Have you heard the one about is, ==, and the £5 note that it’s your pocket?

You may have already come across the fact that is and == are not the same in Python and cannot be used interchangeably… if you haven’t, you heard it now!

But why? Let’s see what the £5 note has got to do with this story

Let’s assume you and I both have a £5 note in our pocket

[replace with currency of choice]
and
[read “bill” instead of “note” if you wish]

We meet up and go to a coffee shop nearby to buy two coffees which cost £5 altogether

Does it matter which £5 note we use to pay?

/1

[this is like a Netflix series you can binge on, no need to wait until next week for the next episode, read on in the next toot in this thread…]

So, does it matter which £5 note we use to pay?

No, all the barista cares about is that our_payment == 5

So _you_ can pay. Thank you!

In this case, the *value* of the £5 note is what matters, not which actual physical note we use.

But let’s assume we’re chatting and we spot a £5 note on the floor.

We both check our pockets and we realise we’ve both lost our £5 note.

Is the note on the floor mine or yours?

/2

Its value is no longer what matters now.

We both had a £5 note.

What matters is whether this is the actual physical note that was in my pocket or yours.

Luckily, we’re both geeks and we keep a record of the serial numbers of all the notes we carry with us on our phone.

I know, that’s quite sad.

And quite unlikely, too.

But bear with me.

So, we both get our phones out to check the serial numbers on record against the serial number on the £5 note on the floor.

You can see the serial number in two places on the £5 note.

/3

Sorry, the serial number of the note shows it’s _mine_!

This *is* my note. In this case, it wasn’t == that matters but is

With a object, the built-in id() function gives you the “serial number”, which is unique for each object, just as the serial number on a bank note is unique.

In , as in the case with bank notes, you often care more about the value of an object rather than its identity.

So, you’ll often need == just like the barista in the coffee shop didn’t care about the £5 note’s serial number.

/4

But, there are times when you want to know that an object is exactly the same object and not just equal in value.

In these cases, you’ll need to use is.

As a general rule, use == by default unless you know you need is.

But now can we stop squabbling about who the £5 note belongs to?!

/5

@s_gruppetta I actually haven’t really ever encountered ‘is’ myself. I gather it’s probably fairly rare to see this?

@trinsec It’s a lot more likely to need ==, but the use of is does come about when you need to check that two objects are the exact same ones. It’s not that uncommon.

Also, some prefer to use is with singletons such as None

if stuff is None:

Although in this case, there’s no difference between == and is as there is only one None

Here’s an example:

You have a number of teams of people:

teams = [["Bob", "Mary"], ["James", "Kate"], ["Bob", "Mary"]]

Unfortunately, Bob and Mary are common names and there are two teams that each have a Bob and a Mary. I know, the coincidence of that!

>>> teams[0] == teams[2]
True

But, they’re not the same team! They’re different people!

>>> teams[0] is teams[2]
False

If you want to know whether these are the same teams, you need `is' in this case

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