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How do I avoid the “self.x = x; self.y = y; self.z = z” pattern in __init__?

Posted by: admin November 1, 2017 Leave a comment

Questions:

I see patterns like

def __init__(self, x, y, z):
    ...
    self.x = x
    self.y = y
    self.z = z
    ...

quite frequently, often with a lot more parameters. Is there a good way to avoid this type of tedious repetitiveness? Should the class inherit from namedtuple instead?

Answers:

A decorator solution that keeps the signature:

import decorator
import inspect
import sys


@decorator.decorator
def simple_init(func, self, *args, **kws):
    """
    @simple_init
    def __init__(self,a,b,...,z)
        dosomething()

    behaves like

    def __init__(self,a,b,...,z)
        self.a = a
        self.b = b
        ...
        self.z = z
        dosomething()
    """

    #init_argumentnames_without_self = ['a','b',...,'z']
    if sys.version_info.major == 2:
        init_argumentnames_without_self = inspect.getargspec(func).args[1:]
    else:
        init_argumentnames_without_self = tuple(inspect.signature(func).parameters.keys())[1:]

    positional_values = args
    keyword_values_in_correct_order = tuple(kws[key] for key in init_argumentnames_without_self if key in kws)
    attribute_values = positional_values + keyword_values_in_correct_order

    for attribute_name,attribute_value in zip(init_argumentnames_without_self,attribute_values):
        setattr(self,attribute_name,attribute_value)

    # call the original __init__
    func(self, *args, **kws)


class Test():
    @simple_init
    def __init__(self,a,b,c,d=4):
        print(self.a,self.b,self.c,self.d)

#prints 1 3 2 4
t = Test(1,c=2,b=3)
#keeps signature
#prints ['self', 'a', 'b', 'c', 'd']
if sys.version_info.major == 2:
    print(inspect.getargspec(Test.__init__).args)
else:
    print(inspect.signature(Test.__init__))

Questions:
Answers:

EDIT

It seems that several people are concerned about presenting this solution, so I will provide a very clear disclaimer. You should not use this solution. I only provide it as information, so you know that the language is capable of this. The rest of the answer is just showing language capabilities, not endorsing using them in this way.

ORIGINAL ANSWER

There isn’t really anything wrong with explicitly copying parameters into attributes. If you have too many parameters in the ctor, it is sometimes considered a code smell and maybe you should group these params into a fewer objects. Other times, it is necessary and there is nothing wrong with it. Anyway, doing it explicitly is the way to go.

However, since you are asking HOW it can be done (and not whether it should be done), then one solution is this:

class A:
    def __init__(self, **kwargs):
        for key in kwargs:
          setattr(self, key, kwargs[key])

a = A(l=1, d=2)
a.l # will return 1
a.d # will return 2

Questions:
Answers:

As others have mentioned, the repetition isn’t bad, but in some cases a namedtuple can be a great fit for this type of issue. This avoids using locals() or kwargs, which are usually a bad idea.

from collections import namedtuple
# declare a new object type with three properties; x y z
# the first arg of namedtuple is a typename
# the second arg is comma-separated or space-separated property names
XYZ = namedtuple("XYZ", "x, y, z")

# create an object of type XYZ. properties are in order
abc = XYZ("one", "two", 3)
print abc.x
print abc.y
print abc.z

I’ve found limited use for it, but you can inherit a namedtuple as with any other object (example continued):

class MySuperXYZ(XYZ):
    """ I add a helper function which returns the original properties """
    def properties(self):
        return self.x, self.y, self.z

abc2 = MySuperXYZ(4, "five", "six")
print abc2.x
print abc2.y
print abc2.z
print abc2.properties()

Questions:
Answers:

explicit is better than implicit …
so sure you could make it more concise:

def __init__(self,a,b,c):
    for k,v in locals().items():
        if k != "self":
             setattr(self,k,v)

The better question is should you?

… that said if you want a named tuple I would recommend using a namedtuple (remember tuples have certain conditions attached to them) … perhaps you want an ordereddict or even just a dict …

Questions:
Answers:

To expand on gruszczys answer, I have used a pattern like:

class X:
    x = None
    y = None
    z = None
    def __init__(self, **kwargs):
        for (k, v) in kwargs.items():
            if hasattr(self, k):
                setattr(self, k, v)
            else:
                raise TypeError('Unknown keyword argument: {:s}'.format(k))

I like this method because it:

  • avoids repetition
  • is resistant against typos when constructing an object
  • works well with subclassing (can just super().__init(...))
  • allows for documentation of the attributes on a class-level (where they belong) rather than in X.__init__

Prior to Python 3.6, this gives no control over the order in which the attributes are set, which could be a problem if some attributes are properties with setters that access other attributes.

It could probably be improved upon a bit, but I’m the only user of my own code so I am not worried about any form of input sanitation. Perhaps an AttributeError would be more appropriate.

Questions:
Answers:

You could also do:

locs = locals()
for arg in inspect.getargspec(self.__init__)[0][1:]:
    setattr(self, arg, locs[arg])

Of course, you would have to import the inspect module.

Questions:
Answers:

This is a solution without any additional imports.

Helper function

A small helper function makes it more convenient and re-usable:

def auto_init(local_name_space):
    """Set instance attributes from arguments.
    """
    self = local_name_space.pop('self')
    for name, value in local_name_space.items():
        setattr(self, name, value)

Application

You need to call it with locals():

class A:
    def __init__(self, x, y, z):
        auto_init(locals())

Test

a = A(1, 2, 3)
print(a.__dict__)

Output:

{'y': 2, 'z': 3, 'x': 1}

Without changing locals()

If you don’t like to change locals() use this version:

def auto_init(local_name_space):
    """Set instance attributes from arguments.
    """
    for name, value in local_name_space.items():
        if name != 'self': 
            setattr(local_name_space['self'], name, value)

Questions:
Answers:

My 0.02$. It is very close to Joran Beasley answer, but more elegant:

def __init__(self, a, b, c, d, e, f):
    vars(self).update((k, v) for k, v in locals().items() if v is not self)

Additionally, Mike Müller’s answer (the best one to my taste) can be reduced with this technique:

def auto_init(ns):
    self = ns.pop('self')
    vars(self).update(ns)

And the just call auto_init(locals()) from your __init__

Questions:
Answers:

It’s a natural way to do things in Python. Don’t try to invent something more clever, it will lead to overly clever code that no one on your team will understand. If you want to be a team player and then keep writing it this way.

Questions:
Answers:

An interesting library that handles this (and avoids a lot of other boilerplate) is attrs. Your example, for instance, could be reduced to this (assume the class is called MyClass):

import attr

@attr.s
class MyClass:
    x = attr.ib()
    y = attr.ib()
    z = attr.ib()

You don’t even need an __init__ method anymore, unless it does other stuff as well. Here’s a nice introduction by Glyph Lefkowitz.