Read-Only properties without getters

Question

I was reading this link about using getters in Python, quote:

The Pythonic way is to not use them. If you must have them then hide them behind a property.

That same page includes various examples, but my question is, if getters are not the Python way, how would I indicate to someone reading my code that a variable should be read-only after construction.

Suppose I have the following class:

class Car:
    def __init__(self, ID, name, tire, engine):
        self.ID = ID
        self.name = name
        self.tire = tire
        self.engine = engine

    def __str__(self):
        return "ID: {0} Name:{1} Tire: {2} Engine: {3}".format(self.ID, self.name, self.tire, self.engine)

If I wanted to indicate to another developer that self.engine is read-only after being set in the constructor, how would I do so? In orders words, if clients violate the rule, and attempt to set it, it's there problem if the implementation of self.engine changes(for example, from object of the class Engine to Dictionary).

Give it a single underscore prefix e.g `self._engine`. It wont stop others from using it but it is a clear indicator its private. You can also use a double underscore prefix to have python mangle the name (see [here](https://stackoverflow.com/questions/1301346/what-is-the-meaning-of-a-single-and-a-double-underscore-before-an-object-name) for details. — Paul Rooney, Dec 08 '17 at 02:30
@PaulRooney No, the underscore indicates that the variable shouldn't be accessed from outside of the class at all (except from derived classes). — Michael Butscher, Dec 08 '17 at 02:31
I don't mind if they get it, just that they aren't allowed to set it afterward(i'm aware there is nothing really stopping them from doing so). — , Dec 08 '17 at 02:32
@MichaelButscher I'm not clear on where your disagreement lies. — Paul Rooney, Dec 08 '17 at 02:35
@S.R. Right. If you don't want to use properties you could list the accessible variables in the doc-string of the class and mention something like "(read-write)" or "(read-only) for each. — Michael Butscher, Dec 08 '17 at 02:36
@PaulRooney - He saying the underscore should be treated as protected variables — , Dec 08 '17 at 02:37
@S.R. and so was I, so it would seem there is no disagreement on that point. — Paul Rooney, Dec 08 '17 at 02:38
@PaulRooney The OP wanted a read-only variable just by documentation (without `property`) but without property, an underscored variable is by convention meant to be `protected` in Java/C++ notion. — Michael Butscher, Dec 08 '17 at 02:39
Why not just define a _docstring_ for your property and treat the users of your class as consenting adults? — zwer, Dec 08 '17 at 02:40
@zwer - I think you've got it, any attempt to set it after being warned with the docstring is there problem, not mine. — , Dec 08 '17 at 02:42
I think the quote from the link is just wrong (or poorly worded). Properties are the Pythonic way and they need of course a hidden getter (and maybe setter, deleter) function behind it. — Michael Butscher, Dec 08 '17 at 02:46
@MichaelButscher - Agreed, that being said I selected the answer below on how to use them. Thanks! — , Dec 08 '17 at 02:47

Galen · Accepted Answer · 2017-12-08T02:49:37.133

2

You can use the @property decorator and not define a setter to communicate that engine should not be modified:

class Car(object):
    def __init__(self, ID, name, tire, engine):
        self.ID = ID
        self.name = name
        self.tire = tire
        self._engine = engine

    def __str__(self):
        return "ID: {0} Name:{1} Tire: {2} Engine: {3}".format(self.ID, self.name, self.tire, self.engine)

    @property
    def engine(self):
        return self._engine

Note that the user can still go around the property and set _engine directly.

Note that the @property decorator only works with new style classes.

edited Dec 08 '17 at 02:49

answered Dec 08 '17 at 02:36

Galen

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the `@properly` keyword will work with classes even if they don't inherit object, at least in Python 3 anyway. As stated in my question and the link `@property` is not the Python way of doing it apparently. – Dec 08 '17 at 02:38
2

@S.R. no, you are not understanding that link. Properties are the Pythonic way. – juanpa.arrivillaga Dec 08 '17 at 02:41
@S.R. I think the answer you linked is referring to defining methods like `get_thing` and `set_thing`, not using `@property`. The documentation for this facility basically describes this use case: "A typical use is to define a managed attribute x". See the specific example for the read only property `voltage`. – Galen Dec 08 '17 at 02:42
@Galen - I think your right, I've been using `property` this way as described by you and the docs, but seeing that other link made me question if I was doing the right thing. – Dec 08 '17 at 02:45
1

@S.R. In Python 3, all classes are new style classes and happen to inherit from `object`. Old style classes were removed. I will edit my answer to avoid confusion. – Galen Dec 08 '17 at 02:48

Read-Only properties without getters

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