What Do Forward Slashes in Python\'s `help()` Output Mean for Method Signatures?

Understanding Slashes in Python Help Output for Method Signatures

When utilizing Python's help() function, users may encounter slashes in the output for method signatures. This article demystifies the meaning of this symbol, providing insights into its significance.

Positional-Only Parameters

The slash (/) denotes the end of positional-only parameters within a method signature. These are parameters that cannot be passed in as keyword arguments, meaning they must be specified in their proper order. Prior to Python 3.8, positional-only parameters were restricted to the C API.

Example: contains Method

For instance, in the help output for the __contains__ method of the range object, the slash indicates that the key argument can only be provided by position:

>>> help(range.__contains__)
...
__contains__(self, key, /)
    Return key in self.
...

This means that syntax such as range(5).__contains__(key=3) is invalid, as the key argument must be passed by position: range(5).__contains__(3).

Argument Clinic Documentation

The Argument Clinic documentation provides further clarification:

To mark all parameters as positional-only in Argument Clinic, add a / on a line by itself after the last parameter, indented the same as the parameter lines.

Python FAQ and PEP 570

Recently, the Python FAQ also addressed positional-only parameters:

A slash in the argument list of a function denotes that the parameters prior to it are positional-only.

Furthermore, PEP 570 codified the syntax for positional-only parameters into the Python language specification as of version 3.8.

Benefits of Positional-Only Parameters

Utilizing positional-only parameters offers several advantages:

• Cleaner APIs: It enforces parameter ordering, making code more readable and less prone to errors.
• Improved Consistency: Pure-Python implementations of C-only modules become more consistent and maintainable.
• Performance: Positional-only parameters require less processing, resulting in enhanced code execution speed.

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