How to Update Values in a Python Dictionary

Assign to the key. That’s the whole trick to update values in a Python dictionary: d[key] = new_value replaces the old value, and d.update() changes several keys in one call:

prices["coffee"] = 3.75
prices.update({"bagel": 2.55, "muffin": 2.66})

A small coffee-shop price list runs through every example, and each output was captured on Python 3.12.5.

In this tutorial, we’ll change one value, several values, only existing keys, every value at once, and values inside nested dictionaries.

Change one dictionary value with square-bracket assignment

Name the key on the left and the new value on the right. Augmented operators like += read the current value first, so price changes stay one line:

prices = {"coffee": 3.50, "bagel": 2.25, "muffin": 2.95}

prices["coffee"] = 3.75          # replace a value
prices["bagel"] += 0.30          # change it based on the old value
prices["muffin"] = round(prices["muffin"] * 0.9, 2)   # 10% off

for item, price in prices.items():
    print(f"{item:<7} ${price:.2f}")

Output:

coffee  $3.75
bagel   $2.55
muffin  $2.66

Counters work the same way; incrementing a value in a Python dictionary has more patterns for tallies.

Update several values at once with update()

update() accepts another dictionary or keyword arguments. Be careful, though: any key it doesn’t find gets added, not rejected:

stock = {"coffee": 40, "bagel": 25, "muffin": 18}

stock.update({"coffee": 36, "bagel": 20})   # several keys in one call
stock.update(muffin=12)                     # keyword form
print(stock)

stock.update(scone=8)                       # a missing key gets ADDED
print(stock)

Output:

{'coffee': 36, 'bagel': 20, 'muffin': 12}
{'coffee': 36, 'bagel': 20, 'muffin': 12, 'scone': 8}

That silent insert is why I don’t use update() for corrections to known fields. Merging with | and |= is covered in my Python dictionary update() guide.

How do I update a value only if the key exists?

Check with in before assigning. For counters, get(key, 0) supplies a starting value. Without either, += on a missing key raises KeyError:

stock = {"coffee": 36, "bagel": 20}

def set_existing(d, key, value):
    if key in d:
        d[key] = value
        return True
    return False

print(set_existing(stock, "bagel", 15), stock)
print(set_existing(stock, "donut", 10), stock)

sales = {}
for item in ["coffee", "bagel", "coffee"]:
    sales[item] = sales.get(item, 0) + 1   # safe even when the key is new
print(sales)

try:
    stock["donut"] += 1
except KeyError as err:
    print("KeyError:", err)

Output:

True {'coffee': 36, 'bagel': 15}
False {'coffee': 36, 'bagel': 15}
{'coffee': 2, 'bagel': 1}
KeyError: 'donut'

The full set of options for missing keys is in handling KeyError with try-except in a Python dictionary.

Update every value in a dictionary with a loop or comprehension

A comprehension builds a fresh dictionary, which suits a blanket change like an 8% price rise. A loop edits the original and handles conditions:

prices = {"coffee": 3.75, "bagel": 2.55, "muffin": 2.66}

# comprehension: builds a NEW dict with every price up 8%
raised = {item: round(p * 1.08, 2) for item, p in prices.items()}
print(raised)

# loop: changes the SAME dict in place, only for cheap items
for item in prices:
    if prices[item] < 3:
        prices[item] = 3.00
print(prices)

Output:

{'coffee': 4.05, 'bagel': 2.75, 'muffin': 2.87}
{'coffee': 3.75, 'bagel': 3.0, 'muffin': 3.0}

Changing values while looping is safe; adding or deleting keys isn’t. More on that in iterating through a Python dictionary, and Python dictionary comprehension goes deeper.

Update values in a nested dictionary

Chain the keys to reach the inner value. A list value can be changed in place with its own methods, with no reassignment needed:

orders = {
    "A-1001": {"customer": "Emily", "status": "pending", "items": ["latte"]},
    "A-1002": {"customer": "James", "status": "pending", "items": ["bagel"]},
}

orders["A-1001"]["status"] = "shipped"        # inner value
orders["A-1002"]["items"].append("muffin")     # list value changed in place
orders["A-1002"].setdefault("shipped_on", "2026-10-18")

for oid, info in orders.items():
    print(oid, info)

Output:

A-1001 {'customer': 'Emily', 'status': 'shipped', 'items': ['latte']}
A-1002 {'customer': 'James', 'status': 'pending', 'items': ['bagel', 'muffin'], 'shipped_on': '2026-10-18'}

Reading deep values that might be missing? See KeyError in a nested Python dictionary.

Keep going with these dictionary tutorials:

• Sum all values in a Python dictionary
• Create a dictionary in Python using a for loop
• Count the keys in a Python dictionary
• Copy a Python dictionary

Frequently asked questions