implicit item assignment in python objects

Question

I would like to be able to assign values to class' attributes without specifying them.

What i would like to make work

A)


class HoldingStuff(object):

    def __init__(self, **args):
        self.a = 1
        self.b = '1'
        self.c = 2
        self.d = '3'
        self.e = 4
        for arg in args:
            self[arg] = args[arg]

first = HoldingStuff(a=5)
second = HoldingStuff(d=2, e='2')

Of course i could just use a dictionary within the class which holds the values. There is a big downside to that however, because i can not use code completion for the attributes then. Which makes them hard to reuse and find. (The attributes are an extensive list of mutable parameters for a simulation.)

What does work (and is unpractical)

B)


class HoldingStuff(object):

    def __init__(self, **args):
        self['a'] = 1
        self['b'] = '1'
        self['c'] = 2
        self['d'] = '3'
        self['e'] = 4
        for arg in args:
            self[arg] = args[arg]

What kinda works

C)


class HoldingStuff(MutableMapping):

    def __init__(self, **args):
        self.a = 1
        self.b = '1'
        self.c = 2
        self.d = '3'
        self.e = 4
        for arg in args:
            self[arg] = args[arg]

    def __setitem__(self, k, v) -> None:
        if k =='a':
            self.a = v
        elif k =='b':
            self.b = v
        elif k =='c':
            self.c = v
        elif k =='d':
            self.d = v
        elif k =='e':
            self.e = v
        else:
            print(k, "Not available for class.")

    def __delitem__(self, v) -> None:
        pass

    def __getitem__(self, k) -> _VT_co:
        if k =='a':
            return self.a
        elif k =='b':
            return self.b
        elif k =='c':
            return self.c
        elif k =='d':
            return self.d
        elif k =='e':
            return self.e
        else:
            print(k, "Not available for class.")

    def __len__(self) -> int:
        return 5

    def __iter__(self):
        print("Not implemented")

The functions __setitem__ and __getitem__ seem obvious and repetitive!

Question Is there a way to reduce the source code of example C) to work in an implicit way of example A) ?

Why are you using **args at all? This issue never would have come up if you just wrote out your argument list normally and assigned attributes from the argument values normally. — user2357112
– user2357112, Commented Oct 15, 2019 at 9:22
It doesn't look like you even want to support arbitrary extra user-provided names, judging by those __getitem__ and __setitem__ methods (and arbitrary user-provided names wouldn't work with the code completion you're after either). Allowing callers to specify arbitrary keywords is the entire design purpose of **. — user2357112
– user2357112, Commented Oct 15, 2019 at 9:24

Jonas · Accepted Answer · 2019-10-15 10:18:57Z

3

Use setattr.

for name, value in args.items():
    setattr(self, name, value)

edited Oct 15, 2019 at 10:18

Jonas

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answered Oct 15, 2019 at 9:39

yllw

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3 Comments

Jonas Over a year ago

ee = HoldingStuff(e='2354345') throws ValueError: not enough values to unpack (expected 2, got 1)

Dion Over a year ago

The first line should probably be for name, value in args.items():.

Jonas Over a year ago

@Dion thanks worked quite nicely! I edited the answer accordingly and accepted it.

Jonas · Accepted Answer · 2019-10-15 09:51:10Z

1

Thank You for Your comments, which helped me find:

https://stackoverflow.com/a/6760593/3707039

delivered an elegant solution


class HoldingStuff(object):

    def __init__(self, **args):
        self.a = 1
        self.b = '1'
        self.c = 2
        self.d = '3'
        self.e = 4
        vars(self).update(args)

answered Oct 15, 2019 at 9:51

Jonas

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Comments

eladgl · Accepted Answer · 2019-10-15 09:45:03Z

0

Can't comment because I have less than 50 reputation. In option a,

class HoldingStuff(object):

    def __init__(self, **args):
        self.a = 1
        self.b = '1'
        self.c = 2
        self.d = '3'
        self.e = 4
        for arg in args:
            self[arg] = args[arg]

first = HoldingStuff(a=5)
second = HoldingStuff(d=2, e='2')

you have the lines:

for arg in args:
    self[arg] = args[arg]

yet you insert a string and trying to find its args' index

second  = HoldingStuff(d=2, e='2') #e='2'

I think you should try using:

for arg in range(len(args)):
    self[args[arg]] = args[arg]

answered Oct 15, 2019 at 9:45

eladgl

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