John Berry - 4 months ago 46

Python Question

I'm trying to make a Point class in python. I already have some of the functions, like __ str__ , or __ getitem__ implemented, and it works great.

The only problem I'm facing is that my implementation of the __ setitem__ does not work, the others are doing fine.

Here is my Point class, and the last function is my __ setitem__:

`class point(object):`

def __init__(self,x=0,y=0):

self.x=x

self.y=y

def __str__(self):

return "point(%s,%s)"%(self.x,self.y)

def __getitem__(self,item):

return (self.x, self.y)[item]

def __setitem__(self,x,y):

[self.x, self.y][x]=y

it should work like this:

`p=point(2,3)`

p[0]=1 #sets the x coordinate to 1

p[1]=10 #sets the y coordinate to 10

(Am I even right, should the setitem work like this?)

Thanks!

Answer

Let `self.data`

and only `self.data`

hold the coordinate values.
If `self.x`

and `self.y`

were to also store these values there is a chance `self.data`

and `self.x`

or `self.y`

will not get updated consistently.

Instead, make `x`

and `y`

properties that look up their values from `self.data`

.

```
class Point(object):
def __init__(self,x=0,y=0):
self.data=[x, y]
def __str__(self):
return "point(%s,%s)"%(self.x,self.y)
def __getitem__(self,item):
return self.data[item]
def __setitem__(self, idx, value):
self.data[idx] = value
@property
def x(self):
return self.data[0]
@property
def y(self):
return self.data[1]
```

The statement

```
[self.x, self.y][x]=y
```

is interesting but problematic. Let pick it apart:

`[self.x, self.y]`

causes Python to build a **new list**, with values `self.x`

and `self.y`

.

`somelist[x]=y`

causes Python to assign value `y`

to the `x`

th index of `somelist`

. So this new list `somelist`

gets updated. But this has no effect on `self.data`

, `self.x`

or `self.y`

. That is why your original code was not working.