How Do Bare Asterisks Define Keyword-Only Parameters in Python?

Understanding the Bare Asterisk in Parameter Lists and Keyword-Only Parameters

In Python, parameter lists can include a bare asterisk to denote keyword-only parameters. This concept differs from the use of asterisks preceding parameters for varying numbers of parameters.

Bare Asterisk and Keyword-Only Parameters

The bare asterisk (*) in a parameter list forces callers to use named arguments. Consider the example below:

def func(*):
    pass

If you attempt to call this function without specifying named arguments, you will encounter a SyntaxError:

>>> func()
  File "<stdin>", line 1
SyntaxError: named arguments must follow bare *

This is because the bare asterisk requires that all arguments after it be named.

Rationale for Keyword-Only Parameters

Keyword-only parameters help in designing functions with improved readability and documentation. By forcing named arguments, it becomes clear which parameters are required and what their purpose is. This can enhance code understanding and reduce the likelihood of errors.

For instance, in the pickle.dump function mentioned in your question, the fix_imports parameter is marked keyword-only. This indicates that it must be explicitly named when calling the function:

pickle.dump(obj, file, protocol=None, *, fix_imports=True)

Additional Information

For further details on bare asterisks and keyword-only parameters, refer to the Python 3 documentation or consider this Stack Overflow answer. These resources provide comprehensive explanations and examples.

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