How do I get a list of all instances of a given class in Python?

The gc or weakref module is used to get a list of all instances of a given class. First, we will install the gc module using pip −

pip install gc

To use the gc module, use the import −

import gc

Get the instances of a class using the gc module

In this example, we have created a Demo class with four instances −

ob1 = Demo()
ob2 = Demo()
ob3 = Demo()
ob4 = Demo()

We loop through the objects in memory −

for ob in gc.get_objects():

Example

Using the isinstance(), each object is checked for an instance of the class Demo. Let us see the complete example −

import gc
# Create a Class
class Demo:
   pass

# Four objects
ob1 = Demo()
ob2 = Demo()
ob3 = Demo()
ob4 = Demo()

# Display all instances of a given class
for ob in gc.get_objects():
   if isinstance(ob, Demo):
      print(ob)

Output

<__main__.Demo object at 0x000001E0A407FC10>
<__main__.Demo object at 0x000001E0A407EBC0>
<__main__.Demo object at 0x000001E0A407EBF0>
<__main__.Demo object at 0x000001E0A407EC20>

Display the count of instances of a class using the gc module

Example

In this example, we will calculate and display the count of instances −

import gc

# Create a Class
class Demo(object):
   pass

# Creating 4 objects
ob1 = Demo()
ob2 = Demo()
ob3 = Demo()
ob4 = Demo()

# Calculating and displaying the count of instances
res = sum(1 for k in gc.get_referrers(Demo) if k.__class__ is Demo)
print("Count the instances = ",res)

Output

Count the instances = 4

Display the instances of a class using weakref module

The weakref module can also be used to get the instances of a class. First, we will install the weakref module using pip −

pip install weakref

To use the gc module, use the import −

import weakref

Example

Let us now see an example −

import weakref

# Create a Demo() function
class Demo:
   instanceArr = []
   def __init__(self, name=None):
      self.__class__.instanceArr.append(weakref.proxy(self))
      self.name = name

# Create 3 objects
ob1 = Demo('ob1')
ob2 = Demo('ob2')
ob3 = Demo('ob3')

# Display the Instances
for i in Demo.instanceArr:
   print(i.name)

Output

ob1
ob2
ob3