Python Dictionaries: How to Change Key and Value

Python dictionaries let you store data as key-value pairs and update them freely after creation. This chapter covers every practical way to modify existing dictionary items: changing a single value, renaming a key, bulk-updating with update(), and handling nested dictionaries — including the edge cases you will encounter in real code.

For background on how dictionaries work, see the Accessing Dictionary Items chapter. To add brand-new entries, see Add Dictionary Items. To delete entries, see Remove Dictionary Items.

Changing a Value

The simplest modification is reassigning the value stored under an existing key. Use the key as the index on the left-hand side of an assignment:

If the key already exists, the value is overwritten in place. The other entries are not affected and the dictionary preserves its insertion order (Python 3.7+).

What happens when the key does not exist?

If you use assignment with a key that is not already in the dictionary, Python silently creates a new entry instead of raising an error:

prices = {'apple': 1.00, 'banana': 0.50}
prices['mango'] = 2.50   # 'mango' did not exist — a new entry is created
print(prices)
# {'apple': 1.00, 'banana': 0.50, 'mango': 2.50}

When you want to update only if a key exists, check first with in:

if 'banana' in prices:
    prices['banana'] = 0.75

Bulk-Updating with update()

dict.update() lets you change several values at once. Pass it a dictionary (or keyword arguments) of key-value pairs to apply:

inventory = {'apple': 10, 'banana': 5, 'orange': 8}
inventory.update({'banana': 12, 'orange': 3})
print(inventory)
# {'apple': 10, 'banana': 12, 'orange': 3}

Keys present in the argument overwrite the existing values; keys not mentioned are left unchanged. You can also pass keyword arguments directly:

inventory.update(apple=20, banana=15)
print(inventory)
# {'apple': 20, 'banana': 15, 'orange': 3}

update() also adds any keys from the argument that do not yet exist in the target dictionary, making it useful for merging two dictionaries together.

Renaming (Changing) a Key

Dictionary keys are immutable references — you cannot rename a key directly. The standard pattern is to add a new key with the old value and delete the old key. The pop() method does both in one expression:

pop('banana') returns 2 and removes the 'banana' entry. Assigning that return value to my_dict['pear'] creates the renamed entry.

Note that the renamed key appears at the end of the dictionary, not in the original position of 'banana', because it is a new key being inserted.

Renaming a key when you are unsure it exists

If the key might be absent, pop() raises a KeyError. Pass a default as the second argument to avoid the exception:

value = my_dict.pop('grape', None)  # returns None if 'grape' is missing
if value is not None:
    my_dict['kiwi'] = value

Changing Both the Key and the Value

Combine the rename pattern with a value reassignment to change both at once:

Or collapse it into a single step by directly assigning the desired value instead of the popped value:

my_dict = {'apple': 1, 'banana': 2, 'orange': 3}
my_dict.pop('banana')          # remove old key (discard value)
my_dict['pear'] = 4            # insert new key with new value
print(my_dict)
# {'apple': 1, 'orange': 3, 'pear': 4}

Updating Items in a Nested Dictionary

When a dictionary contains other dictionaries as values, use chained indexing to reach the inner value:

catalog = {
    'apple':  {'price': 1.00, 'stock': 50},
    'banana': {'price': 0.50, 'stock': 30},
}

# Update only the price of 'banana'
catalog['banana']['price'] = 0.75
print(catalog['banana'])
# {'price': 0.75, 'stock': 30}

You can also call update() on the inner dictionary:

catalog['apple'].update({'price': 1.20, 'stock': 45})
print(catalog['apple'])
# {'price': 1.20, 'stock': 45}

Common Gotchas

Rebuilding to preserve key order

If you need the renamed key to stay in its original position, rebuild the dictionary with a comprehension:

my_dict = {'apple': 1, 'banana': 2, 'orange': 3}
my_dict = {
    ('pear' if k == 'banana' else k): v
    for k, v in my_dict.items()
}
print(my_dict)
# {'apple': 1, 'pear': 2, 'orange': 3}

This iterates over every existing entry and emits the renamed key only where the condition is true, leaving everything else unchanged.

Practice