Use subscript assignment to add a key-value pair: items["apple"] = 3. If the key already exists, this replaces its value. For multiple pairs, use update(); to add a default only when a key is missing, use setdefault().
Add or replace one key-value pair
A Python dictionary stores key-value pairs, and each key is unique within that dictionary. Start with an empty dictionary using {}, then assign a value to a key:
settings = {}
settings["theme"] = "dark"
If you assign to a key that already exists, Python replaces its previous value. For example, after the next line, settings["theme"] is "light":
settings["theme"] = "light"The Python 3.14.8 tutorial explains that when a key is already in use, its old associated value is forgotten. See the official dictionary tutorial.
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Add multiple items with update()
Pass update() a mapping or an iterable of key-value pairs to add several entries at once:
settings.update({"theme": "dark", "font_size": 16})
settings.update([("theme", "light"), ("font_size", 14)])
You can also use keyword arguments when the keys are valid Python identifiers:
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settings.update(theme="dark")update() changes the existing dictionary in place. If an incoming key is already present, the incoming value replaces the current one. It returns None, so do not assign its result back to the dictionary:
settings = settings.update({"font_size": 16}) # Wrong: settings becomes NoneThe Python 3.14 built-in types reference documents the accepted inputs and overwrite behavior.
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Add a key only when it is missing
Use setdefault(key, default) when you want to keep an existing value but insert a default if the key is absent:
settings.setdefault("language", "en")If "language" is already present, setdefault() returns its current value without changing it. Otherwise, it inserts "language": "en" and returns "en". Choose direct assignment instead when replacing an existing value is intended.
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Merge dictionaries (Python 3.9 and later)
Python 3.9 introduced the | and |= dictionary merge operators. Use | to create a new merged dictionary:
defaults = {"theme": "light", "font_size": 14}
user_settings = {"theme": "dark"}
combined = defaults | user_settings
combined is a new dictionary, and the right-hand dictionary wins when both contain the same key. To update the left-hand dictionary itself, use |=:
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defaults |= user_settingsFor Python versions earlier than 3.9, use update() when you need to apply incoming entries to an existing dictionary. The operators and their behavior are covered in the official built-in types reference.
Collect multiple values under one key
If each key needs to hold several values, a defaultdict(list) can create an empty list automatically the first time a missing key is accessed. Append each item to the list:
from collections import defaultdict
groups = defaultdict(list)
for category, item in records:
groups[category].append(item)
This pattern is useful for grouping records by category. The Python collections reference describes how defaultdict supplies a default value for a missing key.
Choose an operation and avoid common errors
| Goal | Use | Behavior |
|---|---|---|
| Add or replace one value | d[key] = value |
Adds a missing key or replaces its existing value. |
| Add or replace several values | d.update(...) |
Modifies the dictionary in place; incoming duplicate keys overwrite existing values. |
| Insert a default only if absent | d.setdefault(key, default) |
Keeps an existing value; otherwise inserts and returns the default. |
| Make a merged copy (Python 3.9+) | d1 | d2 |
Returns a new dictionary; values from the right operand win for duplicate keys. |
| Merge into an existing dictionary (Python 3.9+) | d1 |= d2 |
Updates the left-hand dictionary; values from the right operand win for duplicate keys. |
| Accumulate many values per key | defaultdict(list) |
Creates a list for a missing key so values can be appended. |
Use valid dictionary keys
Keys must be hashable. Strings and numbers are common choices; a tuple can be a key if its contents are hashable. Lists cannot be keys because they are mutable. See the Python dictionary tutorial.
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Distinguish a lookup from an insertion
Reading d[key] when the key is missing raises KeyError. Use d.get(key) or d.get(key, fallback) for a lookup that can tolerate a missing key. get() does not insert anything; use setdefault() if the missing key should be added.
Quick Recap
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