using python! Write a Mapping class that represents a 1-1 mapping between items.

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using 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]

>>> m[2]

>>> 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]

>>> 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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using python! Write a Mapping class that represents a 1-1 mapping between items.

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