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Use my_dict.update(other) to add entries to an existing dictionary. It changes the dictionary in place, returns None, and replaces the old value when an incoming entry has the same key. In Python 3.9 and later, | creates a merged dictionary and |= updates one in place.
Choose the right way to extend a dictionary
| Need | Use | Behavior |
|---|---|---|
| Add entries to an existing dictionary | d.update(other) |
Mutates d, returns None, and incoming values replace conflicts. |
| Merge two dictionaries while keeping both inputs unchanged | merged = left | right |
Creates a new dictionary; values from right win conflicts. Available in Python 3.9 and later. |
| Update an existing dictionary with operator syntax | left |= other |
Mutates left. Available in Python 3.9 and later. |
| Add or replace a single entry | d[key] = value |
Sets the value for that one key. |
Use update() to add or replace entries
Pass a mapping, an iterable of key-value pairs, or keyword arguments whose names are valid Python identifiers. For each key, the incoming value becomes the value stored in the dictionary.
settings = {"theme": "light", "font_size": 12}
settings.update({"theme": "dark", "show_tips": True})
print(settings)
# {'theme': 'dark', 'font_size': 12, 'show_tips': True}Here, theme already existed, so its value changed to "dark"; show_tips was added. The original dictionary is modified.
Pass key-value pairs
update() also accepts an iterable of pairs, such as a list of two-item tuples:
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d = {"a": 1}
d.update([("b", 2), ("c", 3)])
print(d)
# {'a': 1, 'b': 2, 'c': 3}Do not assign the return value
update() changes the dictionary directly and returns None. This common mistake replaces your dictionary variable with None:
d = {"a": 1}
d = d.update({"b": 2}) # d is now NoneCall d.update(...) on its own when you want to modify d.
Use dictionary merge operators in Python 3.9 and later
Create a new merged dictionary with |
The | operator combines two dictionaries without changing either input. When a key occurs in both, the right-hand dictionary supplies the value.
base = {"timeout": 10, "retries": 2}
custom = {"timeout": 30}
merged = base | custom
print(merged)
# {'timeout': 30, 'retries': 2}
print(base)
# {'timeout': 10, 'retries': 2}This operator requires dictionary operands; it does not accept a list of pairs as its right-hand side.
Update in place with |=
Use |= when you want operator syntax but need to modify the dictionary on the left. Like update(), it accepts a mapping or an iterable of key-value pairs.
d = {"a": 1}
d |= [("b", 2)]
print(d)
# {'a': 1, 'b': 2}Both merge operators were added in Python 3.9. Use update() if your code must run on an earlier Python version.
What happens when keys conflict?
For update(), |=, and |, an incoming value replaces the value for an existing key. With the merge operator, the rightmost value wins. These operations do not preserve both values or append one value to another.
When the input mapping has an order, newly added keys follow that mapping’s order. Existing keys retain their position when their values are replaced.
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Dictionary extension operates on the top-level keys and values; it does not recursively merge nested dictionaries or append to collections stored as values. Choose and perform the value-level operation you want explicitly.
Append to a list stored under a key
d = {"items": ["pen"]}
d["items"].append("notebook")Update a nested dictionary explicitly
d = {"options": {"color": "blue", "size": "small"}}
d["options"].update({"size": "large", "enabled": True})This example updates the nested dictionary itself. It does not make dict.update() perform a recursive merge automatically.
Quick Recap
Common mistakes to avoid
- Do not write
d = d.update(other); the method returnsNone. - Do not use
|with a list of key-value pairs. Useupdate()or, in Python 3.9 and later,|=. - Do not expect duplicate keys to keep both values or append collection contents; the incoming value replaces the existing one.
- Do not assume nested dictionaries are merged recursively. Specify the operation for their values explicitly.
References
- Python documentation: Dictionary view objects and dictionary methods
- PEP 584: Add Union Operators To dict
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