How to fill specific positional arguments with partial in python?

Question

Basically, what I\'d like to do is:

>>> from functools import partial
>>> partial(str.startswith, prefix=\'a\')(\'a\')
Traceback (most rece         


        

Answer 1

Use this code:

# See: functoolspartial for binding...

class Function(object):
    def __init__(self, fn):
        self.fn = fn

    def __call__(self, *args, **kwargs):
        return self.fn(*args, **kwargs)

    def __add__(self, other):
        def subfn(*args, **kwargs):
            return self(*args, **kwargs) + other(*args, **kwargs)
        return subfn


class arg(object):
    """Tagging class"""
    def __init__(self, index):
        self.index = index


class bind(Function):
    """Reorder positional arguments.
    Eg: g = f('yp', _1, 17, _0, dp=23)
    Then g('a', 'b', another=55) --> f('yp', 'b', 17, 'a', dp=23, another=55)
    """

    def __init__(self, fn, *pargs, **pkwargs):
        # Maximum index referred to by the user.
        # Inputs to f above this index will be passed through
        self.fn = fn
        self.pargs = pargs
        self.pkwargs = pkwargs
        self.max_gindex = max(
            (x.index if isinstance(x, arg) else -1 for x in pargs),
            default=-1)

    def __call__(self, *gargs, **gkwargs):
        fargs = \
            [gargs[x.index] if isinstance(x, arg) else x for x in self.pargs] + \
            list(gargs[self.max_gindex+1:])

        fkwargs = dict(self.pkwargs)
        fkwargs.update(gkwargs)    # Overwrite keys
        return self.fn(*fargs, *fkwargs)

Then try it out like:

def myfn(a,b,c):
print(a,b,c)

bind(myfn, arg(1), 17, arg(0))(19,14)