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Description:

positionaldefaults 2023.4.19

positional_defaults
Python package to set defaults for any positional-only parameter
This tiny Python package contains a decorator @defaults which lets you
specify default arguments for any positional-only parameter, no matter where
it appears in the argument list.
Installation
pip install positional_defaults

Usage
Use the @defaults decorator to set default values for positional-only
parameters (i.e. those before the positional-only indicator /):
from positional_defaults import defaults

@defaults(start=0)
def myrange(start, stop, /, step=1):
...

# now these are equivalent
myrange(4)
myrange(0, 4)
myrange(0, 4, 1)

This works on methods as well:
class A:
@defaults(start=0)
def myrange(self, start, stop, /, step=1):
...

Multiple defaults can be set, which are filled in the order in which they are
specified:
@defaults(forename='Alice', greeting='Welcome', prefix='Mrs')
def greet(greeting, prefix, forename, surname, /, suffix='Esq'):
...

# these are now equivalent
greet('Smith')
greet('Alice', 'Smith')
greet('Welcome', 'Alice', 'Smith')
greet('Welcome', 'Mrs', 'Alice', 'Smith')
greet('Welcome', 'Mrs', 'Alice', 'Smith', 'Esq')

Signatures
Left-defaulted functions come with the correct signature:
>>> from inspect import signature
>>> signature(myrange)
<Signature (start=0, stop, /, step=1)>
>>> signature(greet)
<Signature (greeting='Welcome', prefix='Mrs', forename='Alice', surname, /, suffix='Esq')>

These show up correctly in the usual places such as help():
>>> help(myrange)

Help on function myrange:

myrange(start=0, stop, /, step=1)

>>> help(greet)

Help on function greet:

greet(greeting='Welcome', prefix='Mrs', forename='Alice', surname, /, suffix='Esq')

Performance
When the package is compiled as a native extension (which is what pip will
deliver in the majority of cases), functions with positional defaults have
comparable performance with respect to their undecorated functions.