On: First Impressions of Python from a Java Developer

[Jabbles]

Argh, the complexities of Java!

Integer a = new Integer(10);

Integer b = new Integer(10);

Integer c = new Integer(1000);

Integer d = new Integer(1000);

a==b is true

c==d is false

[reinhardt → Jabbles]

Sorry to break it out to you but you picked a bad example; the equivalent Python code has the same WTF effect:

Python 2.6.6 (r266:84292, Oct 17 2010, 15:53:05)

[GCC 4.4.3] on linux2

>>> a = int(10)

>>> b = int(10)

>>> c = int(1000)

>>> d = int(1000)

>>> a is b

True

>>> c is d

False

[Jabbles → reinhardt]

Wow! Really? (I'm not a Pythonator.)

Same reasoning as Java?

[reinhardt → Jabbles]

Basically yes, although it is not part of the language definition and should not be relied on. It's just an implementation artifact of CPython, and not only that but the threshold of which integers are cached changes for different Python versions.

[jrockway → Jabbles]

It's simple, right? The result of <Integer> == <Integer> is undefined with respect to the value that is stored in the Integer object.

[RodgerTheGreat → jrockway]

A bit more subtle than that- Java reserves singleton instances of Integer for values in the range -128 to 127. In these cases, the Integers will contain the same value and refer to the same object, so == works. It's not actually undefined behavior.