How to prevent the use of shared memory / pointers using multiple class instances in Python?

Question

How can I prevent the creation of shared memory between multiple instances of a class?

Or, how can I prevent the following?

mwe.py

#!/usr/bin/python3

class Entry:
  content = []

sample = {
    'C1':  Entry(),
    'C2':  Entry(),
    }

sample['C1'].content.append('test')

print(sample['C2'].content)

$ ./mwe.py 
['test']