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Python dictionaries have no append() method. To add a key-value pair, assign a value with d[key] = value. If the key already exists, the assignment replaces its value; it does not create a duplicate key.
append() is for a list value, not a dictionary
A dictionary maps unique keys to values. That means adding an item to the dictionary means creating a key-value pair, which you do with assignment. The name “append” is often used informally to mean “add,” but calling append() on a dictionary raises an error because dictionaries do not provide that method. See the Python dictionary documentation.
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scores = {}
scores["Ada"] = 95
scores["Ada"] = 98 # Replaces 95; it does not add another "Ada" entry
Add one key-value pair
Use square-bracket assignment when you want a key to point to a value:
settings = {"theme": "light"}
settings["language"] = "en" # Adds a new key
settings["theme"] = "dark" # Replaces the existing value
Dictionary keys are unique. Assigning to a key that is already present updates the value associated with that key.
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Add or update several pairs
Use update() to apply pairs from another mapping or an iterable of key-value pairs. It changes the dictionary in place and returns None; matching keys are overwritten.
settings.update({"theme": "dark", "language": "en"})Collect multiple values under one key
If your goal is to keep several values associated with one key, make the dictionary value a list. Then call append() on that list—not on the dictionary.
groups = {}
groups.setdefault("fruit", []).append("pear")
groups.setdefault("fruit", []).append("plum")
# {"fruit": ["pear", "plum"]}
setdefault(key, default) returns the existing value for a key, or inserts and returns default when the key is absent. Here, it provides a list that can receive the new item.
When you repeatedly build lists for keys that may not exist yet, collections.defaultdict(list) is another option:
from collections import defaultdict
groups = defaultdict(list)
groups["fruit"].append("pear")
groups["fruit"].append("plum")
Use setdefault() when you want to keep a regular dictionary explicit; use defaultdict(list) when automatic list creation for missing keys suits the code. Both patterns are documented in the Python collections documentation.
Read a missing key without inserting it
Square-bracket lookup raises KeyError when the key is absent. If absence is expected and you want a fallback value without changing the dictionary, use get():
value = settings.get("font", "system")If "font" is missing, value is "system"; get() does not add a "font" entry. By contrast, setdefault("font", "system") would insert that key when it is absent.
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Choose the operation that matches the task
| What you want | Use | Effect |
|---|---|---|
| Add or replace one pair | d[key] = value |
Creates the key if absent; replaces its value if present. |
| Apply several pairs | d.update(other) |
Mutates d; overwrites colliding keys. |
| Insert a fallback only if absent | d.setdefault(key, default) |
Returns the existing value or inserts and returns the default. |
| Collect repeated items under one key | d[key].append(item) |
Appends to a list stored as that key’s value; initialize the list with setdefault() or defaultdict(list) if needed. |
| Read with a fallback, without inserting | d.get(key, default) |
Returns the stored value or the fallback; leaves the dictionary unchanged. |
What happens to dictionary order?
Current Python dictionaries preserve insertion order. Updating an existing key does not move it; deleting a key and then inserting it again places it at the end. See the dictionary type documentation for these ordering details.
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