How do I avoid the “self.x = x; self.y = y; self.z

Question

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?

Answer 1

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.

Answer 2

Python 3.7 onwards

In Python 3.7, you may (ab)use the dataclass decorator, available from the dataclasses module. From the documentation:

This module provides a decorator and functions for automatically adding generated special methods such as __init__() and __repr__() to user-defined classes. It was originally described in PEP 557.

The member variables to use in these generated methods are defined using PEP 526 type annotations. For example this code:

@dataclass
class InventoryItem:
    '''Class for keeping track of an item in inventory.'''
    name: str
    unit_price: float
    quantity_on_hand: int = 0

    def total_cost(self) -> float:
        return self.unit_price * self.quantity_on_hand

Will add, among other things, a __init__() that looks like:

def __init__(self, name: str, unit_price: float, quantity_on_hand: int=0):
      self.name = name
      self.unit_price = unit_price
      self.quantity_on_hand = quantity_on_hand

Note that this method is automatically added to the class: it is not directly specified in the InventoryItem definition shown above.

If your class is large and complex, it may be inappropriate to use a dataclass. I'm writing this on the day of release of Python 3.7.0, so usage patterns are not yet well established.

Answer 3

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.

Answer 4

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.

Answer 5

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()