Python’s dict() function creates a new dictionary. Call it with no arguments for an empty dictionary, or give it a mapping, an iterable of key-value pairs, keyword arguments, or a combination of those inputs. If a key appears more than once, the last value supplied is kept.
What does dict() do?
The Python 3.14 Built-in Types documentation defines the constructor this way: “Return a new dictionary initialized from an optional positional argument and a possibly empty set of keyword arguments.” A dictionary stores values under keys; keys must be unique, and assigning a key again replaces its value.
The constructor accepts an optional positional argument and keyword arguments. The positional input may be a mapping or an iterable of pairs.
Create a dictionary with dict()
Start with an empty dictionary
Call dict() without arguments to create an empty dictionary:
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settings = dict()The empty dictionary literal {} creates the same kind of object and is often the shorter choice:
settings = {}Build one from a mapping
Pass a mapping, such as an existing dictionary, to copy its key-value data into a new dictionary:
defaults = {"debug": True, "retries": 3}
options = dict(defaults)Build one from key-value pairs
A non-mapping positional input must be an iterable whose items each contain exactly two elements. Each item supplies one key and its value:
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person = dict([("name", "Ada"), ("language", "Python")])This form also works when keys cannot be written as keyword names:
labels = dict([("display name", "Ada"), ("user-id", 17)])An item with fewer or more than two elements does not meet the constructor’s pair requirement.
Build one from keyword arguments
Keyword arguments provide a compact way to create entries when every key is a valid Python identifier:
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options = dict(debug=True, retries=3)Keyword syntax cannot express keys such as "display name" or "user-id"; use a mapping or pairs for those keys.
Combine positional input and keywords
You can provide a mapping or pair iterable and then keyword arguments. The keyword arguments are applied after the positional input:
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options = dict({"debug": True}, retries=3)What happens when a key appears more than once?
When the same key is supplied multiple times, later values replace earlier ones. This applies both to repeated pairs and to keywords that duplicate a key from the positional input:
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options = dict([("mode", "safe"), ("mode", "fast")])
# {'mode': 'fast'}
options = dict({"mode": "safe"}, mode="fast")
# {'mode': 'fast'}Repeated keys do not cause dict() to raise an error. That behavior is useful when later values should override defaults, but it can also hide accidental duplicate data.
When should you use dict() instead of another form?
Choose the form that best matches how you already have the data:
| Form | Best fit | Example |
|---|---|---|
| Dictionary literal | Inline key-value data written directly in the code | {"name": "Ada", "language": "Python"} |
| Dictionary comprehension | Values or keys generated from an iterable | {number: number * number for number in range(4)} |
dict() |
Converting a mapping or iterable of pairs, or using valid-identifier keyword keys | dict([("name", "Ada")]) |
A literal is usually easy to read when the entries are visible and fixed. A comprehension makes the generation rule explicit. dict() is especially useful when the input already comes in mapping or pair form.
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How does dict() differ from update() and merge operators?
dict() returns a new dictionary. To combine data with an existing dictionary, choose an operation based on whether the original should change:
| Operation | Result | Changes the target? | Version note |
|---|---|---|---|
update() |
Returns None |
Yes; overwrites matching keys in place | No version qualification stated in the Python 3.14 documentation |
| |
A new merged dictionary | No | Added in Python 3.9 |
|= |
Updates the existing dictionary | Yes | Added in Python 3.9 |
update() accepts a mapping or an iterable of key-value pairs. Like dict(), it overwrites a value when a key already exists. The merge operators also apply later values over earlier ones; use | when you want a merged result without updating the original, and |= when you do want to update it.
Do dictionaries keep insertion order?
Yes. Dictionaries preserve insertion order. Replacing the value of an existing key does not change that key’s position. If you delete a key and insert it again, it appears at the end.
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