Question

I try to define the function up with 1 parameter in class X, like this:

class X:

    def __init__(self):
        self.elem=[]

    def i(self, new):
        self.elem.append(new)
        self.up(self,len(self.elem)-1)                    <= error here

    def up(self, n):
        print n

border = X()

a = [2,4,3,1]

for i in a:
    border.i (i)

The error looks like this:

$ Traceback (most recent call last):
  File "p.py", line 60, in <module>
    border.i (i)
  File "prim.py", line 50, in i
    self.up(self,len(self.elem)-1)
TypeError: up() takes exactly 2 arguments (3 given)
$ 

if I call in i like this self.up(self) , it compiles, and print n displays so:

$ <__main__.X instance at 0x7f7a7ddf0128>
<__main__.X instance at 0x7f7a7ddf0128>
<__main__.X instance at 0x7f7a7ddf0128>
<__main__.X instance at 0x7f7a7ddf0128>
$ 

Answer 1

You do not need to pass self on to up():

def i(self, new):
    self.elem.append(new)
    self.up(len(self.elem)-1)

Python does this for you; by looking up self.up, you are given a bound method, which means that Python will automatically add in self when you call up().

You were, in effect, calling up(self, self, len(self.elem)-1); three elements where only 2 were expected.