Python `map` and arguments unpacking

问题:

I know, that

map(function, arguments)

is equivalent to

for argument in arguments:
    function(argument)

Is it possible to use map function to do the following?

for arg, kwargs in arguments:
    function(arg, **kwargs)

回答1:

You can with a lambda:

map(lambda a: function(a[0], **a[1]), arguments)

or you could use a generator expression or list comprehension, depending on what you want:

(function(a, **k) for a, k in arguments)
[function(a, **k) for a, k in arguments]

In Python 2, map() returns a list (so the list comprehension is the equivalent), in Python 3, map() is a generator (so the generator expression can replace it).

There is no built-in or standard library method that does this directly; the use case is too specialised.

回答2:

For the case of positional arguments only, you can use itertools.starmap(fun, args):

Return an iterator whose values are returned from the function evaluated with a argument tuple taken from the given sequence.

Example:

from itertools import starmap

def f(i, arg):
    print(arg * (i+1))

for _ in starmap(f, enumerate(["a", "b", "c"])):
    pass

prints:

a
bb
ccc

回答3:

You just have to remember that map will pass the arguments to the function as one tuple rather than separate arguments. If you can't change your original function, you can call it with a helper function:

def f(tup):
    args, kwargs = tup
    function(args, **kwargs)

map(f, arguments)

回答4:

I kept running into the same need and ended up making the following function:

def kwarg_map(element_constructor, **kwarg_lists):
    """
    A helper function for when you want to construct a chain of objects with individual arguments for each one.  Can
    be easier to read than a list expansion.

    :param element_constructor: A function of the form object = fcn(**kwargs)
    :param kwarg_lists: A dict of lists, where the index identifies two which element its corresponding value will go.
    :return: A list of objects.

    e.g. Initializing a chain of layers:
        layer_sizes = [784, 240, 240, 10]
        layers = kwarg_map(
            Layer,
            n_in = layer_sizes[:-1],
            n_out = layer_sizes[1:],
            activation = ['tanh', 'tanh', 'softmax'],
            )

    is equivalent to:
        layers = [Layer(n_in=784, n_out=240, activation='tanh'), Layer(n_in=240, n_out=240, activation='tanh'), Layer(n_in=240, n_out=10, activation='softmax')]
    """
    all_lens = [len(v) for v in kwarg_lists.values()]
    assert len(kwarg_lists)>0, "You need to specify at least list of arguments (otherwise you don't need this function)"
    n_elements = all_lens[0]
    assert all(n_elements == le for le in all_lens), 'Inconsistent lengths: %s' % (all_lens, )
    return [element_constructor(**{k: v[i] for k, v in kwarg_lists.iteritems()}) for i in xrange(n_elements)]