Python Forum

I tried out like this

class Spam:
    pass
def fun(a):
    a.foo=5
    print(a.foo)
    print(id(a.foo))

foo=6
id1=5
s=Spam()
fun(s)
print(s.foo)
print(foo)
print(id(foo))
print(id1)
print(id(id1))

And the output is

Output:
5
1918656864
5
6
1918656896
5
1918656864

Interesting.. looks like the same memory reference for value 5 is being shared by both variables a.foo and id1

Malt Wrote:looks like the same memory reference for value 5 is being shared by both variables a.foo and id1

Yes, python keeps an array of instances for the small integers and reuses them. See here. This is an implementation detail.

λ python                                                                                          
Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)] on win32 
Type "help", "copyright", "credits" or "license" for more information.                            
>>> hash(2)                                                                                       
2                                                                                                 
>>> hash(1)                                                                                       
1                                                                                                 
>>> hash(0)                                                                                       
0                                                                                                 
>>> hash(-1)                                                                                      
-2                                                                                                
>>> hash(-2)                                                                                      
-2                                                                                                
>>> hash(-3)                                                                                      
-3                                                                                                
>>>                                                                                               

If you check values for equality, don't use identity checks (is/is not). Use equality check for integers and comparison for floats.
If you want to check for Types of objects, which exists only once in memory, it's save to use the is operator.

*to check floats of equality, use math.isclose.