Strange Python Behavior From Inappropriate Usage Of 'is Not' Comparison?

I (incorrectly?) used 'is not' in a comparison and found this curious behavior: >>> a = 256 >>> b = int('256') >>> c = 300 >>> d = int('300') &g

Solution 1:

"is" is not a check of equality of value, but a check that two variables point to the same instance of an object.

ints and strings are confusing for this as is and == can happen to give the same result due to how the internals of the language work.

Solution 2:

For small numbers, Python is reusing the object instances, but for larger numbers, it creates new instances for them.

See this:

>>> a=256
>>> b=int('256')
>>> c=300       
>>> d=int('300')

>>> id(a)
158013588
>>> id(b)
158013588
>>> id(c)
158151472
>>> id(d)
158151436

which is exactly why a is b, but c isn't d.

Solution 3:

Int is an object in python, and python caches small integer between [-5,256] by default, so where you use int in [-5,256], they are identical.

a = 256
b = 256

a is b # True

If you declare two integers not in [-5,256], python will create two objects which are not the same(though they have the same value).

a = 257
b = 257

a is b # False

In your case, using != instead to compare the value is the right way.

a = 257
b = 257

a != b # False

Solution 4:

For more understanding why this occurs take a look to Python-2.6.5/Objects/intobject.c:78:small_ints array and Python-2.6.5/Objects/intobject.c:1292:_PyInt_Init function in python sources.

Also similar thing occurs with lists:

>>> a = [12]
>>> id_a = id(a)
>>> del(a)
>>> id([1,2,34]) == id_a
True
>>> 

Removed lists are not destroyed. They are reused