Dictionaries are quite useful and commonly used in python. It may happen that you require to add new keys or update an existing one in a dictionary. In this tutorial, we’ll look at how you can add or update items in a dictionary.

Before we proceed, here’s a quick refresher on dictionaries in python – Dictionaries are a collection of items used for storing key to value mappings. They are mutable and hence we can update the dictionary by adding new key-value pairs, removing existing key-value pairs, or changing the value corresponding to a key. For more, check out our guide on dictionaries and other data structures in python.

## How to add or update items in a dictionary ?

You can use the subscript notation, which is, accessing the key or creating a new one using square brackets `[]`

and then providing the corresponding value. Also, you can use the dictionary method `update()`

to add or update a key in an existing dictionary. There are other ways as well but for the purpose of this tutorial we’ll be limiting to these two are they are the common ones.

### Using the subscript notation

The subscript notation is used to refer the value corresponding a key in a dictionary. For example, in the dictionary `d = {'a':1, 'b':2}`

, the subscript notation `d['a']`

will give you the value corresponding to the key `'a'`

in the dictionary.

**To add a new key to the dictionary** you can simply use the subscript notation with the new key and assign its respective value using the assignment operator `=`

.

**Example**: Add a new key to the dictionary using subscript notation

```
# add item to a dictionary
# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50}
# print
print("Shares in your portfolio:", shares)
# add 80 shares of TSLA to the portfolio using subscript notation
shares['TSLA'] = 80
# print
print("Shares in your portfolio:", shares)
```

Output:

```
Shares in your portfolio: {'APPL': 100, 'GOOG': 50}
Shares in your portfolio: {'APPL': 100, 'GOOG': 50, 'TSLA': 80}
```

In the above example, the dictionary `shares`

stores the number of shares of different companies in a sample portfolio. The key represents the company and the value represents the number of shares of the company in the portfolio. A new key `TSLA`

is added to the dictionary with the value `80`

using the subscript notation.

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**To update an existing key** you can simply refer to it using the subscript notation and they change its value.

**Example:** Update an existing key using the subscript notation

```
# update item in a dictionary
# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50}
# print
print("Shares in your portfolio:", shares)
# update the shares of 'GOOG' to 150
shares['GOOG'] = 150
# print
print("Shares in your portfolio:", shares)
```

Output:

```
Shares in your portfolio: {'APPL': 100, 'GOOG': 50}
Shares in your portfolio: {'APPL': 100, 'GOOG': 150}
```

In the above example, the existing value corresponding to the key `'GOOG'`

is changed to `150`

using the subscript notation.

The subscript notation, that is accessing and changing keys using the square brackets `[]`

is a simple and intuitive way of adding or updating a dictionary.

### Using the update() function

`update()`

is a dictionary function in python used to update a dictionary by changing the values of existing keys or adding new keys.

**Example**: Add a new key to the dictionary using the update function

```
# add item to a dictionary
# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50}
# print
print("Shares in your portfolio:", shares)
# add 80 shares of TSLA to the portfolio using the update function
shares.update({'TSLA': 80})
# print
print("Shares in your portfolio:", shares)
```

Output:

```
Shares in your portfolio: {'APPL': 100, 'GOOG': 50}
Shares in your portfolio: {'APPL': 100, 'GOOG': 50, 'TSLA': 80}
```

In the above example, the new key `TSLA`

with the value `80`

is added to the dictionary using the dictionary function `update()`

. Note that the `update()`

function modifies the dict **in-place**.

**Example:** Update an existing key using the update function

```
# add item to a dictionary
# dictionary of a sample portfolio
shares = {'APPL': 100, 'GOOG': 50}
# print
print("Shares in your portfolio:", shares)
# update the shares of 'GOOG' to 150
shares.update({'GOOG': 150})
# print
print("Shares in your portfolio:", shares)
```

Output:

```
Shares in your portfolio: {'APPL': 100, 'GOOG': 50}
Shares in your portfolio: {'APPL': 100, 'GOOG': 150}
```

In the above example, the `update()`

function is used to update the value of the existing key `'GOOG'`

to `150`

.

For more on the update function refer to the python docs.

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