- [Let's see if you can guess this?](#lets-see-if-you-can-guess-this)

- [💡 Explanation:](#-explanation-18)

Originally, suggested by @PiaFraus in [this](https://github.com/satwikkansal/wtfPython/issues/9) issue.

a, b = a[b] = {}, 5

**Output:**

>>> a

{5: ({...}, 5)}

* According to [Python language reference](https://docs.python.org/2/reference/simple_stmts.html#assignment-statements), assignment statements have the form

```

(target_list "=")+ (expression_list | yield_expression)

```

and

> An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.

* The `+` in `(target_list "=")+` means there can be **one or more** target lists. In this case, target lists are `a, b` and `a[b]` (note the expression list is exactly one, which in our case is `{}, 5`).

* After the expression list is evaluated, it's value is unpacked to the target lists from **left to right**. So, in our case, first the `{}, 5` tuple is unpacked to `a, b` and we now have `a = {}` and `b = 5`.

* `a` is now assigned to `{}` which is a mutable object.

* The second target list is `a[b]` (you may expect this to throw an error because both `a` and `b` have not been defined in the statements before. But remember, we just assigned `a` to `{}` and `b` to `5`).

* Now, we are setting the key `5` in the dictionary to the tuple `({}, 5)` creating a circular reference (the `{...}` in the output refers to the same object that `a` is already referencing). Another simpler example of circular reference could be

```py

>>> some_list = some_list[0] = [0]

>>> some_list

[[...]]

>>> some_list[0]

[[...]]

>>> some_list is some_list[0]

[[...]]

```

Similar is the case in our example (`a[b][0]` is the same object as `a`)

* So to sum it up, you can break the example down to

```py

a, b = {}, 5

a[b] = a, b

```

And the circular reference can be justified by the fact that `a[b][0]` is the same object as `a`

```py

>>> a[b][0] is a

True

```