Shared reference and equality

[zyo]

Hi everybody,
I am starting to learn Python in these days: I came across the topic of shared references and equality, and I thought I had understood its basics. However, I came across an example where things were not so clear to me.
Here it is:

a = 'a'
b = 'b'
ab = 'ab'
a is 'a'     # returns True
b is 'b'     # returns True
ab is 'ab'   # returns False

Why does the 3rd statement return false? Is it linked to the lenght of the string 'ab'? Thanks for helping

[woooee]

Printing the id will show if you are referencing the same block of memory (is)

print(id("ab"))
## 139660486771968

x="a"
x += "b"
print(x)
## "ab"
print(id(x))
## 139660486772024  different memory location 

[volcano63]

Small integers - -5 to 255, ASCII letters, Booleans and None have dedicated constant objects associated with them. Still, the only case when is operator is recommended is a test for None value

[ljmetzger]

Like @volcano63 said, is tests if two variables point to the same object.

To check if values are the same use ==. In the example below mylist1 and mylist2 point to the same object. mylist3 is a copy of mylist1.

Python 3.7.0 (v3.7.0:1bf9cc5093, Jun 27 2018, 04:06:47) [MSC v.1914 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> a = 'a'
>>> b = 'b'
>>> ab = 'ab'
>>> a == 'a'
True
>>> b == 'b'
True
>>> ab == 'ab'
True

>>> mylist1 = [1, 2, 3]
>>> mylist2 = mylist1
>>> mylist1 == mylist2
True
>>> mylist1 is mylist2
True

>>> mylist3 = list(mylist1)
>>> mylist3 == mylist1
True
>>> mylist3 is mylist1
False

For other methods to create a copy see https://stackoverflow.com/questions/2612...opy-a-list
Other methods include:

import copy
 
b = a[:]
b = list(a)
b = copy.copy(a)
b = copy.deepcopy(a)   

Lewis