[Python-ideas] JavaScript-Style Object Creation in Python (using a constructor function instead of a class to create objects)

Brendan Barnwell brenbarn at brenbarn.net
Sun May 14 00:53:51 EDT 2017

On 2017-05-13 21:07, Simon Ramstedt wrote:
> Hi, do you have an opinion on the following?

	My general opinion is that imitating JavaScript is almost always a bad 
idea.  :-)

> Wouldn't it be nice to define classes via a simple constructor function
> (as below) instead of a conventional class definition?
> *conventional*:
> |
> classMyClass(ParentClass):
> def__init__(x):
> self._x=x
> defmy_method(y):
>          z=self._x+y
> returnz
> |
> *proposed*:
> |
> defMyClass(x):
> self=ParentClass()
> defmy_method(y):
>          z=x+y
> returnz
> self.my_method=my_method # that's cumbersome (see comments below)
> returnself
> |
> Here are the pros and cons I could come up with for the proposed method:
> (+) Simpler and more explicit.

	I don't really see how that's simpler or more explicit.  In one respect 
it's clearly less explicit, as the "self" is implicit.

> (+) No need to create attributes (like `self._x`) just to pass something
> from `__init__` to another method.

	Attributes aren't just for passing things to other methods.  They're 
for storing state.  In your proposed system, how would an object mutate 
one of its own attributes?  It looks like "x" here is just stored in a 
function closure, which wouldn't allow easy mutation.  Also, how would 
another object access the attribute from outside (as we currently do 
with self.x)?  You can say we'd only use this new attribute-free 
approach when we want to pass a constructor argument that's used but 
never mutated or accessed from outside, but that severely restricts the 
potential use cases, and all it saves you is typing "self".

	Relatedly, how is ParentClass itself defined?  I don't see how you 
could bootstrap this without having a real class at the bottom of it 
somehow (as your test script in fact does).

> (+) Default arguments / annotations for methods could be different for
> each class instance. Adaptive defaults wouldn't have to simulated with a
> None.

	That seems as likely to be a negative as a positive.  Having different 
instances with different default values could be confusing.  This would 
even allow different instances to define totally different methods (with 
if-logic inside the function constructor), which would definitely be 

> (+) Class/instance level imports would work.

	How often is that really needed?

> (-/+) Speed: The `def`-based objects take 0.6 μs to create while the
> `class`-based objects take only 0.4 μs. For method execution however the
> closure takes only 0.15 μs while the proper method takes 0.22 μs (script
> <https://gist.github.com/rmst/78b2b0f56a3d9ec13b1ec6f3bd50aa9c>).

	I don't think you can really evaluate the performance impact of this 
alternative just based on a trivial example like that.

> (-/+) Checking types: In the proposed example above the returned object
> wouldn't know that it has been created by `MyClass`. There are a couple
> of solutions to that, though. The easiest to implement would be to
> change the first line to `self = subclass(ParentClass())` where the
> subclass function looks at the next item in the call stack (i.e.
> `MyClass`) and makes it the type of the object. Another solution would
> be to have a special rule for functions with capital first letter
> returning a single object to append itself to the list of types of the
> returned object. Alternatively there could be a special keyword e.g.
> `classdef` that would be used instead of `def` if we wouldn't want to
> rely on the name.

	Those special rules sound very hackish to me.

> (-) The current syntax for adding a function to an object is
> cumbersome.  That's what is preventing me from actually using the
> proposed pattern. But is this really the only reason for not using it?
> And if so, wouldn't that be a good argument for enabling something like
> below?
> *
> *
> *attribute function definitions*:
> |
> defMyClass(x):
> self=ParentClass()
> defself.my_method(y):
>          z=x+y
> returnz
> returnself
> |
> or alternatively*multiline lambdas*:
> |
> defMyClass(x):
> self=ParentClass()
> self.my_method=(y):
>          z=x+y
> returnz
> returnself
> |

	To be honest, from all your examples, I don't really see what the point 
is.  It's a different syntax for doing some of the same things the 
existing class syntax does, while providing no apparent way to do some 
important things (like mutating attributes).  I think Python's existing 
class syntax is simple, clear, and quite nice overall, and creating 
class instances by calling a function instead of a class doesn't add 
anything.  In fact, even JavaScript has recently added classes to allow 
programmers to move away from the old approach that you describe here. 
Also, as I alluded to above, JavaScript is so terrible in so many ways 
that the mere idea of imitating it inherently makes me skeptical; 
there's almost nothing about JavaScript's design that couldn't be done 
better, and most of what it does are things that Python already does 
better and has done better for years.  In short, I don't see any 
advantages at all to doing classes this way, and there are some 
non-negligible disadvantages.

Brendan Barnwell
"Do not follow where the path may lead.  Go, instead, where there is no 
path, and leave a trail."
    --author unknown

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