[Tutor] Learning Python

Grigor Kolev grigor.kolev at gmail.com
Sun Dec 13 13:36:26 CET 2009

It is from book Learning Python
Operator overloading. Write a class called Mylist that shadows (“wraps”)
a Python
list: it should overload most list operators and operations, including
+, indexing,
iteration, slicing, and list methods such as append and sort. See the
reference manual for a list of all possible methods to support. Also,
provide a con-
structor for your class that takes an existing list (or a Mylist
instance) and copies
its components into an instance member. Experiment with your class
tively. Things to explore:
  a. Why is copying the initial value important here?
  b. Can you use an empty slice (e.g., start[:]) to copy the initial
value if it’s a
      Mylist instance?
c. Is there a general way to route list method calls to the wrapped
d. Can you add a Mylist and a regular list? How about a list and a
e. What type of object should operations like + and slicing return? What
   indexing operations?
f. If you are working with a more recent Python release (version 2.2 or
   you may implement this sort of wrapper class by embedding a real list
in a
   standalone class, or by extending the built-in list type with a
   Which is easier, and why?
My answer is:
class MyList ():
	def __init__(self, value=[]):
		for i in value:
	def __add__(self , other):
		return self.list
	def __mul__(self , other):
		return self .list
	def __delitem__(self , other):
		return self .list
	def __geritem__(self , other):	
		return self.list 	
	def __repeat__(self , other):
		return self.list 
	def sort(self  ):
		self.list = self.list.sort()
		return self.list 
	def append(self , other):
		return self.list 

This is work
In the book answer is this:

class MyList:
  	 def _ _init_ _(self, start):
        #self.wrapped = start[:]         # Copy start: no side effects
        	self.wrapped = []                # Make sure it's a list here
		for x in start: self.wrapped.append(x)
	def _ _add_ _(self, other):
		return MyList(self.wrapped + other)
	def _ _mul_ _(self, time):
		return MyList(self.wrapped * time)
	def _ _getitem_ _(self, offset):
		return self.wrapped[offset]
	def _ _len_ _(self):
		return len(self.wrapped)
	def _ _getslice_ _(self, low, high):
		return MyList(self.wrapped[low:high])
	def append(self, node):
		def _ _getattr_ _(self, name):       # Other members: sort/reverse/etc
		return getattr(self.wrapped, name)
	def _ _repr_ _(self):
		return repr(self.wrapped)

Answer in the book does not work
Where is the error
Grigor Kolev <grigor.kolev at gmail.com>

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