Reflection/List properties

Task

The goal is to get the properties of an object, as names, values or both.

Some languages support dynamic properties, which in general can only be inspected if a class' public API includes a way of listing them.

Java

<lang java>import java.lang.reflect.Field;

public class ListFields {

   public int examplePublicField = 42;
   private boolean examplePrivateField = true;
   
   public static void main(String[] args) throws IllegalAccessException {
       ListFields obj = new ListFields();
       Class clazz = obj.getClass();

       System.out.println("All public fields (including inherited):");
       for (Field f : clazz.getFields()) {
           System.out.printf("%s\t%s\n", f, f.get(obj));
       }
       System.out.println();
       System.out.println("All declared fields (excluding inherited):");
       for (Field f : clazz.getDeclaredFields()) {
           System.out.printf("%s\t%s\n", f, f.get(obj));
       }
   }

}</lang>

Output:

All public fields (including inherited):
public int ListFields.examplePublicField	42

All declared fields (excluding inherited):
public int ListFields.examplePublicField	42
private boolean ListFields.examplePrivateField	true

There are multiple ways of getting property names, each of which include different subsets of an object's properties, such as enumerable or inherited properties. Properties in JavaScript can be enumerable or non-enumerable; enumerable properties are accessable when looping over the object with for. Object.getOwnPropertyNames().

<lang javascript>var obj = Object.create({

   name: 'proto',
   proto: true,
   doNothing: function() {}
 }, {
   name: {value: 'obj', writable: true, configurable: true, enumerable: true},
   obj: {value: true, writable: true, configurable: true, enumerable: true},
   'non-enum': {value: 'non-enumerable', writable: true, enumerable: false},
   doStuff: {value: function() {}, enumerable: true}

});

// get enumerable properties on an object and its ancestors function get_property_names(obj) {

   var properties = [];
   for (var p in obj) {
       properties.push(p);
   }
   return properties;

}

get_property_names(obj); //["name", "obj", "doStuff", "proto", "doNothing"]

Object.getOwnPropertyNames(obj); //["name", "obj", "non-enum", "doStuff"]

Object.keys(obj); //["name", "obj", "doStuff"]

Object.entries(obj); //[["name", "obj"], ["obj", true], ["doStuff", function()]] </lang>

Objective-C

<lang objc>#import <Foundation/Foundation.h>

import <objc/runtime.h>

@interface Foo : NSObject {

 int exampleIvar;

} @property (nonatomic) double exampleProperty; @end @implementation Foo - (instancetype)init {

 self = [super init];
 if (self) {
   exampleIvar = 42;
   _exampleProperty = 3.14;
 }
 return self;

} @end

int main() {

 id obj = [[Foo alloc] init];
 Class clazz = [obj class];

 NSLog(@"\Instance variables:");
 unsigned int ivarCount;
 Ivar *ivars = class_copyIvarList(clazz, &ivarCount);
 for (unsigned int i = 0; i < ivarCount; i++) {
   Ivar ivar = ivars[i];
   const char *name = ivar_getName(ivar);
   const char *typeEncoding = ivar_getTypeEncoding(ivar);
   // for simple types we can use Key-Value Coding to access it
   // but in general we will have to use object_getIvar and cast it to the right type of function
   // corresponding to the type of the instance variable
   id value = [obj valueForKey:@(name)];
   NSLog(@"%s\t%s\t%@", name, typeEncoding, value);
 }
 free(ivars);

 NSLog(@"");
 NSLog(@"Properties:");
 unsigned int propCount;
 objc_property_t *properties = class_copyPropertyList([Foo class], &propCount);
 for (unsigned int i = 0; i < propCount; i++) {
   objc_property_t p = properties[i];
   const char *name = property_getName(p);
   const char *attributes = property_getAttributes(p);
   // for simple types we can use Key-Value Coding to access it
   // but in general we will have to use object_getIvar and cast it to the right type of function
   // corresponding to the type of the instance variable
   id value = [obj valueForKey:@(name)];
   NSLog(@"%s\t%s\t%@", name, attributes, value);
 }
 free(properties);

 return 0;

}</lang>

Output:

Instance variables:
exampleIvar	i	42
_exampleProperty	d	3.14

Properties:
exampleProperty	Td,N,V_exampleProperty	3.14

Python

The dir() function and Python's inspect module both will list properties.

<lang python>class Parent(object):

   __priv = 'private'
   
   def __init__(self, name):
       self.name = name
   
   def __repr__(self):
       return '%s(%s)' % (type(self).__name__, self.name)
   
   def doNothing(self):
       pass

import re

class Child(Parent):

   # prefix for "private" fields
   __rePrivate = re.compile('^_(Child|Parent)__')
   # used when setting dynamic property values
   __reBleh = re.compile('\Wbleh$')
   @property
   def reBleh(self):
       return self.__reBleh
   
   def __init__(self, name, *args):
       super(Child, self).__init__(name)
       self.args = args
   
   def __dir__(self):
       myDir = filter(
           # filter out private fields
           lambda p: not self.__rePrivate.match(p),
           list(set( \
               sum([dir(base) for base in type(self).__bases__], []) \
               + type(self).__dict__.keys() \
               + self.__dict__.keys() \
           )))
       return myDir + map(
           # dynamic properties
           lambda p: p + '_bleh',
           filter(
               # don't add dynamic properties for methods and other special properties
               lambda p: (p[:2] != '__' or p[-2:] != '__') and not callable(getattr(self, p)),
               myDir))
   
   def __getattr__(self, name):
       if name[-5:] == '_bleh':
           # dynamic '_bleh' properties
           return str(getattr(self, name[:-5])) + ' bleh'
       if hasattr(super(Child, chld), '__getattr__'):
           return super(Child, self).__getattr__(name)
       raise AttributeError("'%s' object has no attribute '%s'" % (type(self).__name__, name))
   
   def __setattr__(self, name, value):
       if name[-5:] == '_bleh':
           # skip backing properties that are methods
           if not (hasattr(self, name[:-5]) and callable(getattr(self, name[:-5]))):
               setattr(self, name[:-5], self.reBleh.sub(, value))
       elif hasattr(super(Child, self), '__setattr__'):
           super(Child, self).__setattr__(name, value)
       elif hasattr(self, '__dict__'):
           self.__dict__[name] = value
   
   def __repr__(self):
       return '%s(%s, %s)' % (type(self).__name__, self.name, str(self.args).strip('[]()'))
   
   def doStuff(self):
       return (1+1.0/1e6) ** 1e6

par = Parent('par') par.parent = True dir(par)

['_Parent__priv', '__class__', ..., 'doNothing', 'name', 'parent']

inspect.getmembers(par)

[('_Parent__priv', 'private'), ('__class__', <class '__main__.Parent'>), ..., ('doNothing', <bound method Parent.doNothing of <__main__.Parent object at 0x100777650>>), ('name', 'par'), ('parent', True)]

chld = Child('chld', 0, 'I', 'two') chld.own = "chld's own" dir(chld)

['__class__', ..., 'args', 'args_bleh', 'doNothing', 'doStuff', 'name', 'name_bleh', 'own', 'own_bleh', 'reBleh', 'reBleh_bleh']

inspect.getmembers(chld)

[('__class__', <class '__main__.Child'>), ..., ('args', (0, 'I', 'two')), ('args_bleh', "(0, 'I', 'two') bleh"), ('doNothing', <bound method Child.doNothing of Child(chld, 0, 'I', 'two')>), ('doStuff', <bound method Child.doStuff of Child(chld, 0, 'I', 'two')>), ('name', 'chld'), ('name_bleh', 'chld bleh'), ('own', "chld's own"), ('own_bleh', "chld's own bleh"), ('reBleh', <_sre.SRE_Pattern object at 0x10067bd20>), ('reBleh_bleh', '<_sre.SRE_Pattern object at 0x10067bd20> bleh')]

</lang>