title: Cannot set field value in assignment expression
body: <p>With Python 3.8 <a href="https://www.python.org/dev/peps/pep-0572/" rel="noreferrer">Assignment Expressions</a> have been introduced, allowing to assign values in conditionals and lambdas as such:</p>
  
  <pre><code>if x := True:
      print(x)
  
  </code></pre>
  
  <p>However it appears this does not extends to attribute assignment, as trying to do something like this</p>
  
  <pre><code>from typing import NamedTuple 
  
  class Test(NamedTuple): 
      field : bool
  
  test = Test(field=False) 
  
  if test.field := True: 
      print(test.field)
  </code></pre>
  
  <p>Will result in the following error:</p>
  
  <pre><code>SyntaxError: cannot use named assignment with attribute 
  </code></pre>
  
  <p>Is it really only possible to update attribute in assignment <strong>statements</strong> (as opposed to assignment <strong>expressions</strong>) and if yes why this limitation?</p>