I want to translate if possible the following from Python to D. Please help.

---
class cached_property(object):
    """A version of @property which caches the value.  On access, it calls the
    underlying function and sets the value in `__dict__` so future accesses
    will not re-call the property.
    """
    def __init__(self, f):
        self._fname = f.__name__
        self._f = f

    def __get__(self, obj, owner):
        assert obj is not None, 'call {} on an instance'.format(self._fname)
        ret = obj.__dict__[self._fname] = self._f(obj)
        return ret
---

Maybe https://dlang.org/phobos/std_functional.html#memoize should helped you

On Tue, Jan 29, 2019 at 4:15 PM Victor Porton via Digitalmars-d < digitalmars-d@puremagic.com> wrote:

> I want to translate if possible the following from Python to D. Please help.
>
> ---
> class cached_property(object):
>      """A version of @property which caches the value.  On access,
> it calls the
>      underlying function and sets the value in `__dict__` so
> future accesses
>      will not re-call the property.
>      """
>      def __init__(self, f):
>          self._fname = f.__name__
>          self._f = f
>
>      def __get__(self, obj, owner):
>          assert obj is not None, 'call {} on an
> instance'.format(self._fname)
>          ret = obj.__dict__[self._fname] = self._f(obj)
>          return ret
> ---
>

On Tuesday, 29 January 2019 at 15:12:37 UTC, Victor Porton wrote:
> I want to translate if possible the following from Python to D. Please help.
>
> ---
> class cached_property(object):
>     """A version of @property which caches the value.  On access, it calls the
>     underlying function and sets the value in `__dict__` so future accesses
>     will not re-call the property.
>     """
>     def __init__(self, f):
>         self._fname = f.__name__
>         self._f = f
>
>     def __get__(self, obj, owner):
>         assert obj is not None, 'call {} on an instance'.format(self._fname)
>         ret = obj.__dict__[self._fname] = self._f(obj)
>         return ret
> ---

Please use the learn forum for such questions. Thanks!

On Tuesday, 29 January 2019 at 15:26:13 UTC, Daniel Kozak wrote:
> Maybe https://dlang.org/phobos/std_functional.html#memoize should helped you

No, this does not compile:

struct C {
    int _f() { writeln("Got!"); return 2; }
    alias f = memoize!_f;
}

I've successfully implemented it myself:

---
import std.stdio;

mixin template Cached(string name, string baseName = '_' ~ name) {
    mixin("private typeof(" ~ baseName ~ ") " ~ name ~ "Cache;");
    mixin("private bool " ~ name ~ "IsCached = false;");
    mixin("@property typeof(" ~ baseName ~ ") " ~ name ~ "() {\n" ~
          "if (" ~ name ~ "IsCached" ~ ") return " ~ name ~ "Cache;\n" ~
          name ~ "IsCached = true;\n" ~
          "return " ~ name ~ "Cache = " ~ baseName ~ ";\n" ~
          '}');
}

struct C {
    @property int _f() { writeln("Got!"); return 2; }
    mixin Cached!"f";
}

void main(string[] args)
{
    C x;
    writeln(x.f);
    writeln(x.f);
}
---

I think, we should add this to the standard library. What do you think?

On Tue, Jan 29, 2019 at 05:13:16PM +0000, Victor Porton via Digitalmars-d wrote:
> I've successfully implemented it myself:
> 
> ---
> import std.stdio;
> 
> mixin template Cached(string name, string baseName = '_' ~ name) {
>     mixin("private typeof(" ~ baseName ~ ") " ~ name ~ "Cache;");
>     mixin("private bool " ~ name ~ "IsCached = false;");
>     mixin("@property typeof(" ~ baseName ~ ") " ~ name ~ "() {\n" ~
>           "if (" ~ name ~ "IsCached" ~ ") return " ~ name ~ "Cache;\n" ~
>           name ~ "IsCached = true;\n" ~
>           "return " ~ name ~ "Cache = " ~ baseName ~ ";\n" ~
>           '}');
> }
> 
> struct C {
>     @property int _f() { writeln("Got!"); return 2; }
>     mixin Cached!"f";
> }
> 
> void main(string[] args)
> {
>     C x;
>     writeln(x.f);
>     writeln(x.f);
> }
> ---
> 
> I think, we should add this to the standard library. What do you think?

You could have just used std.typecons.Nullable instead of reimplementing it yourself:

	private Nullable!T _myField;
	@property T myField() {
		if (_myField.isNull)
			_myField = ...; // initialize it here
		return _myField;
	}


T

-- 
Elegant or ugly code as well as fine or rude sentences have something in common: they don't depend on the language. -- Luca De Vitis

On Tuesday, 29 January 2019 at 18:00:36 UTC, H. S. Teoh wrote:
> You could have just used std.typecons.Nullable instead of reimplementing it yourself:
>
> 	private Nullable!T _myField;
> 	@property T myField() {
> 		if (_myField.isNull)
> 			_myField = ...; // initialize it here
> 		return _myField;
> 	}

I know, but how it is better than my code?