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# Python Keyword-Only Parameter
Keyword-only parameters are function arguments that can only be supplied using keyword syntax (`parameter_name=value`) during invocation. They cannot be bound to positional arguments. If a caller attempts to pass a keyword-only parameter positionally, the Python interpreter raises a `TypeError`.
In a function signature, keyword-only parameters are defined by placing them after a var-positional parameter (typically `*args`) or after a bare asterisk (`*`).
## Syntax Visualization
**1. Using a Bare Asterisk (`*`)**
The bare asterisk consumes no arguments but dictates that all subsequent parameters in the signature are keyword-only.
```python theme={"dark"}
def configure_server(host, port, *, secure, timeout=30):
# 'host' and 'port' can be positional or keyword.
# 'secure' and 'timeout' are strictly keyword-only.
pass
# Valid Invocation
configure_server("127.0.0.1", 8080, secure=True)
configure_server("127.0.0.1", 8080, secure=True, timeout=60)
# Invalid Invocation: Raises TypeError (takes 2 positional arguments but 3 were given)
configure_server("127.0.0.1", 8080, True)
```
**2. Following a Var-Positional Parameter (`*args`)**
When a function accepts an arbitrary number of positional arguments via `*args`, any standard parameters defined after it automatically become keyword-only.
```python theme={"dark"}
def concatenate_strings(*strings, delimiter=","):
# 'strings' captures all positional arguments as a tuple.
# 'delimiter' is keyword-only.
return delimiter.join(strings)
# Valid Invocation
concatenate_strings("apple", "banana", "cherry", delimiter="|")
# Syntactically valid, but "->" is absorbed into the *strings tuple,
# leaving 'delimiter' at its default value.
concatenate_strings("apple", "banana", "cherry", "->")
```
## Parameter Ordering Rules
When constructing a function signature, Python enforces a strict parameter resolution order. Keyword-only parameters must be placed according to the following hierarchy:
1. Positional-only parameters (preceding `/`)
2. Standard parameters (positional or keyword)
3. Var-positional parameter (`*args`) OR bare asterisk (`*`)
4. **Keyword-only parameters**
5. Var-keyword parameter (`**kwargs`)
```python theme={"dark"}
# Comprehensive Signature Example
def complex_function(pos_only, /, standard, *args, kw_only1, kw_only2="default", **kwargs):
pass
```
## Required vs. Optional
Keyword-only parameters can be required or optional.
* If a keyword-only parameter is defined **without** a default value (e.g., `secure` in the first example), it is a *required* keyword-only parameter. Omitting it during invocation raises a `TypeError: missing 1 required keyword-only argument`.
* If it is defined **with** a default value (e.g., `timeout=30`), it is optional.
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