when to use in/not in/is/is not in Python ??

First let's cover in/not in.

in is an easy way to check if an element is in a list or not. not in is similar, but instead it checks if an element is not in a list.

>>> my_list = [1, 2, 3]
>>> 0 in my_list
False
>>> 1 in my_list
True
>>> 0 not in my_list
True

is/is not is a little more difficult to understand.

is is similar to ==, but it checks for exact equivalence. == checks if all of the attributes of one object and all of the methods of one object is equal to all of the attributes and methods of another.

But, is checks if two objects are exactly equivalent. This means that the memory location (aka ID) must be equal too.

>>> 1 is 1
True
>>> "abc" is "abc"
True
>>> a = 1
>>> b = 1
>>> a == b
True
>>> a is b
True
>>> a = [1, 2, 3]
>>> b = [1, 2, 3]
>>> a == b
True
>>> a is b
False

Python is pretty memory-efficient, so every time you make a number/string/boolean, it points to the same location in memory. However, with lists, every time you make a new list, it actually does make a brand new list and assigns the value to its own place in memory.

1 ----> 4481526416
       ^
      /
     /
    /
1 -

4481526416 is a location in memory. Both times you refer to 1 it points to that location in memory.

[1, 2, 3] -----> 4444834184

[1, 2, 3] -----> 4444832776

Lists are less efficient. When you make a list, it links it to a new location in memory.

Therefore [1, 2, 3] is [1, 2, 3] is False and 1 is 1 is True!

I hope this wasn't too technical for you! is is a more advanced keyword, so don't worry if you don't quite understand it.

Just make you understand in, and you're fine.