Compare if two variables reference the same object in python

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How to check whether two variables reference the same object?

x = ['a', 'b', 'c']
y = x                 # x and y reference the same object
z = ['a', 'b', 'c']   # x and z reference different objects

That’s what is is for.

In the example, x is y returns True because it is the same object while x is z returns False because it are different objects (which happen to hold identical data).

While the two correct solutions x is z and id(x) == id(z) have already been posted, I want to point out an implementation detail of python. Python stores integers as objects, as an optimization it generates a bunch of small integers at its start (-5 to 256) and points EVERY variable holding an integer with a small value to these preinitialized objects. More Info

This means that for integer objects initialized to the same small numbers (-5 to 256) checking if two objects are the same will return true (ON C-Pyhon, as far as I am aware this is an implementation detail), while for larger numbers this only returns true if one object is initialized form the other.

> i = 13
> j = 13
> i is j
True

> a = 280
> b = 280
> a is b
False

> a = b
> a
280
> a is b
True

y is x will be True, y is z will be False.

You can also use id() to check which unique object each variable name refers to.

In [1]: x1, x2 = 'foo', 'foo'

In [2]: x1 == x2
Out[2]: True

In [3]: id(x1), id(x2)
Out[3]: (4509849040, 4509849040)

In [4]: x2 = 'foobar'[0:3]

In [5]: x2
Out[5]: 'foo'

In [6]: x1 == x2
Out[6]: True

In [7]: x1 is x2
Out[7]: False

In [8]: id(x1), id(x2)
Out[8]: (4509849040, 4526514944)

I really like to have a visual feedback, that’s why I sometimes just open up http://www.pythontutor.com/visualize.html#mode=edit to see how the memory is allocated and what is referencing what.

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Added this awesome gif as this reply is about visualizing..

This is from docs.python.org: “Every object has an identity, a type and a value. An object’s identity never changes once it has been created; you may think of it as the object’s address in memory. The ‘is’ operator compares the identity of two objects; the id() function returns an integer representing its identity.”

Apparently every time you change the value the object is recreated as indicated by the identity changing. The line x=3 followed by the line x=3.14 gives no error & gives different identities, types and values for x.


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