Bare Forward Slash In Python Function Definition?

In the Python 3.8 Programming FAQ, I saw the following function definition: class callByRef: def __init__(self, /, **args): for key, value in args.items():

Solution 1:

Introduction as syntax

The / as syntax was introduced in Python 3.8.

The rationale for / in an argument list is given in PEP 570 -- Python Positional-Only Parameters:

The new syntax will enable library authors to further control how their API can be called. It will allow designating which parameters must be called as positional-only, while preventing them from being called as keyword arguments.

Previously, (informational) PEP 457 defined the syntax, but with a much more vague scope. This PEP takes the original proposal a step further by justifying the syntax and providing an implementation for the / syntax in function definitions.

Comparison of syntax and annotational PEPs

Similarities

For all intents and purposes, if you understand help()'s / notation, then that's what is formally included as Python syntax in v3.8 via PEP 570.

Differences

• PEP 570 -- Python Positional-Only Parameters

  • Defines the syntax of Python 3.8+
  • Formal grammatical specification of the syntax
  • Type: Accepted

• PEP 457 -- Notation For Positional-Only Parameters

  • Defines notation (not syntax) used in help() annotations
  • Informal English language description
  • Type: Informational

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Explanation and example

There are already excellent answers on the meaning and usage of / in arguments.

To save you the click through:

A / means that all preceding parameters are positional-only parameters. Positional-only parameters before a /cannot be passed as name=value when calling the function.

Python 3.8 What's New gives the following example:

defpow(x, y, z=None, /):
    r = x**y
    if z isnotNone:
        r %= z
    return r

Valid function calls:

• pow(2, 10)
• pow(2, 10, 17)

Invalid function calls:

• pow(x=2, y=10)
• pow(2, 10, z=17)