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Assignment operator
===================

`Python library reference
<http://docs.python.org/reference/simple_stmts.html#assignment-statements>`_
says:

  Assignment statements are used to (re)bind names to values and to
  modify attributes or items of mutable objects.

In short, it works as follows (simple assignment):

#. an expression on the right hand side is evaluated, the corresponding
   object is created/obtained
#. a **name** on the left hand side is assigned, or bound, to the
   r.h.s. object

Things to note:

* a single object can have several names bound to it:

    .. ipython::
        
        In [1]: a = [1, 2, 3]
        
        In [1]: b = a
        
        In [1]: a
        
        In [1]: b
        
        In [1]: a is b
    
        In [1]: b[1] = 'hi!'
        
        In [1]: a

* By modifying `b`, we have also modified `a`! This is because `a` and
  `b` are just labels that point to the same object. If we want to
  make a copy, we can explicitly ask for one by using the ``list``
  command:

    .. ipython::
        
        In [1]: a = [1, 2, 3]
    
        In [1]: b = list(a)
   
        In [1]: a is b
        
        In [1]: b[1] = 'hi!'
   
        In [1]: a

* Note that taking slices of lists does perform a copy:

    .. ipython::
        
        In [1]: a = [1, 2, 3]
        
        In [1]: b = a[:2]
        
        In [1]: b
        
        In [1]: b[0] = 'hi!'
        
        In [1]: b
        
        In [1]: a

* The key concept here is **mutable vs. immutable**

    * mutable objects can be changed in place.
    * immutable objects cannot be modified once created.

A very good and detailed explanation of the above issues can be found
in David M. Beazley's article `Types and Objects in Python
<http://www.informit.com/articles/article.aspx?p=453682>`_.