New submission from Benjamin S Wolf <[email protected]>:
Positional-only arguments come before position-or-keyword arguments.
def f(pos1, pos2, /, pos_or_kwd, *, kwd1, kwd2):
However, the posonlyargs are defined to come after args in the AST:
arguments = (arg* args, arg* posonlyargs, arg? vararg, arg* kwonlyargs,
expr* kw_defaults, arg? kwarg, expr* defaults)
which results in confusing ast.dump output because they share defaults:
>>> r = ast.parse('lambda a=1,/,b=2:a+b', mode='eval')
>>> ast.dump(r.body.args)
"arguments(
args=[arg(arg='b', annotation=None, type_comment=None)],
posonlyargs=[arg(arg='a', annotation=None, type_comment=None)],
vararg=None, kwonlyargs=[], kw_defaults=[], kwarg=None,
defaults=[Constant(value=1, kind=None), Constant(value=2, kind=None)])"
[manually prettified]
Note how the ordering is 'args b', then 'posonlyargs a', but the defaults are
still 1 then 2. This can be confusing to someone building an ast.arguments
using keywords because the elements in 'defaults' have to be supplied in a
specific order, but the keyword args 'args' and 'posonlyargs' do not, or to
someone building an ast.arguments using positional arguments (because, maybe
ironically, they're not keyword-only arguments) because 'posonlyargs' and
'args' must be supplied in a different order than the ordering of elements in
'defaults' would imply.
Potential solutions:
1. Swap posonlyargs and args.
2. Add a separate pos_defaults list.
----------
messages: 347932
nosy: Benjamin.S.Wolf
priority: normal
severity: normal
status: open
title: ast.arguments has confusing args/posonlyargs ordering
type: behavior
versions: Python 3.8
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Python tracker <[email protected]>
<https://bugs.python.org/issue37593>
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