How come python's object gets referenced through another by assigning object to another object

Question

I'm curious, how to best explain why when you do:

R = []
R.append(3)

and you do this:

P = R
R.append(4)

why does P's (as an obj) also gets R's values:

(the numbered list represent each line)

R = []

R.append(3)

P = R

R.append(4)

print P

It says: [3, 4]

Answer 1

4

The line

P = R

Just performs reference assignment. It doesn't clone or copy the object(s). This is the same behavior in a lot of popular languages, including C/C++, Java and Ruby.

You can think of it as simply declaring another 'name' (or reference) for the object 'held' by the current variable named R. Both variables P and R now refer to the same object.

Note that primitive variables behave differently from variables containing references to objects, which is likely the source of your confusion. In Ruby, for example:

a = 1    # a => 1
b = a    # b => 1
a += 1   # a => 2
b        # b => 1

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answered Sep 13 '11 at 10:00

Alistair A. Israel
3.1k●2●10

edited Sep 13 '11 at 21:29

2

+1 on the "it's how practically all mainstream languages do it".

Ruby nitpicking, though: Ruby doesn't have a ++ operator, and doing the += only makes the incrementing appear like an operation on primitives by creating a copy of the object.

A better way to show that all entities in Ruby are objects/references is:

a = "test string"
b = a
puts b      # outputs "test string"
a.reverse
puts b      # outputs "test string"
a.reverse!
puts b      # outputs "gnirts tset"

(Sep 13 '11 at 12:22) Bryan Bibat

Answer 2

2

Python's objects are all pass by reference. If you need to pass by value you can do:

from copy import deepcopy
...
P = deepcopy(R)

link

answered Sep 12 '11 at 22:53

Marconi
928●9

edited Sep 12 '11 at 22:54

It's not even about "pass by reference". He's just doing reference copying, not object 'cloning' or deep copying as you've noted.

(Sep 13 '11 at 07:01) Alistair A. Israel

Yes I know, I was giving an example on how you might mimic pass by value.

(Sep 13 '11 at 10:16) Marconi

How come python's object gets referenced through another by assigning object to another object

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