Changing class attribute not working as expected

Here is a simplified running code to show my problem:

class c_event():
  cl_attr = None
  
  def __init__(self, mystring):  
    print(self.cl_attr)
    if self.cl_attr is None:
      self.cl_attr = mystring
    print(self.cl_attr)
    
var1 = c_event('123')
var2 = c_event('456')    
print(var1.cl_attr)
print(var2.cl_attr)
print(c_event.cl_attr)

I am under the assumption that a class attribute defined like I did is common, read- and writeable by all instances of that class. So I expected to get an output of:

None
123
123
123
123
123
123

Instead I get:

None
123
None
456
123
456
None

What is the (thinking) mistake I made?
***Edit: Chepners answer helped understanding. Here is the code that does what I want:

class c_event():
  cl_attr = None
  
  def __init__(self, mystring):  
    print(c_event.cl_attr)
    if c_event.cl_attr is None:
      c_event.cl_attr = mystring
    print(c_event.cl_attr)
    
var1 = c_event('123')
var2 = c_event('456')    
print(var1.cl_attr)
print(var2.cl_attr)
print(c_event.cl_attr)

>Solution :

You can only change a class attribute by assigning to the attribute via the class itself, not an instance. __init__ is creating a new instance attribute that subsequently shadows the class attribute.

class c_event():
  cl_attr = None
  
  def __init__(self, mystring):  
    print(self.cl_attr)
    if self.cl_attr is None:
      type(self).cl_attr = mystring
    print(self.cl_attr)

It’s not clear that you should be modifying a class attribute like this in the first place. Basically, cl_attr is remembering the value used to create the first instance of c_event.

If you really just want to set the instance attribute, the class attribute isn’t necessary at all:

class c_event:
    def __init__(self, mystring):
        self.attr = mystring