foo.py

class Foo:
  x = {}
  
  def __init__(self, id):
    self.x = { id: id }

f1 = Foo(5)
print f1.x     # { 5:5 }, as expected
f2 = Foo(6)
print f2.x     # { 6:6 }, as expected
print Foo.x    # {}, didn't intend this!