For the basics on function arguments, please see the [function concept][function concept].
## Parameter Names
Paramater names, like variable names, must start with a letter or underscore and may contain letters, underscores, or numbers.
Parameter names should not contain spaces or punctuation.
## Positional Arguments
Positional arguments are for parameters defined without a [default argument][default arguments].
Positional arguments can optionally be called by their keyword.
Following is an example of positional arguments being called by position and by their keyword:
```python
>>> def concat(x, y):
return f"{x}{y}"
>>> print(concat("Hello, ", "Bob"))
Hello, Bob
>>> print(concat(y="Bob", x="Hello, "))
Hello, Bob
```
The first call to `concat` passes the arguments by position.
The second call to `concat` passes the arguments by keyword, allowing their positions to be changed.
Arguments can be forced to be positional-only through the use of the `/` operator.
Following is an example of positional-only arguments:
```python
>>> def concat(x, y, /):
return f"{x}{y}"
>>> print(concat("Hello, ", "Bob"))
Hello, Bob
>>> print(concat(y="Bob", x="Hello, "))
Traceback (most recent call last):
print(concat(y="Bob", x="Hello, "))
TypeError: concat() got some positional-only arguments passed as keyword arguments: 'x, y'
```
## Keyword Arguments
Keyword arguments are for parameters defined with a [default argument][default arguments].
Keyword arguments can optionally be called by their position.
Following is an example of keyword arguments being called by their keyword and by position:
```python
>>> def concat(x="Hello, ", y="you"):
return f"{x}{y}"
>>> print(concat(y="Bob", x="Hello, "))
Hello, Bob
>>> print(concat("Hello, ", "Bob"))
Hello, Bob
```
Arguments can be forced to be keyword--only through the use of the `*` operator.
Following is an example of keyword-only arguments:
```python
>>> def concat(*, x="Hello, ", y="you"):
return f"{x}{y}"
>>> print(concat(y="Bob", x="Hello, "))
Hello, Bob
>>> print(concat("Hello, ", "Bob"))
Traceback (most recent call last):
print(concat("Hello, ", "Bob"))
TypeError: concat() takes 0 positional arguments but 2 were given
```
## Positional or Keyword Arguments
Arguments can be positional or keyword if neither the `/` nor `*` operators are used in the parameter definitions.
Alternately, the positional-or-keyword arguemtns can be placed between the positional-only parameters on the left and the keyword-only parameters on the right.
Following is an example of positonal-only, positional-or-keyword, and keyword-only arguments:
```python
>>> def concat(x, /, y="you", *, z="."):
return f"{x}{y}{z}"
>>> print(concat("Hello, ", "Bob", z="!"))
Hello, Bob!
>>> print(concat("Hello, ", y="Bob", z="!"))
Hello, Bob!
>>> print(concat(x="Hello, ", y="Bob", z="!"))
Traceback (most recent call last):
print(concat(x="Hello, ", y="Bob", z="!"))
TypeError: concat() got some positional-only arguments passed as keyword arguments: 'x'
