Is

== Equality of Value

The == operator checks if two values are equal. However it is important to understand that equal does not always mean the same type. For example '1' == 1 returns False because even though they look similar, '1' is a string and 1 is an integer. They are not the same type so Python considers them unequal.

is - Equality of Memory

The is keyword does something completely different. Instead of asking "are these values equal?", it asks "are these the exact same object in memory?" Every time Python creates a value it stores it somewhere in memory. is checks whether two variables are pointing to the exact same location in memory, not just whether they contain the same value.

This is why '1' is '1' returns True and True is True returns True. Python stores simple values like short strings and Booleans in a shared memory location and reuses them, so they point to the same place.

But [] is [] returns False. This surprises a lot of beginners. Even though both lists are empty and look identical, every time you create a list Python creates a brand new object in a brand new location in memory. The two lists may contain the same values but they live in different places, so is returns False.

The same applies to any list regardless of its contents. Even [1, 2, 3] is [1, 2, 3] returns False because the two lists were created separately and occupy different locations in memory.

To summarise:

• == checks if the values are the same
• is checks if the location in memory is the same

Try each of the following in the box below and run it each time to see what Python returns:

Exercise 1: Predict the Output

• Task: Before running the code, write a comment predicting what each expression will return. Then run each one and check if you were right.

• Goal: Write a comment next to each result explaining why Python returned that value.

Exercise 2: The List Trap

• Task: Create two variables list_a = [1, 2, 3] and list_b = [1, 2, 3].
• Goal: Run the following checks and write a comment explaining why each one returns what it does:

Exercise 3: Same Object

• Task: Create a variable list_a = [1, 2, 3] and assign it to a second variable list_b = list_a.
• Goal: Run the following checks and write a comment explaining why the result is different from Exercise 2:

Exercise 4: Spot the Difference

• Task: Look at the two blocks of code below and predict what each will print before running them.

• Goal: Write a comment explaining why a is b and a is c may return different results even though all three variables contain the word "hello".

Exercise 5: When to use is and When to use ==

• Task: Create a variable result = None.
• Goal: Write two checks. First use == to check if result is None and then use is to check the same thing. Print both results and write a comment explaining which one is the correct and recommended way to check for None in Python and why.