How can I know how many instances were made from a class?

Question

I tried this program:

class Test():
    def __init__(self):
        self.value = 10
        print(self.value)

t = Test()
t2 = Test()

I would like to know how many instances were made from the Test class.

Where would you like to track this? A file? In memory? Etc – JacobIRR Jul 12 '20 at 17:54 — JacobIRR, Jul 12 '20 at 17:54
I would track this in .py file – Paul Kocian Jul 12 '20 at 17:58 — Paul Kocian, Jul 12 '20 at 17:58
What? You aren't doing anything to track this. – juanpa.arrivillaga Jul 12 '20 at 22:57 — juanpa.arrivillaga, Jul 12 '20 at 22:57

score 1 · Answer 1 · edited Nov 21 '20 at 11:39

1

This should do the job, which you're looking for. Rest you can make a file and print the response. I am just representing the thing which you want for now:

class Test:
    # this is responsible for the count
    counter = 0
    def __init__(self):
        Test.counter += 1
        self.id = Test.counter

t = Test()
t2 = Test()
print(Test.counter)

# OUTPUT => 2

edited Nov 21 '20 at 11:39

halfer

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answered Jul 12 '20 at 18:02

Alok

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score 1 · Accepted Answer · answered Jul 12 '20 at 18:24

The idea to create a counter for the class and increment them if a new instance is created, works in most cases.

However, you should keep some aspects in mind. What if a instance is removed? There is no mechanism to decrease this counter. To do this, you could use the __del__ method, which is called when the instance is about to be destroyed.

class Test:
    counter = 0
    def __init__(self):
        Test.counter += 1
        self.id = Test.counter
    def __del__(self):
        Test.counter -= 1

But sometimes it can be problematic to find out, when an instance is deleted. In this blog post you can find some more information if needed.

I find that this answer is more complete – Paul Kocian Nov 07 '21 at 09:22 — Paul Kocian, Nov 07 '21 at 09:22

How can I know how many instances were made from a class?

2 Answers2