What Do Bare Asterisks Mean in Python Function Parameters?

Unveiling the Role of Bare Asterisks in Function Parameters

Within the realm of function parameter lists, a bare asterisk denotes a conspicuous distinction known as "keyword-only" parameters. This syntax dictates that callers must explicitly specify the names of arguments when invoking the function, effectively barring the use of positional arguments.

Origins and Purpose

This unique syntax emerged in Python 3 as a mechanism to enhance code clarity and avoid ambiguity. Keyword-only parameters ensure that argument names are always present, eliminating the potential pitfalls of relying on positional ordering that can lead to errors.

Syntactic Structure

A bare asterisk is placed directly after a variable number of non-keyword arguments to delineate the start of keyword-only parameters, as exemplified by the following snippet from the pickle module:

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

In this example, fix_imports is a keyword-only argument that must be invoked by name, reducing the likelihood of inadvertently providing it as a positional parameter.

Enforcement and Benefits

The interpreter strictly enforces this syntax rule, issuing a SyntaxError if keyword arguments follow the bare asterisk without a preceding positional parameter. This enforced structure compels callers to diligently specify argument names, promoting code clarity and reducing the risk of hard-to-detect errors.

Conclusion

The bare asterisk in function parameters serves as a pivotal tool for defining keyword-only parameters, compelling callers to specify argument names explicitly. This syntax ensures code clarity and eliminates the ambiguities inherent in positional argumentation, leading to more reliable, maintainable, and error-free programs.

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