setitem and getitem — python

问题:

I've created a python program that makes a vector. Now i want to set an item using a function__setitem__ and__getitem__. So for example, ifvector = Vec()andvector[3] = 26would change an empty vector to[0, 0, 0, 26]. I need to override the__getitem__and__setitem__ I've listed the code below, but i'm having troubles with the get and set functions. Any advice?

class Vec:
    def __init__(self, length = 0):
        self.vector = [0]*length

    def __str__(self):
        return '[{}]'.format(', '.join(str(i) for i in self.vector))
        #This formats the output according to the homework.
        #Adds '[' and ']' and commas between each 0

    def __len__(self):
        return len(self.vector)

    def extend(self, newLen):
        self.vector.append([0]*newLen)
        return (', '.join(str(j) for j in self.vector))

    def __setitem__(self, key, item):
        self.vector[key] = item

    def __getitem__(self, key):
        return self.vector[key]

回答1:

You have several problems:

1. extend is appending essentially a new vector to the end of the original, rather than increasing the length of the original. It's not clear that it needs to return a string representation of the modified vector (unless it's just for debugging purposes).

   def extend(self, newlength):
       # Assume newlength is greater than the old
       self.vector.extend([0] * (newlength - len(self)))
   

2. __setitem__ needs to call extend if the key is too large.

   def __setitem__(self, key, item):
       if key >= len(self):
           self.vector.extend(key+1)
       self.vector[key] = item
   

3. __getitem__ needs to access the underlying list, not use an undefined attribute

   def __getitem__(self, key):
       # It's probably better to catch any IndexError to at least provide
       # a class-specific exception
       return self.vector[key]
   

回答2:

The problem is the vector you created has no length because of the default value given the length keyword argument in the __init__() method definition. Try this:

vector = Vec(4)
vector[3] = 26
print vector  # --> [0, 0, 0, 26]

回答3:

You need to adapt your __getitem__ and __setitem__ to delegate to the underlying list:

def __setitem__(self, key, item):
    self.vector[key] = item 
    # return doesn't make sense here

def __getitem__(self, key):
    # not sure what self.__key is ? Let's return value from index in `self.vector` instead
    return self.vector[key]

回答4:

It would be easier in __str__ if you do this

return str(self.vector)