USE!!!!!!! PYTHON!!! Write a Mapping class that represents a 1-1 mapping between

Question

USE!!!!!!! PYTHON!!!

Write a Mapping class that represents a 1-1 mapping between items. A 1-1 mapping is like a dictionary, but it has key1:key2 pairs (instead of key:value pairs). You can look up either key. For example, if m[1] is equal to 2, then m[2] is equal to 1.    Your implementation MUST inherit from (subclass) dict. Basically, whenever you create key11:key2 pair, you also make the corresponding key2:key1 pair.  

>>> m = Mapping()

>>> m[1]=2

>>> m

Mapping({1: 2, 2: 1})

>>> m[1]

2

>>> m[2]

1

>>> m[3]=4

>>> m

Mapping({1: 2, 2: 1, 3: 4, 4: 3})

>>> m[5]=5

>>> m

Mapping({1: 2, 2: 1, 3: 4, 4: 3, 5: 5})

>>> m[5]

5

>>> m.pop(1)

>>> m

Mapping({3: 4, 4: 3, 5: 5})

>>> m.pop(5)

>>> m

Mapping({3: 4, 4: 3})

>>> m = Mapping( {1:2,2:3,4:4})

>>> m

Mapping({2: 3, 3: 2, 4: 4})

>>>

Implementation notes: You MUST inherit from (subclass dict).   You will get some functionality for free.

Write the class first without worrying about self mapping keys (m[5]=5).Once you get that to work, worry about self mapping keys (notes below).

__repr__ - see test code for output

__getitem__ - get this for free! No need to implement.

pop()

@when a key is popped you, must also pop the key it maps to

@For self mapping keys, you need to make sure that this does not create an error!!!!!Don’t worry about this until you get everything else working.

__setitem__

@given key1:key2 make sure you also add key2:key1

@before adding the key1:key2 and key2:key1 pairs, you should check whether either key1 or key2 exists already.If so, they should be popped before adding.

__init__ you should be able to construct

@An empty Mapping

@A Mapping constructed from a given dictionary, see test runs towards bottom.

Explanation / Answer

class Mapping(dict):
   di = {}
   def __init__(self, l = {}):
       self.di = l
   def __getitem__(self, index):
       return self.di[index]
   def __setitem__(self, index, value):
       self.di[index] = value
       self.di[value] = index
   def __str__(self):
       return 'Mapping('+str(self.di)+')'
   def pop(self, index):
       del self.di[self.di[index]]
       if index in self.di:
           del self.di[index]

m = Mapping()
m[1] = 2
print m
m[1]
m[2]
m[3]=4
print m
m[5]=5
print m
print m[5]
m.pop(1)
print m
m.pop(5)
print m
m = Mapping( {1:2,2:3,4:4})
print m

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