Python – Why is _replace Method of Namedtuple Classes Protected?

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In the section Method Names and Instance Variables, the Python Style Guide (aka PEP 0008) says, "Use one leading underscore only for non-public methods and instance variables." Why then, does the _replace method on namedtuple have a leading underscore? namedtuple classes themselves do not invoke _replace at all, and meanwhile the existence of _replace is advertised in the docstring for namedtuple() (see here, for example).

Best Answer

namedtuple is a bit unusual thing in python. It is not a base class, it is a class generator. Invoking it creates a new class that uses a tuple for its storage and can be manipulated as either tuple or normal class with named fields.

The generated class is intended to be used as a base for your own types that will add methods, and constraints. Including constraints that tie the values of individual fields together.

So the namedtuple might be used as base for classes that don't support replacing. And since derived class can't remove methods its base provides, the method must not be public.

However some derived classes might want that functionality. So it is provided, as internal method. The subclass can export it if it has a good use for it.

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I think it is a bad idea.

Its a very odd use of meta classes which will contribute to confusion of anyone trying to follow your code. In order to make it worth using this technique, you need the advantages to outweigh that confusion.

As for your reasons:

Creating an accesible Foo class that can be instantiated, but doesn't actually work like a typical object is much more prone to misuse than an internal class that you know you aren't supposed to call.
Don't name the base class with an underscore. If its intended to be inherited by other modules, it shouldn't be named that way.
There already is a method to implement static classes in python, staticmethod or classmethod. As noted, its not hard to implement a staticproperty

In general, there is a theme in your approach of trying to prevent client code from doing the wrong thing. That's not a pythonic attitude. The pythonic approach is to trust your client code. If they want to do something crazy, let them. Don't try to prevent client from instantiating your internal object, there may well be a good reason to do that.

I'm a bit of a purist and think that you shouldn't be storing state like this. So the very fact that you are asking the question means you're already doing it wrong. If you have state, I think it should be in an object. I think storing any sort of state at the module level should be avoided. I think its only a good idea to implement something like this if you have no changing state.

To close off, I'd like to plug the Code Review Stack Exchange, where I'm a moderator. If you post your code there you can get suggestions on how to make it better. I think you may get some helpful suggestions for how to restructure your code to avoid this question from even coming up.

Python – Replace Function for Classes Similar to Namedtuple _replace

namedtuple has such a method because it itself is immutable. Other immutable types in the standard library have one too, like datetime.datetime for example.

It is not a common pattern to use with mutable objects. So no, there is no built-in version for custom types for this. Custom classes invariably require custom handling anyway.

Note that your utility is a function, and not a method either. You'd usually make it a method on custom classes:

class cab(object):
    a = None
    b = None

    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __repr__(self):
        return "cab(a=%s, b=%s)" % (self.a, self.b)

    def replace(self, **kw):
        return type(self)(kw.get('a', self.a), kw.get('b', self.b))

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