Keyword-Only Arguments
In Python, it is sometimes necessary to specify that certain arguments to a function should be passed by keyword only (or named). This means that these arguments cannot be provided as positional arguments, but only by using their name in the function call. This feature helps make the code more readable and less error-prone.
Syntax for Keyword-Only Arguments
To indicate that certain arguments should be passed by keyword only, you use the * syntax in the function definition. Arguments that appear after the * must be passed as named arguments.
Syntax:
def function_name(arg1, arg2, *, keyword_only1, keyword_only2): # Function body
In this syntax, arg1 and arg2 can be passed by position or keyword, while keyword_only1 and keyword_only2 must be passed by keyword.
Example of a Function with Keyword-Only Arguments
Here is an example of a function using keyword-only arguments:
def configure_system(env, level, *, verbose=False, debug=False):
print(f"Environment: {env}")
print(f"Level: {level}")
print(f"Verbose: {verbose}")
print(f"Debug: {debug}")
# Correct usage
configure_system('production', 3, verbose=True, debug=True)
# Incorrect usage (keyword-only arguments must be specified by name)
configure_system('production', 3, True, True) # TypeError: configure_system() takes 2 positional arguments but 4 were given
In this example, verbose and debug must be passed by keyword, which means that you must use their names when providing them in the function call.
Reasons to Use Keyword-Only Arguments
. Clarity: Keyword-only arguments make code more readable and easier to understand, because the argument names are explicitly specified.
. Error Prevention: By forcing users to name certain arguments, you can avoid positional errors that can occur with positional arguments.
. Flexibility: Allows you to add additional parameters to functions without changing existing function calls.
Keyword-Only Arguments in Class Methods
Keyword-only arguments can also be used in class methods to ensure that certain arguments are passed by keyword.
Example:
class User:
def __init__(self, username, email, *, age=None):
self.username = username
self.email = email
self.age = age
# Correct usage
user = User('john_doe', 'john@example.com', age=30)
# Incorrect usage (keyword-only arguments must be specified by name)
user = User('john_doe', 'john@example.com', 30) # TypeError: User() got an unexpected positional argument
Restrictions and Limitations
• Compatibility: Keyword-only arguments are available starting with Python 3.0. Make sure your Python environment supports them.
• Signature Clarity: While this can improve clarity, it can also make the function signature more complex if used excessively.
Summary
Keyword-only arguments allow you to specify that some arguments to a function should be passed only by name, not by position. Here are the key points:
• Syntax with *: Arguments after the * should be passed by keyword.
• Usage: They help make code more readable, prevent errors, and add flexibility to functions.
• Class Methods: They can also be applied to class methods.
• Limitations: Available since Python 3.0 and can sometimes complicate function signatures.