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  • Scope and Ambiguous Assignments In Python

    March 21st, 2011
    python, tech
    Consider the following two Python snippets:
        name='Mary'
        def print_name():
          print name
        print_name()
        print name
      
        name='Mary'
        def print_name():
          name='John'
        print_name()
        print name
      
    The first will print 'Mary', twice. The second will print 'Mary' once. This happens because while python interprets reads as looking outside the current scope, writes can't be [1] anything but local. So the assignment to 'name' inside 'print_name' creates a new variable that disappears when the function exits.

    So now consider:

        name='Mary'
        def print_name():
          print name
          name='John'
        print_name()
        print name
      
    This code will generate an error:
        Traceback (most recent call last):
          File "tmp.py", line 5, in <module>
            print_name()
          File "tmp.py", line 3, in print_name
            print name
        UnboundLocalError: local variable 'name' referenced before assignment
      
    The error is because within a function a variable must be either local or global. If it's local, the 'print name' is illegal because 'name' isn't defined yet. If it's global, the 'name="John"' is illegal because you can't assign outside your scope. So python chooses "local" and decides that the 'print name' line is invalid.

    [1] well, you could use the 'global' keyword

    Comment via: r/Python

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