add method to disallow duplicate items

Question

I have created a new class under UserList called UList that overrides the __add__ method - it does not allow duplicate items to be added to a list. But when I try to run it, I don't get the expected results:

from collections import UserList

class UList(UserList):
  def append(self,ap_item):
    for ap in ap_item:
        if ap in self:
          raise ValueError(repr(ap) + " already exists in list")
        else:
          return UserList.append(self, ap_item)
  def __add__(self,ad_item):
    for ad in ad_item:
        if ad in self:
          raise ValueError(repr(ad) + " already exists in list")
        else:
          return UserList.__add__(self,ad_item)
  def extend(self, ex_item):
    for ex in ex_item:
        if ex in self:
          raise ValueError(repr(i) + " already exists in list")
        else:
          return UserList.extend(self,ex_item)

How can I get this method to execute correctly?

add just overrides the `+` operator `x__add__(y)` is equivelent to `x+y` — Joran Beasley, Sep 13 '13 at 18:36
I expected the items in y to be added to z, unless of the items already exists in z. when i run this it prints None. By the way, I am just learning. — kflaw, Sep 13 '13 at 18:49
When you run it it raises an exception: `ValueError: 5 already exists in list`. — Martijn Pieters, Sep 13 '13 at 18:58
Also, there is a logical error - you are adding the ad_item list once for each item, in your add_fucntion. The code in the `else` clause should be outside the `for` loop. — jsbueno, Sep 13 '13 at 19:08
yes but when i change the values in y to something other than what is in z it returns none. I will look at the logic error — kflaw, Sep 13 '13 at 19:34
1. `append()` is supposed to add single items, not whole lists. So the `for` loop in them is not needed. — glglgl, Sep 14 '13 at 11:23
2. You forgot to override `__iadd__`; this will be called when doing `a += [4]`. — glglgl, Sep 14 '13 at 11:23

score 1 · Accepted Answer · answered Sep 13 '13 at 18:55

1

Your __add__ function lacks a return statement. Any function so lacking implicitly returns None. That explains why your function returns None.

Try this:

return UserList.__add__(self,ad_item)

answered Sep 13 '13 at 18:55

Robᵩ

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thank you. in the same class i am also overrriding the extend and append methods in the same way, i'll edit my code above to show it, why do those methods work without using a return statement? – kflaw Sep 13 '13 at 19:00
I added in return statements after the extend and append methods, but when I tested on extend, I still got None. – kflaw Sep 13 '13 at 19:03
`None` is the correct return value from `.extend()` and `.append()`. Try this, for example, `print ([1].append([2]))`. Those functions modify their object in place. – Robᵩ Sep 13 '13 at 19:33

__add__ method to disallow duplicate items

1 Answers1

add method to disallow duplicate items