I need help in my assignment plz. kindly guide me any technique/idea to do the following. m not at advance level of programming.

The program should take function defnitions and those definitions can be used in future expressions
for eg:

def add(x,y)=(x+y)
>>> ok
def inv(x)=1/x
>>> ok
>>> ans : 0.25

Also four additional commands:
show; to prints all de?nitions on screen
save flename; to save all defnitions in a fle
load flename; to load all defnitions from a fle
delete funname; to delete the defnition corresponding to the function named funname