Michael Blahay <[email protected]> added the comment:
With the optional arguments, the determination about whether to use the default
value is made based on whether the flag is present or not. When positional
arguments are involved, the need for the defaults seems to in part be
determined based on whether the argument exists. The fact that * and REMAINDER
are zero-to-many in nature add some ambiguity into the situation. For the *, it
seems that the positional argument only exists if there is at least one actual
argument value that it can consume.
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs=1,default=['none'])
#parser.add_argument('bar', nargs=argparse.REMAINDER,default=['nothing'])
parser.add_argument('baz', nargs='*', default=['nada'])
parser.parse_args('a b'.split())
Out[25]: Namespace(baz=['b'], foo=['a'])
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs=1,default=['none'])
#parser.add_argument('bar', nargs=argparse.REMAINDER,default=['nothing'])
parser.add_argument('baz', nargs='*', default=['nada'])
parser.parse_args('a'.split())
Out[26]: Namespace(baz=['nada'], foo=['a'])
Mean while, the REMAINDER option makes the argument act as if it exists
regardless of whether an actual argument value exists.
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs=1,default=['none'])
parser.add_argument('bar', nargs=argparse.REMAINDER,default=['nothing'])
#parser.add_argument('baz', nargs='*', default=['nada'])
parser.parse_args('a b'.split())
Out[27]: Namespace(bar=['b'], foo=['a'])
import argparse
parser = argparse.ArgumentParser()
parser.add_argument('foo', nargs=1,default=['none'])
parser.add_argument('bar', nargs=argparse.REMAINDER,default=['nothing'])
#parser.add_argument('baz', nargs='*', default=['nada'])
parser.parse_args('a'.split())
Out[28]: Namespace(bar=[], foo=['a'])
To conclude, * and REMAINDER perform similar, but different, roles when used
with positional arguments. With edge cases like the ones laid out above, it can
be hard to conceptualize what the exact behavior should be. I will recommend
that the documentation be updated to convey the following message: "When used
with positional arguments, REMAINDER will never use the designated default
value list. It will instead return an empty list if there are no values for the
argument to consume. If the use of default values is desired, then * must be
used."
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Python tracker <[email protected]>
<https://bugs.python.org/issue35495>
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