Dynamically create class attributes

Question

I need to dynamically create class attributes from a DEFAULTS dictionary.

defaults = {
    'default_value1':True,
    'default_value2':True,
    'default_value3':True,
}

class Settings(object):
    default_value1 = some_complex_init_function(defaults[default_value1], ...)
    default_value2 = some_complex_init_function(defaults[default_value2], ...)
    default_value3 = some_complex_init_function(defaults[default_value3], ...)

I could also achive this by having sth. like __init__ for class creation, in order to dynamically create these attributes from dictionary and save a lot of code and stupid work.

How would you do this?

Thank you very much in advance!

score 31 · Answer 1 · edited Jun 15 '16 at 07:13

You could do it without metaclasses using decorators. This way is a bit more clear IMO:

def apply_defaults(cls):
    defaults = {
        'default_value1':True,
        'default_value2':True,
        'default_value3':True,
    }
    for name, value in defaults.items():
        setattr(cls, name, some_complex_init_function(value, ...))
    return cls

@apply_defaults
class Settings(object):
    pass

Prior to Python 2.6 class decorators were unavailable. So you can write:

class Settings(object):
    pass
Settings = apply_defaults(Settings)

in older versions of python.

In the provided example apply_defaults is reusable… Well, except that the defaults are hard-coded in the decorator's body :) If you have just a single case you can even simplify your code to this:

defaults = {
    'default_value1':True,
    'default_value2':True,
    'default_value3':True,
}

class Settings(object):
    """Your implementation goes here as usual"""

for name, value in defaults.items():
    setattr(Settings, name, some_complex_init_function(value, ...))

This is possible since classes (in the sense of types) are objects themselves in Python.

You also need to return the "decorated" class (`cls` in the first example) at the end of the decorator function, don't you? Took me a while to figure out, but unless I do this the class will equal `None` after definition. Am I missing something, like for example some way of supplying the class by reference instead? — Simon, Mar 03 '13 at 10:42
How do i pass defaults to the file like this `a = Settings(defaults)` `settings.py` is a particular file — TomSawyer, Oct 07 '17 at 07:32

score 5 · Answer 2 · answered Apr 06 '10 at 08:54

5

I think that's case for metaclass:

class SettingsMeta(type):
    def __new__(cls, name, bases, dct):
        for name, value in defaults.items():
            dct[name] = some_complex_init_function(value, ...)
        return type.__new__(cls, name, bases, dct)

class Settings(object):
    __metaclass__ = SettingsMeta

answered Apr 06 '10 at 08:54

Łukasz

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how do we pass defaults, or how can we initialize any object of this Settings Class – vijay shanker Nov 11 '15 at 11:22
1

same question with @vijayshanker – TomSawyer Oct 09 '17 at 08:48
The code depends on the `defaults` dictionary being in scope when defining `Settings` class. – Łukasz Oct 09 '17 at 11:33
`SettingsMeta` and `Settings` in the setting.py file? How do i pass defaults parameter to it? – TomSawyer Oct 09 '17 at 17:50
With the current code, you cannot pass anything, the `defaults` dictionary is taken from the scope, defined before `Settings` class. To be able to pass the defaults, you would need to restructure it. But it feels like a different question to the one asked originally. – Łukasz Oct 09 '17 at 19:27

score 2 · Answer 3 · answered Apr 06 '10 at 11:42

2

When defining a class, the local namespace will be converted into the class namespace at the conclusion of the class body. As such, you can accomplish this with:

class Settings(object):
    for key, val in defaults.iteritems():
        locals()[key] = some_complex_init_function(val, ...)

answered Apr 06 '10 at 11:42

Brian

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-1 The [documentation](http://docs.python.org/library/functions.html?highlight=locals#locals) for `locals()` says the contents of the dictionary returned should not be modified. – martineau Nov 16 '10 at 12:26

Dynamically create class attributes

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