Python create new object with the same value [duplicate]

Question

I come from java world where I expect following things

int a = valueassignedbyfunction();
int b = a;
a = a + 1; 

after this a is 1 greater than b. But in python the b automatically gets incremented by one once the a = a + 1 operation is done because this b is referencing to the same object as a does. How can I copy only the value of a and assign it to a new object called b?

Thanks!

Legalese

I think you are wrong here. Integer objects are immutable in Python. They can't be changed. – Albuminoid

Answer 1

Assuming integers, I cannot reproduce your issue:

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

If we assume objects instead:

class Test(object):
...     def __init__(self, v):
...         self.v = v
...         
>>> a = Test(1)
>>> b = a.v
>>> a.v += 1
>>> print a.v, b
2 1
# No issues so far
# Let's copy the object instead
>>> b = a
>>> a.v += 1
>>> print a.v, b.v
3 3
# Ah, there we go
# Using user252462's suggestion
>>> from copy import deepcopy
>>> b = deepcopy(a)
>>> a.v += 1
>>> print a.v, b.v
4 3

Decennial

Note that this is exactly how it works in Java and most other languages. Nothing unusual here. – Wellspoken

@delnan: In C++, b = a creates copies of user defined classes by default (unless you overload the assignment operator). – Albuminoid

@Sven: Yes, but C++ is rather unique among widely used high-level languages (in several areas, not only here). For example, Java, C#, VB.NET, Ruby, and propably many others have similar semantics. Admittedly, should be s/most/many/ in my first comment, but still applies for OP. – Wellspoken

Answer 2

This documentation might help out: http://docs.python.org/library/copy.html

You can use the copy library to deepcopy objects:

import copy
b = copy.deepcopy(a)

Catherin

I think this is wrong, per the answers from Sean, Sven and dfb, integers in Python are immutable, you'd only use copy on mutable objects. – Caret

Answer 3

I think the main confusion here is the following: In Java, a line like

int i = 5;

allocates memory for an integer and associates the name i with this memory location. You can somehow identify the name i with this memory location and its type and call the whole thing "the integer variable i".

In Python, the line

i = 5

evaluates the expression on the right hand side, which will yield a Python object (in this case, the expression is really simple and will yield the integer object 5). The assignment statement makes the name i point to that object, but the relation between the name and the object is a completely different one than in Java. Names are always just references to objects, and there may be many names referencing the same object or no name at all.

Albuminoid

Note that for Objects, Java (as well as several other languages) too only store a reference with all of the observable effects. It's only the special casing for primitive types that differs so radically. Still +1'd as it provides a decent writeup on the variables and object model. – Wellspoken

Answer 4

I'm not sure what you're seeing here.

>>> a = 1 
>>> b = a
>>> a = a + 1
>>> b
1
>>> a
2
>>> a is b
False

Python Integers are immutable, the + operation assigns creates a new object with value a+1. There are some weird reference issues with integers (http://distilledb.com/blog/archives/date/2009/06/18/python-gotcha-integer-equality.page), but you should get the same thing you expected in Java

Ebert

Answer 5

How about just doing

a = 1
b = a*1

Traction