Python lets you control how callers provide each function argument: a parameter can be positional-only, positional-or-keyword, or keyword-only, and a default lets callers omit it. In a signature such as def render(item, /, format="text", *, strict=False):, the slashes and asterisk mark those boundaries. This guide explains how to define and call each kind, avoid common binding errors, and choose parameter forms that make an API clear and stable.
How Python classifies function parameters
Unless a signature marks otherwise, an ordinary parameter is positional-or-keyword: a caller can pass its value by position or by parameter name. A default value makes a parameter optional at the call site; Python uses that default only when the caller omits the argument.
Python’s special markers divide a function signature into regions:
- Before
/: positional-only parameters. Supply these by position, not by name. - Between
/and*: positional-or-keyword parameters. Supply these by position or name. - After a bare
*or after*args: keyword-only parameters. Supply these by name.
For example:
def render(item, /, format="text", *, strict=False):
...Here, item is positional-only, format is positional-or-keyword with a default, and strict is keyword-only with a default. The / and bare * are syntax markers, not parameters callers pass.
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Positional-only syntax using / is available from Python 3.8 onward. If your code must run on an older interpreter, check its minimum supported version before using it. See the Python 3.12 language reference.
How to call a function with these parameter kinds
Given the render definition above, these calls are valid:
render("report")
render("report", "json", strict=True)
render("report", format="json", strict=True)The first call omits both optional parameters, so Python uses "text" and False. The second supplies format positionally and strict by name. The third supplies both by name where allowed, while still passing positional-only item by position.
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A keyword-only parameter does not have to be optional. Omitting a default makes it required:
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def connect(host, *, timeout):
...
connect("example.com", timeout=10)In contrast, def connect(host, *, timeout=10): allows a call such as connect("example.com"); Python uses the default of 10.
What do / and * mean in a function definition?
The slash marks positional-only parameters
Every parameter to the left of / must be passed by position. In render, render(item="report") is invalid because item is positional-only. Positional-only parameters are useful when the parameter name should not be part of the calling interface, or when you want to retain freedom to rename it later.
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They can also matter when a function accepts arbitrary keyword arguments. With def foo(name, /, **kwds):, the call foo(1, name=2) can bind the positional parameter to 1 and leave name=2 inside kwds. Without the slash, def foo(name, **kwds):, that call fails because name is already bound to the positional value.
The asterisk marks keyword-only parameters
Parameters after a bare * must be passed by name. In render, render("report", "json", True) is invalid because the third value would have to be supplied as strict=True. The same keyword-only rule applies to parameters after a variadic positional parameter such as *args.
How to choose parameter kinds
Choose based on whether the name is part of the public interface and whether a call is clearer by position or by a descriptive keyword.
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| Parameter kind | Choose it when | What callers can do |
|---|---|---|
| Positional-only | The name has no meaningful public value, order is the intended convention, arbitrary keywords need to remain available, or you want freedom to rename the parameter without breaking callers who use its name. | Pass the argument by position only. |
| Positional-or-keyword | Both positional convenience and named clarity are useful. | Pass it by position or by its parameter name. |
| Keyword-only | The name explains the argument, or requiring a named argument makes a call easier to understand than relying on position. | Pass it by name only. |
The Python tutorial summarizes one positional-only benefit this way: “For an API, use positional-only to prevent breaking API changes if the parameter’s name is modified in the future.” — Python Software Foundation, Python Tutorial, “Special parameters”.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Defaults and the mutable-default pitfall
A default expression is evaluated when Python defines the function, and the resulting object is reused on later calls. This can surprise you if the default is mutable, such as a list: changes made during one call can affect later calls that omit the argument.
To create a fresh list for each call, use None as a sentinel and build the list inside the function:
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def append_item(item, items=None):
if items is None:
items = []
items.append(item)
return itemsWhen a caller supplies items, this function appends to that supplied list; when the caller omits it, the function creates a new list for that call. The Python tutorial’s function-parameter guidance demonstrates this pattern.
Common argument-binding errors
Python raises TypeError when a call does not match the function’s parameter rules. Typical causes include:
- Passing a positional-only parameter by name:
render(item="report"). - Passing a keyword-only parameter by position:
render("report", "json", True). - Supplying one parameter twice: for example,
render("report", format="json", strict=True, **{"strict": False}). - Omitting a required parameter, including a required keyword-only parameter such as
timeoutinconnect(host, *, timeout). - Using an unrecognized keyword when the function does not accept arbitrary keyword arguments.
To diagnose an error, compare the call with the signature: check which parameters are required, whether each argument is passed using an allowed form, and whether any parameter receives more than one value.
How to inspect parameter kinds
For tooling that needs to examine a callable, Python’s inspect module provides inspect.signature(). Its Signature.parameters mapping preserves parameter order, and each parameter has a kind such as POSITIONAL_ONLY, POSITIONAL_OR_KEYWORD, VAR_POSITIONAL, KEYWORD_ONLY, or VAR_KEYWORD. The Python 3.12 inspect documentation describes these kinds and signature objects.
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