Why does “a is b” behave differently on Interactive mode and when it’s ran from script? [duplicate]

by

While trying to answer a question about the use of the is keyword, I figured out that this code:

Script:

a = 123456
b = 123456
print a is b # True

Interactive mode:

>>> a = 123456
>>> b = 123456
>>> a is b
False

was giving different outputs on Python Interactive mode and when it was ran from a script.

From this answer:

The current implementation keeps an array of integer objects for all integers between -5 and 256, when you create an int in that range you actually just get back a reference to the existing object.

So, I would expect that a is b returned True only for integers in the range [-5, 256]. But it is only true on Interactive mode, not when it is ran from a script.

Question: Why does a is b behaves differently on Interactive mode and when it’s ran from script?

Note: Tested in Python 2.7 and Python 3

  • 2

    Results are the same on Python 3. False for interactive, True for script.

    – 




  • Python 3.3 behaves as 2.7. Interestingly, >>> a=b; >>> a is b returns True.

    – 

The difference is, how constants are handled. In interactive mode, there is no way to say, if a number constant is already there or not. But for compiled code, every constant is internally saved to a table, and duplicates are removed. But this is a implementation detail, and need not be true for every python version.

Leave a Comment