Injecting global imports/locals

class_factory.py

# In an actual use case, pkg1 and pkg2 would be packages, 
# but to make this runnable I used classes 
# to make a namespace instead (same idea regardless)

# make a dummy class so we can use it as a namespace
class Dummy(object): pass

pkg1, pkg2 = Dummy(), Dummy()
pkg1.average = lambda *args : sum(args) * 1.0 / len(args)
pkg2.get_lengths = lambda *args : map(len, args)

### Use defaults with boolean or to set locals
# (this gets *really* long with many local variables)

def get_average_class(**kwargs):

    # note that if you used average = kwargs.get("average") or average here
    # you'd get an error about using an unbound local.
    # (and using globals would kinda defeat the purpose entirely)

    average = kwargs.get("average") or pkg1.average
    get_lengths = kwargs.get("get_lengths") or pkg2.get_lengths

    class AverageList(object):
        def __init__(self, *args):
            self.items = args
        @property
        def average(self):
            return average(*get_lengths(*self.items))
    return AverageList

adjusted_lengths = lambda *args: map(len, args) + [15]
print (get_average_class())([1,2], [10, 4, 5, 6]).average == 3 # True
print (get_average_class(get_lengths=adjusted_lengths))([1,2], [10, 4, 5, 6]).average  == 7 # True

# This example shows how you *can't* add to the locals dict

average = pkg1.average
get_lengths = pkg2.get_lengths

def get_average_class2(**kwargs):
    # this doesn't work because adding to localvars does nothing
    localvars = locals()
    for k in ("average", "get_lengths"):
        if kwargs.get(k, None):
            localvars[k] = kwargs[k]

    class AverageList2(object):
        def __init__(self, *args):
            self.items = args
        @property
        def average(self):
            return average(*get_lengths(*self.items))
    return AverageList2

adjusted_lengths = lambda *args: map(len, args) + [15]
print (get_average_class2())([1,2], [10, 4, 5, 6]).average == 3 # True
print (get_average_class2(get_lengths=adjusted_lengths))([1,2], [10, 4, 5, 6]).average == 7 # False