# 
# .. contents::
#     
# Download this page as:
#     
#     - :download:`a commented Python script <assignment.py>`
#     - :download:`a minimal Python script <assignment_bare.py>`
# 
# 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:
# 

a = [1, 2, 3]
b = a
a
b
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:
# 

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:
# 

a = [1, 2, 3]
b = a[:2]
b
b[0] = 'hi!'
b
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>`_.