Develop class Polynomial. The internal representation of a Polynomial is a dictionary of terms. Each term is a key–value pair that contains an exponent and a coefficient. The term 2x4 has the coefficient 2 and the exponent 4. For simplicity, assume the polynomial contains only nonne- gative exponents. Develop the class with a dictionary-based interface for accessing terms that includes the

following elements:

a) The class’s constructor accepts a dictionary of exponent:coefficient pairs.
b) Coefficient values in a Polynomial are accessed by exponent keys
(e.g., polynomial[ exponent ] = coefficient). If a polynomial does not
have a coefficient for a specified exponent, the expression polynomial[ exponent ] evaluates to 0.
c) The length of a Polynomial is the value of its highest exponent.
d) Define method __str__ for representing a Polynomial as a string with terms of the form cxy.
e) Include an overloaded addition operator (+) to add two Polynomials.
f) Include an overloaded subtraction operator (-) to subtract two Polynomials.

```
# Definition of class Polynomial.

import copy

class Polynomial:
"""Class to represent a Polynomial as a dictionary"""

def __init__( self, initialDictionary = None ):
"""Initializes Polynomial instance"""

if not initialDictionary:
initialDictionary = {}

self._polyDict = copy.deepcopy( initialDictionary )

def __getitem__( self, power ):
"""Retrieve the coefficient for a power"""

if power in self.exponents():
return self._polyDict[ power ]
else:
return 0

def __setitem__( self, power, coefficient ):
"""Set a coefficient for a power"""

self._polyDict[ power ] = coefficient

def __len__( self ):
"""Returns highest power of polynomial"""

return max( self._polyDict.keys() )

def __str__( self ):
"""String representation of polynomial"""

terms = self._polyDict.items()
terms.sort()
terms.reverse()

termList = []

for power, coefficient in terms:
termList.append( "%dx^%d" % ( coefficient, power ) )

return " + ".join( termList )

def __add__( self, other ):
"""Add two Polynomial instances"""

newDict = copy.deepcopy( self._polyDict )

for power in range( max( len( self ), len( other ) ) + 1 ):
newDict[ power ] = self[ power ] + other[ power ]

return Polynomial( newDict )

def __sub__( self, other ):
"""Subtract two Polynomial instances"""

newDict = copy.deepcopy( self._polyDict )

for power in range( max( len( self ), len( other ) ) + 1 ):
newDict[ power ] = self[ power ] - other[ power ]

return Polynomial( newDict )

def __mul__( self, other ):
"""Multiply two Polynomial instances"""

newPoly = Polynomial()

for powerA, coefficientA in self.terms():

for powerB, coefficientB in other.terms():
newPoly[ powerA + powerB ] += \
coefficientA * coefficientB

return newPoly

def terms( self ):
"""Return list of terms in polynomial"""

return self._polyDict.items()

def exponents( self ):
"""Return list of exponents in polynomial"""

return self._polyDict.keys()

def coefficients( self ):
"""Return list of coefficients in polynomial"""

return self._polyDict.values()
```
>>> p1 = Polynomial( { 3 : 3, 2 : 2, 1 : 1, 0 : -1 } )
>>> p2 = Polynomial( { 2 : 4, 1 : 2 } )
>>> print p1
3x^3 + 2x^2 + 1x^1 + –1x^0
>>> print p2
4x^2 + 2x^1
>>> print p1 + p2
3x^3 + 6x^2 + 3x^1 + -1x^0
>>> print p1 - p2
3x^3 + -2x^2 + -1x^1 + -1x^0
>>> print p1 * p2
12x^5 + 14x^4 + 8x^3 + -2x^2 + -2x^1