Use lambda or helper method in python to avoid for loop duplication

Question

I have the following simple class and am wondering if there is a simple way to use a lambda, decorator, or helper method, etc... to avoid the duplicated for loop that appears in each method in CODENAMES and ALL_DEFAULTS?

class classproperty(object):
    """
    When used to decorate a method in a class, that method will behave
    like as a class property.
    """
    def __init__(self, f):
        # f - the func that's being decorated
        self.f = f

    def __get__(self, obj, cls):
        # call the func on the class
        return self.f(cls)

class PermissionInfo(object):

    MODELS = ['ticket', 'person', 'role']
    PERMS = ['view', 'add', 'change', 'delete']

    @classproperty
    def CODENAMES(cls):
        codenames = []
        for p in cls.PERMS:
            for m in cls.MODELS:
                codenames.append('{}_{}'.format(p, m))
        return codenames

    @classproperty
    def ALL_DEFAULTS(cls):
        ret = {}

        for p in cls.PERMS:
            for m in cls.MODELS:
                ret["{}_{}".format(p, m)] = False

        return ret

Duplicated for loop is this section of each method:

# ...
for p in cls.PERMS:
    for m in cls.MODELS:
#...

niemmi · Accepted Answer · 2016-12-13 01:32:42Z

5

You could use itertools.product within helper method to generate the names:

from itertools import product

def names(cls):
    yield from ('_'.join(x) for x in product(cls.PERMS, cls.MODELS))

Then you could change your class to utilize it:

@classproperty
def CODENAMES(cls):
    return list(names(cls))

@classproperty
def ALL_DEFAULTS(cls):
    return dict.fromkeys(names(cls), False)

edited Dec 13, 2016 at 1:32

answered Dec 13, 2016 at 1:25

niemmi

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Comments

Aske Doerge · Accepted Answer · 2016-12-13 01:25:57Z

0

You could perhaps use something like:

MODEL_PERMS = list(map(lambda t: "{}_{}".format(t[0],t[1]), itertools.product(MODELS, PERMS)))

answered Dec 13, 2016 at 1:25

Aske Doerge

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1 Comment

Aske Doerge Over a year ago

Meh. @niemmi's answer is better.

Stephane Martin · Accepted Answer · 2016-12-13 01:32:36Z

0

from itertools import product


class PermissionInfo(object):

    MODELS = ['ticket', 'person', 'role']
    PERMS = ['view', 'add', 'change', 'delete']
    PRODUCT = product(PERMS, MODELS)
    CODENAMES = ['{}_{}'.format(p, m) for (p, m) in PRODUCT]
    ALL_DEFAULTS = {codename: False for codename in CODENAMES}

ohhh shiny :-}

answered Dec 13, 2016 at 1:32

Stephane Martin

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1 Comment

niemmi Over a year ago

Note that this is different from the original code since ALL_DEFAULTS can be easily mutated. Assume you generate custom permissions like this: perm = PermissionInfo.ALL_DEFAULTS; perm['view_ticket'] = True. With original implementation this is not an issue but with your implementation defaults will change.

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