how to create class variable dynamically in python

Question

I need to make a bunch of class variables and I would like to do it by looping through a list like that:

vars=('tx','ty','tz') #plus plenty more

class Foo():
    for v in vars:
        setattr(no_idea_what_should_go_here,v,0)

is it possible? I don’t want to make them for an instance (using self in the __init__) but as class variables.

Asked By: pawel

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Source

Answer 1

You can run the insertion code immediately after a class is created:

class Foo():
     ...

vars=('tx', 'ty', 'tz')  # plus plenty more
for v in vars:
    setattr(Foo, v, 0)

Also, you can dynamically store the variable while the class is being created:

class Bar:
    locals()['tx'] = 'texas'

Answered By: Raymond Hettinger

Answer 2

If for any reason you can’t use Raymond’s answer of setting them up after the class creation then perhaps you could use a metaclass:

class MetaFoo(type):
    def __new__(mcs, classname, bases, dictionary):
        for name in dictionary.get('_extra_vars', ()):
            dictionary[name] = 0
        return type.__new__(mcs, classname, bases, dictionary)

class Foo(): # For python 3.x use 'class Foo(metaclass=MetaFoo):'
    __metaclass__=MetaFoo # For Python 2.x only
    _extra_vars = 'tx ty tz'.split()

Answered By: Duncan

Answer 3

Late to the party but use the type class constructor!

Foo = type("Foo", (), {k: 0 for k in ("tx", "ty", "tz")})

Answered By: costrouc

Answer 4

The locals() version did not work for me in a class.

The following can be used to dynamically create the attributes of the class:

class namePerson:
    def __init__(self, value):
        exec("self.{} = '{}'".format("name", value)

me = namePerson(value='my name')
me.name # returns 'my name'

Answered By: Frederick Chesneau

Answer 5

setattr(object, name, value)
This is the counterpart of getattr(). The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it.
For example, setattr(x, 'name', value) is equivalent to x.name = value.

Answered By: afghani

Answer 6

The function you need is:

setattr(obj, name, value)

This allows you to set named attributes for a given class (this can be self).

The built in documentation for this function is pretty self-explanatory:

Signature: setattr(obj, name, value, /)
Docstring:
Sets the named attribute on the given object to the specified value.

setattr(x, 'y', v) is equivalent to ``x.y = v''
Type:      builtin_function_or_method

Example use

One use of this is to use a dictionary to set multiple class attributes, in my case this was from xpath definitions. I felt this improved maintainability by keeping potentially more fragile xpath definitions all in one place:

class Person:
    def _extract_fields(self):
        ''' Process the page using XPath definitions '''
        logging.debug("_extract_fields(): {}".format(repr(self)))
        # describe how to extract data from the publicly available site
        # (kept together for maintainability)
        fields = {
            'staff_name':
            '//div[@id="staff_name"]//text()',
            'staff_dob':
            '(//div[@id="staff_dob"]//text())[1]'
        }
        # populate named class attributes from the dict
        for key in fields:
            setattr(self, key, self._parsed_content.xpath(fields[key]))

    def __init__(self):
        self._extract_fields()

Answered By: Mark

how to create class variable dynamically in python

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