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dict.update()

Merges another mapping or key/value pairs into a dict in-place.

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data.update(other)
data.update(key=value)
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What it does

Overview

Merges another mapping or key/value pairs into a dict in-place.

The dict.update() method merges a dictionary with another mapping or an iterable of key-value pairs (such as a list of tuples). It performs an in-place mutation, meaning it modifies the original object and returns None. If keyword arguments are provided, they are processed after the first argument, allowing for specific overrides. Performance is typically O(K), where K is the number of elements in the incoming data, as it must iterate and hash each key. A critical edge case involves key collisions: if a key exists in both the target and the source, the target's value is overwritten by the source's value. This method is preferred over manual looping for efficiency and readability when combining configuration objects or applying default values.

Quick reference

Syntax

data.update(other)
data.update(key=value)

Inputs

Parameters

other (optional)Mapping | Iterable[tuple] · Dict or iterable of (key, value) pairs.
**kwargs (optional)Any · Extra key=value pairs to add/overwrite.

See it in practice

Examples

1

Merging Two Dictionaries

settings = {'theme': 'light', 'notify': True}
extra_config = {'theme': 'dark', 'version': 2.0}
settings.update(extra_config)
print(settings)
Output:
{'theme': 'dark', 'notify': True, 'version': 2.0}

Existing keys like 'theme' are overwritten, while new keys like 'version' are added.

2

Updating with Keyword Arguments

profile = {'user': 'admin'}
profile.update(status='active', attempts=3)
print(profile)
Output:
{'user': 'admin', 'status': 'active', 'attempts': 3}

Keyword arguments allow adding or updating specific keys directly without creating a temporary dictionary.

3

Using an Iterable of Tuples

counts = {'a': 1}
updates = [('b', 2), ('c', 3)]
counts.update(updates)
print(counts)
Output:
{'a': 1, 'b': 2, 'c': 3}

The method accepts any iterable containing pairs of items, making it compatible with list of tuples or generated sequences.

Debug faster

Common Errors

1

LogicError

Cause: Assuming update() returns the modified dictionary for chaining or assignment.

Fix: Call update() on a separate line; do not assign its result to a variable.

data = {'x': 1}
new_data = data.update({'y': 2})  # new_data is now None
2

TypeError

Cause: Passing multiple dictionaries as positional arguments or passing an incompatible iterable.

Fix: Pass a single dictionary/iterable as the first argument, or use keyword arguments.

d = {}
d.update({'a': 1}, {'b': 2})  # Raises TypeError

Runtime support

Compatibility

Python 3.8+

Source: Python Software Foundation

Common questions

Frequently Asked Questions

Merges another mapping or key/value pairs into a dict in-place.

other: Dict or iterable of (key, value) pairs. **kwargs: Extra key=value pairs to add/overwrite.

LogicError: Call update() on a separate line; do not assign its result to a variable. TypeError: Pass a single dictionary/iterable as the first argument, or use keyword arguments.