Python variable overview

# This statement means that x is now a reference to the object 4
>>> x = 4

# Set y to refer to what x refers to i.e. y is a reference to the object 4 as well
>>> y = x

# Set x to refer to the object 5
>>> x = 5

# x and y now have references to different objects
>>> x != y
True

# Set orig to refer to a new list object, which has the values 2, 7, 12
>>> orig = [2, 7, 12]

# Set cur to refer to what orig refers to, which is the same list object
>>> cur = orig

>>> print(orig)
[2, 7, 12]

>>> print(cur)
[2, 7, 12]

# Now pop the last value off of cur and print it
>>> print(cur.pop())
12

>>> print(cur)
[2, 7]

# But...
>>> print(orig)
[2, 7]

# Because cur and orig both have a reference to the same object they are the same!
>>> cur is orig
True

# The is operator is only True when both sides have a reference to the same object,
# whereas == checks that the value of each side is equal:
>>> False == 0
True

>>> False is 0
False

# If you want to be able to update the value of cur without affecting orig, you need them
# to refer to different objects. So you make a *copy* of the list orig refers to, and then
# set cur to refer to this new copy.

# Using slice to get a new copy
>>> cur = orig[:]

# Using the copy module
>>> import copy
>>> cur = copy.copy(orig)

# In both cases:
>>> cur == orig
True
>>> cur is orig
False

# Something to keep aware of though is that both methods just copy the items in orig to
# the new list, and so each list contains references to the same objects, i.e.
>>> cur[0] is orig[0]
True
>>> cur[1] is orig[1]
True

# etc. So this can happen:
>>> orig2 = ['x', 1.4, { 'f': 'Bob', 'l': 'Bobby' }]
>>> cur2 = orig2[:]
>>> cur2[0] = 'zzz'
>>> cur2
['zzz', 1.4, {'f': 'Bob', 'l': 'Bobby'}]
>>> orig2
['x', 1.4, {'f': 'Bob', 'l': 'Bobby'}]

# But...

>>> cur2[2]['f'] = 'Robert'
>>> cur2
['zzz', 1.4, {'f': 'Robert', 'l': 'Bobby'}]
>>> orig2
['x', 1.4, {'f': 'Robert', 'l': 'Bobby'}]

# Both have been changed, because the last items of each list were referring to the same dictionary object!

# To avoid, use the function deepcopy() in the copy module:
>>> import copy
>>> orig2 = ['x', 1.4, { 'f': 'Bob', 'l': 'Bobby' }]
>>> cur2 = copy.deepcopy(orig2)
>>> cur2[2]['f'] = 'Robert'
>>> cur2
['x', 1.4, {'f': 'Robert', 'l': 'Bobby'}]
>>> orig2
['x', 1.4, {'f': 'Bob', 'l': 'Bobby'}]

# Hope that's helpful! If you're not using IPython for everything Python-related, do so - then I can just 
# type it into the console, then save the inputs and outputs for that to a file you could download and
# load and re-run or edit in IPython yourself.

# http://ipython.org/ipython-doc/stable/interactive/tutorial.html
    
# https://splash.riverbed.com/community/product-lines/flyscript/blog/2013/03/08/interactive-python-coding-with-ipython

# http://www.slideshare.net/ioggstream/will-ipythonreplacebash

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