Python Dictionary Update: 5 Ways With Examples

If you’ve tried a Python dictionary update like prices = prices.update(feed), you’ve probably seen your whole dictionary turn into None.

That’s because dict.update() changes the dictionary in place and returns nothing. Call it on its own line: prices.update(feed).

In this tutorial, we’ll do a python dictionary update five ways on one price list, from update() to |= and nested data. I ran it all on Python 3.12.5.

The sample price list for our Python dictionary update

A store keeps prices by SKU, and a vendor feed sends changes:

prices = {"SKU-101": 19.99, "SKU-102": 4.50, "SKU-103": 129.00}
feed = {"SKU-102": 4.75, "SKU-104": 59.99}

SKU-102 should change to $4.75 and SKU-104 is new. New to dictionaries? Start with how Python dictionaries work.

Here’s that mistake on the sample:

from inventory import prices, feed

prices = prices.update(feed)
print(prices)
print(prices["SKU-102"])

Output:

None
Traceback (most recent call last):
  File "C:\pyguides\assign_update.py", line 5, in <module>
    print(prices["SKU-102"])
          ~~~~~~^^^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable

Method 1: Update a dictionary with dict.update() (Recommended)

Use update() when another dictionary holds the new values. Same price list and feed:

from inventory import prices, feed

print("Before:", prices)
prices.update(feed)
print("After: ", prices)
print(f"{len(prices)} SKUs, SKU-102 now ${prices['SKU-102']:.2f}")

Output:

Before: {'SKU-101': 19.99, 'SKU-102': 4.5, 'SKU-103': 129.0}
After:  {'SKU-101': 19.99, 'SKU-102': 4.75, 'SKU-103': 129.0, 'SKU-104': 59.99}
4 SKUs, SKU-102 now $4.75

How does this code work?

For each key in feed, Python sets prices[key] = value, overwriting existing keys and adding missing ones at the end.

That’s why the method is also the answer to “how do I extend a dictionary”. To change single values instead, see updating values in a Python dictionary.

How to update a dictionary from tuples, zip() or keyword arguments

update() also takes any iterable of key-value pairs, plus keyword arguments:

from inventory import prices, feed

rows = [("SKU-101", 18.49), ("SKU-105", 7.25)]
prices.update(rows)
prices.update(zip(["SKU-103"], [119.00]))
print(prices)

settings = {"store": "Austin, TX", "tax_rate": 0.0625}
settings.update(tax_rate=0.0825, currency="USD")
print(settings)

Output:

{'SKU-101': 18.49, 'SKU-102': 4.5, 'SKU-103': 119.0, 'SKU-105': 7.25}
{'store': 'Austin, TX', 'tax_rate': 0.0825, 'currency': 'USD'}

SKU-101 drops to $18.49, SKU-103 to $119.00, SKU-105 is added, and the tax rate becomes 8.25%.

How does this code work?

Each tuple must be exactly one key and one value. The zip() function builds those pairs from two lists.

Important: keyword arguments need keys that are valid Python names; SKU-101=18.49 is a syntax error.

Method 2: Use the | and |= dictionary merge operators (Python 3.9+)

Python 3.9 added |, which builds a new dictionary, and |=, which updates in place. Same price list:

from inventory import prices, feed

merged = prices | feed
print("New dict:", merged)
print("Original:", prices)

prices |= {"SKU-101": 17.99}
print("After |=:", prices)

Output:

New dict: {'SKU-101': 19.99, 'SKU-102': 4.75, 'SKU-103': 129.0, 'SKU-104': 59.99}
Original: {'SKU-101': 19.99, 'SKU-102': 4.5, 'SKU-103': 129.0}
After |=: {'SKU-101': 17.99, 'SKU-102': 4.5, 'SKU-103': 129.0}

How does this code work?

The right side wins on duplicate keys, just like update().

More ways to join two dictionaries: concatenating dictionaries in Python.

Method 3: Update only existing keys, or only add missing ones

Sometimes the feed mustn’t add SKUs, or defaults mustn’t overwrite real prices. Same sample data, filtered first:

from inventory import prices, feed

known = {k: v for k, v in feed.items() if k in prices}
prices.update(known)
print("Existing keys only:", prices)

catalog = {"SKU-101": 0.00, "SKU-106": 12.00}
for sku, price in catalog.items():
    prices.setdefault(sku, price)
print("Missing keys only: ", prices)

Output:

Existing keys only: {'SKU-101': 19.99, 'SKU-102': 4.75, 'SKU-103': 129.0}
Missing keys only:  {'SKU-101': 19.99, 'SKU-102': 4.75, 'SKU-103': 129.0, 'SKU-106': 12.0}

Pass one changes only SKU-102. Pass two adds SKU-106 at $12.00 and keeps SKU-101 at $19.99.

How does this code work?

The dictionary comprehension keeps only feed keys already in prices.

setdefault() writes only when the key is missing, a safe way to extend a dict. For adding keys in general, see Python dictionary append.

Why does update() replace a nested dictionary?

update() is shallow, so an inner dictionary gets replaced, not merged:

store = {"TX": {"tax": 0.0625, "open": True}}
store.update({"TX": {"tax": 0.0825}})
print("update():  ", store)

def deep_update(target, changes):
    for key, value in changes.items():
        if isinstance(value, dict) and isinstance(target.get(key), dict):
            deep_update(target[key], value)
        else:
            target[key] = value

store = {"TX": {"tax": 0.0625, "open": True}}
deep_update(store, {"TX": {"tax": 0.0825}})
print("deep merge:", store)

Output:

update():   {'TX': {'tax': 0.0825}}
deep merge: {'TX': {'tax': 0.0825, 'open': True}}

How does this code work?

deep_update() recurses when both sides hold a dictionary and assigns otherwise.

Which Python dictionary update method should you use?

Situation	Use	Why
Change or add from another dict	d.update(other)	Any Python 3
Pairs from a CSV or zip()	d.update(pairs)	No dict needed
Keep the original	new = d | other	New dict (3.9+)
Only existing / only missing keys	Comprehension / setdefault()	Picks the keys
Nested dictionaries	Recursive helper	update() is shallow

I default to update(), and | when the original must survive.

Things to keep in mind when you update a dictionary

• Don’t assign it. update() returns None.
• Don’t update while looping over the same dict. Adding keys mid-loop raises RuntimeError; iterate over a copy.
• Watch key spelling. A typo silently adds a key; later lookups raise a dictionary KeyError.

Keep going with these dictionary guides:

• Increment a value in a Python dictionary
• Remove an item from a Python dictionary
• Change a key in a Python dictionary
• Copy a Python dictionary

Frequently asked questions