint; neither any specific instance of dataPoint or the class itself tracks information about other instances. — chepner
– chepner, Commented Sep 29, 2017 at 13:50
Put another way, getRef would be a method of another class that can track groups of related dataPoint instances. (As Tom Dalton shows in his answer, DataPoint can be instrumented to act as the instance tracker.) — chepner
– chepner, Commented Sep 29, 2017 at 13:51
Is this the common way to solve that kind of problems or is this only a particular solution for my problem? — Stefan Jaritz
– Stefan Jaritz, Commented Sep 29, 2017 at 14:13

Tom Dalton · Accepted Answer · 2017-09-29 13:50:58Z

1

You could put the "instance-container" in the class itself:

class DataPoint:
    instances = {}

    def __init__(self, name, typeUUID, value=None):
        self.name = name
        self.typeUUID = typeUUID
        self.value = value

        self.instances[name] = self

    @classmethod
    def get(cls, name):
        return cls.instances[name]

Then you can use it like this:

>>> d1 = DataPoint("foo", "12345")
>>> d2 = DataPoint("bar", "67890")

>>> DataPoint.get("foo")
<DataPoint object at 0x.........>

answered Sep 29, 2017 at 13:50

Tom Dalton

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2 Comments

Stefan Jaritz Over a year ago

In you approach 'DataPoint' holds all instances of it and managing them. Looks like a solution. What is with a more C/C++ concept of passing (smart) pointers through the system?

Tom Dalton Over a year ago

All python objects are 'smart pointers' e.g. all variables are references.

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