Python language basics 56: adding and removing items in a dictionary

Introduction

In the previous post we saw how to get hold of the values and keys in a dictionary. We looked at the three functions to retrieve either the key, the value or both: keys, values, items. All functions have return values that can be iterated in a for-each loop.

In this post we’ll discuss how to update a dictionary by adding new items to it or removing one.

The update method

We’ve already seen how to add a new item to a dictionary using the key indexer:

sizes = {"S": "Small", "M": "Medium", "L": "Large", "XL": "X-Large"}
print("Before: {0}".format(sizes))
sizes["XS"] = "Extra-small"
print("After: {0}".format(sizes))

…which prints:

Before: {‘S’: ‘Small’, ‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘L’: ‘Large’}
After: {‘S’: ‘Small’, ‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘XS’: ‘Extra-small’, ‘L’: ‘Large’}

If you’d like to add several items at once to an existing dictionary you can use the update method which accepts another dictionary:

sizes = {"S": "Small", "M": "Medium", "L": "Large", "XL": "X-Large"}
more_sizes = {"XS": "Extra-Small", "XXL": "Double-extra-large", "S": "Smallish"}
print("Before: {0}".format(sizes))
sizes.update(more_sizes)
print("After: {0}".format(sizes))

Note that existing keys will be updated, so the key “S” will have the value “Smallish” in the updated dictionary:

Before: {‘L’: ‘Large’, ‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘S’: ‘Small’}
After: {‘L’: ‘Large’, ‘XL’: ‘X-Large’, ‘S’: ‘Smallish’, ‘XXL’: ‘Double-extra-large’, ‘XS’: ‘Extra-Small’, ‘M’: ‘Medium’}

The update method even accepts a list of tuples where each tuple can be transformed into a key-value pair:

sizes = {"S": "Small", "M": "Medium", "L": "Large", "XL": "X-Large"}
more_sizes = [("XS", "Extra-small"), ("XXL", "Double-extra-large")]
print("Before: {0}".format(sizes))
sizes.update(more_sizes)
print("After: {0}".format(sizes))

Before: {‘L’: ‘Large’, ‘M’: ‘Medium’, ‘S’: ‘Small’, ‘XL’: ‘X-Large’}
After: {‘XXL’: ‘Double-extra-large’, ‘XS’: ‘Extra-small’, ‘L’: ‘Large’, ‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘S’: ‘Small’}

You can furthermore supply a comma-separated keyword argument list directly in the update method:

sizes = {"S": "Small", "M": "Medium", "L": "Large", "XL": "X-Large"}
print("Before: {0}".format(sizes))
sizes.update(XS = "Extra-small", XXL = "Double-extra-large", S = "Smallish")
print("After: {0}".format(sizes))

Before: {‘M’: ‘Medium’, ‘L’: ‘Large’, ‘XL’: ‘X-Large’, ‘S’: ‘Small’}
After: {‘L’: ‘Large’, ‘XL’: ‘X-Large’, ‘S’: ‘Smallish’, ‘M’: ‘Medium’, ‘XS’: ‘Extra-small’, ‘XXL’: ‘Double-extra-large’}

Item deletion

A key-value pair can be deleted using the ‘del’ keyword which we saw previously where we applied it to remove an item from a list. Del accepts a key-reference as follows:

sizes = {"S": "Small", "M": "Medium", "L": "Large", "XL": "X-Large"}
print("Before: {0}".format(sizes))
del sizes["S"]
print("After: {0}".format(sizes))

Before: {‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘S’: ‘Small’, ‘L’: ‘Large’}
After: {‘M’: ‘Medium’, ‘XL’: ‘X-Large’, ‘L’: ‘Large’}

Read the next post here where we start discussing exceptions.

Read all Python-related posts on this blog here.