def nochange(y):
	y = 1

x = 2
nochange(x)
print x # prints 2

def nochange(y):
	y = ['hello']

x = ['not hello']
nochange(x)
print x # prints ['not hello']

# What is happening in the above is the following,
#
# You are creating a list which has the content ['not hello'] and then telling
# the varible 'x' to point at this list.
#
# When you call nochange, the y argument is set to point to what x is also
# pointing to. The next line then tells y to point somewhere else!


# So why does the following work?
def changeme(y):
	y.append('hello')

x = []
changeme(x)
print x # prints ['hello']

# You created an empty list and tell the varible 'x' to point to this list.
# You then call the changeme function, the y argument is set to point to what x
# is also pointing to. Then you say "call the append function on this thing
# which y points to".
#
# The append function then changes the contents of the object that y and x are
# both pointing too.

# So why doesn't the following work
def changeme(y):
	y += 1

x = 2
changeme(x)
print x # prints 2

# The reason this does not work is because an integer is immutable. You can not
# change an int object. 
#
# This means that y += 1 is mapped to y = y+1
#
# Which means, take the object at y, add one and assign y the resulting object.
#