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0 pointsthe_mitsuhiko13y ago0 comments

> Classes hold state. More state adds more complexity, and typical OO design add lots of layers of indirection which also add complexity. Most of the time, you don't need the flexibility that the extra indirection gives you. So you get complexity for little benefit.

Functions hold state as well, they just encapsulate it. You can still break up your algorithm into a class with multiple template methods and then encapsulate it in a function if you're afraid of leaking state elsewhere.

What I see instead is that people can't get rid of state and put it as global variables into Python modules. Case in point: pickle. It uses sys.modules and pickle.dispatch_table to store some of its state.

> If you don't need to hold state, then a collection of functions will usually suffice. In Python, you can use modules for this.

Classes have another advantage: virtual method calls. If one function in a module calls into another function in the module I can only do two things: a) copy/paste the library and change one of the functions or b) a monkeypatch which modifies a shared resource someone else might want to use with the old semantics.

A class I can subclass and change the behavior for each method separately.

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4 comments · 2 top-level

nessus4213y ago· 2 in thread

> You can still break up your algorithm into a class with multiple template methods

Template methods are an abomination. They are only widely used because many OO languages don't have first-class functions. A template method means that the method is being parameterized by one or more functions. Parameters should be expressed as parameters! You wouldn't design a Circle class so that you need to subclass it to specify the radius of the circle. The radius is a parameter. Likewise for the callback functions used by a template method. Template methods are intrinsically difficult to understand and spaghetti-like, unless well-documented, because it is not immediately clear what the parameters to it are. And when overriding a callback method, it's not immediately clear what template method the overridden method is paramaterizing. Or that it is even parameterizing anything. (Caveat: Sometimes template methods are warranted, but they are greatly overused.)

> Classes have another advantage: virtual method calls.

You don't need virtual method calls in order to parameterize functionality. See previous paragraph. Furthermore, it is not safe to override a method without knowing if it has been designed for overriding, and what the class expects of the overriding method. See Effective Java, "Item 17: Design and document for inheritance or else prohibit it". Also see "Item 16: Favor composition over inheritance", and a similar discussion in the Gang of Four book.

> If one function in a module calls into another function in the module I can only do two things: a) copy/paste the library and change one of the functions or b) a monkeypatch which modifies a shared resource someone else might want to use with the old semantics.

If a method is not designed to be overridden, you shouldn't override it anyway. (See previous paragraph.) When using a functional approach (in a language with first-class functions), you can usually design for flexibility just as easily or more easily than when using an inheritance-based OO approach for flexibility.

dragonwriter13y ago

> Furthermore, it is not safe to override a method without knowing if it has been designed for overriding, and what the class expects of the overriding method.

This is true, but likewise, its not safe to parameterize functionality by passing callbacks unless you know what the function you are passing the callback to expects of callback functions (in terms of arguments, return values, and side effects.)

Both of these, really, are the same principle: its not safe to call code without knowing what the code expects of you when calling it.

nessus4213y ago

> This is true, but likewise, its not safe to parameterize functionality by passing callbacks unless you know what the function you are passing the callback to expects of callback functions

Of course; this goes without saying. But when programming in an OO style, programmers often override methods thinking that they are only changing the behavior of that one method, when in fact they could be inadvertently changing the behavior of other methods in the class.

When passing in functions as arguments, on the other hand, this makes it much more explicit what is supported and what isn't.

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rlpb13y ago

> What I see instead is that people can't get rid of state and put it as global variables into Python modules.

What I see is people writing classes that hold state in instances for far longer than is needed. Other code written around these classes then need to take this into account. This tends to the same place where global variables are. A tangled mess.

> If one function in a module calls into another function in the module I can only do two things...

3) Adjust the function to include the behaviour that you need. In Python, default parameters often mean that you can do this without breaking backwards compatibility. If a function is deficient in some way, why not fix the function rather than working around it?

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