Using property versus getters and setters

Question

Here is a pure Python-specific design question    class MyClass object               def get my attr self                    def set my attr self  value                 and   class MyClass object                        property     def my attr self                     my attr setter     def my attr self  value                 Python lets us to do it either way  If you would design a Python program  which approach would you use and why

User · Answer

In complex projects I prefer using read-only properties (or getters) with explicit setter function:

class MyClass(object):
...        
@property
def my_attr(self):
    ...

def set_my_attr(self, value):
    ...

In long living projects debugging and refactoring takes more time than writing the code itself. There are several downsides for using @property.setter that makes debugging even harder:

1) python allows creating new attributes for an existing object. This makes a following misprint very hard to track:

my_object.my_atttr = 4.

If your object is a complicated algorithm then you will spend quite some time trying to find out why it doesn't converge (notice an extra 't' in the line above)

2) setter sometimes might evolve to a complicated and slow method (e.g. hitting a database). It would be quite hard for another developer to figure out why the following function is very slow. He might spend a lot of time on profiling do_something() method, while my_object.my_attr = 4. is actually the cause of slowdown:

def slow_function(my_object):
    my_object.my_attr = 4.
    my_object.do_something()

User · Answer

Using properties is to me more intuitive and fits better into most code   Comparing  o x   5 ox   o x   vs   o setX 5  ox   o getX     is to me quite obvious which is easier to read  Also properties allows for private variables much easier

User · Answer

TL DR   You can skip to the end for a code example    I actually prefer to use a different idiom  which is a little involved for using as a one off  but is nice if you have a more complex use case   A bit of background first   Properties are useful in that they allow us to handle both setting and getting values in a programmatic way but still allow attributes to be accessed as attributes   We can turn  gets  into  computations   essentially  and we can turn  sets  into  events    So let s say we have the following class  which I ve coded with Java-like getters and setters   class Example object       def   init   self  x None  y None           self x   x         self y   y      def getX self           return self x or self defaultX        def getY self           return self y or self defaultY        def setX self  x           self x   x      def setY self  y           self y   y      def defaultX self           return someDefaultComputationForX        def defaultY self           return someDefaultComputationForY     You may be wondering why I didn t call defaultX and defaultY in the object s   init   method   The reason is that for our case I want to assume that the someDefaultComputation methods return values that vary over time  say a timestamp  and whenever x  or y  is not set  where  for the purpose of this example   not set  means  set to None   I want the value of x s  or y s  default computation   So this is lame for a number of reasons describe above   I ll rewrite it using properties   class Example object       def   init   self  x None  y None           self  x   x         self  y   y       property     def x self           return self x or self defaultX         x setter     def x self  value           self  x   value       property     def y self           return self y or self defaultY         y setter     def y self  value           self  y   value        default XY  as before    What have we gained   We ve gained the ability to refer to these attributes as attributes even though  behind the scenes  we end up running methods   Of course the real power of properties is that we generally want these methods to do something in addition to just getting and setting values  otherwise there is no point in using properties    I did this in my getter example   We are basically running a function body to pick up a default whenever the value isn t set   This is a very common pattern   But what are we losing  and what can t we do   The main annoyance  in my view  is that if you define a getter  as we do here  you also have to define a setter  1   That s extra noise that clutters the code   Another annoyance is that we still have to initialize the x and y values in   init      Well  of course we could add them using setattr   but that is more extra code    Third  unlike in the Java-like example  getters cannot accept other parameters   Now I can hear you saying already  well  if it s taking parameters it s not a getter   In an official sense  that is true   But in a practical sense there is no reason we shouldn t be able to parameterize an named attribute -- like x -- and set its value for some specific parameters   It d be nice if we could do something like   e x a b c    10 e x d e f    20   for example   The closest we can get is to override the assignment to imply some special semantics   e x    a b c 10  e x    d e f 30    and of course ensure that our setter knows how to extract the first three values as a key to a dictionary and set its value to a number or something   But even if we did that we still couldn t support it with properties because there is no way to get the value because we can t pass parameters at all to the getter   So we ve had to return everything  introducing an asymmetry   The Java-style getter setter does let us handle this  but we re back to needing getter setters   In my mind what we really want is something that capture the following requirements    Users define just one method for a given attribute and can indicate there whether the attribute is read-only or read-write   Properties fail this test if the attribute writable  There is no need for the user to define an extra variable underlying the function  so we don t need the   init   or setattr in the code   The variable just exists by the fact we ve created this new-style attribute  Any default code for the attribute executes in the method body itself  We can set the attribute as an attribute and reference it as an attribute  We can parameterize the attribute    In terms of code  we want a way to write   def x self   args       return defaultX     and be able to then do   print e x     - gt  The default at time T0 e x   1 print e x     - gt  1 e x   None print e x     - gt  The default at time T1   and so forth   We also want a way to do this for the special case of a parameterizable attribute  but still allow the default assign case to work   You ll see how I tackled this below   Now to the point  yay  the point     The solution I came up for for this is as follows   We create a new object to replace the notion of a property   The object is intended to store the value of a variable set to it  but also maintains a handle on code that knows how to calculate a default   Its job is to store the set value or to run the method if that value is not set   Let s call it an UberProperty   class UberProperty object        def   init   self  method           self method   method         self value   None         self isSet   False      def setValue self  value           self value   value         self isSet   True      def clearValue self           self value   None         self isSet   False   I assume method here is a class method  value is the value of the UberProperty  and I have added isSet because None may be a real value and this allows us a clean way to declare there really is  no value    Another way is a sentinel of some sort   This basically gives us an object that can do what we want  but how do we actually put it on our class   Well  properties use decorators  why can t we   Let s see how it might look  from here on I m going to stick to using just a single  attribute   x    class Example object         uberProperty     def x self           return defaultX     This doesn t actually work yet  of course   We have to implement uberProperty and make sure it handles both gets and sets   Let s start with gets   My first attempt was to simply create a new UberProperty object and return it   def uberProperty f       return UberProperty f    I quickly discovered  of course  that this doens t work  Python never binds the callable to the object and I need the object in order to call the function   Even creating the decorator in the class doesn t work  as although now we have the class  we still don t have an object to work with   So we re going to need to be able to do more here   We do know that a method need only be represented the one time  so let s go ahead and keep our decorator  but modify UberProperty to only store the method reference   class UberProperty object        def   init   self  method           self method   method   It is also not callable  so at the moment nothing is working   How do we complete the picture   Well  what do we end up with when we create the example class using our new decorator   class Example object         uberProperty     def x self           return defaultX    print Example x      lt   main   UberProperty object at 0x10e1fb8d0 gt  print Example   x    lt   main   UberProperty object at 0x10e1fb8d0 gt    in both cases we get back the UberProperty which of course is not a callable  so this isn t of much use   What we need is some way to dynamically bind the UberProperty instance created by the decorator after the class has been created to an object of the class before that object has been returned to that user for use   Um  yeah  that s an   init   call  dude   Let s write up what we want our find result to be first   We re binding an UberProperty to an instance  so an obvious thing to return would be a BoundUberProperty   This is where we ll actually maintain state for the x attribute   class BoundUberProperty object       def   init   self  obj  uberProperty           self obj   obj         self uberProperty   uberProperty         self isSet   False      def setValue self  value           self value   value         self isSet   True      def getValue self           return self value if self isSet else self uberProperty method self obj       def clearValue self           del self value         self isSet   False   Now we the representation  how do get these on to an object   There are a few approaches  but the easiest one to explain just uses the   init   method to do that mapping   By the time   init   is called our decorators have run  so just need to look through the object s   dict   and update any attributes where the value of the attribute is of type UberProperty   Now  uber-properties are cool and we ll probably want to use them a lot  so it makes sense to just create a base class that does this for all subclasses   I think you know what the base class is going to be called   class UberObject object       def   init   self           for k in dir self               v   getattr self  k              if isinstance v  UberProperty                   v   BoundUberProperty self  v                  setattr self  k  v    We add this  change our example to inherit from UberObject  and      e   Example   print e x               - gt   lt   main   BoundUberProperty object at 0x104604c90 gt    After modifying x to be    uberProperty def x self       return  datetime datetime now      We can run a simple test   print e x getValue   print e x getValue   e x setValue datetime date 2013  5  31   print e x getValue   e x clearValue   print e x getValue     And we get the output we wanted   2013-05-31 00 05 13 985813 2013-05-31 00 05 13 986290 2013-05-31 2013-05-31 00 05 13 986310    Gee  I m working late    Note that I have used getValue  setValue  and clearValue here   This is because I haven t yet linked in the means to have these automatically returned   But I think this is a good place to stop for now  because I m getting tired   You can also see that the core functionality we wanted is in place  the rest is window dressing   Important usability window dressing  but that can wait until I have a change to update the post   I ll finish up the example in the next posting by addressing these things    We need to make sure UberObject s   init   is always called by subclasses    So we either force it be called somewhere or we prevent it from being implemented  We ll see how to do this with a metaclass   We need to make sure we handle the common case where someone  aliases  a function to something else  such as     class Example object          uberProperty       def x self                        y   x  We need e x to return e x getValue   by default    What we ll actually see is this is one area where the model fails  It turns out we ll always need to use a function call to get the value  But we can make it look like a regular function call and avoid having to use e x getValue     Doing this one is obvious  if you haven t already fixed it out    We need to support setting e x directly  as in e x    lt newvalue gt    We can do this in the parent class too  but we ll need to update our   init   code to handle it  Finally  we ll add parameterized attributes   It should be pretty obvious how we ll do this  too    Here s the code as it exists up to now   import datetime  class UberObject object       def uberSetter self  value           print  setting       def uberGetter self           return self      def   init   self           for k in dir self               v   getattr self  k              if isinstance v  UberProperty                   v   BoundUberProperty self  v                  setattr self  k  v    class UberProperty object       def   init   self  method           self method   method  class BoundUberProperty object       def   init   self  obj  uberProperty           self obj   obj         self uberProperty   uberProperty         self isSet   False      def setValue self  value           self value   value         self isSet   True      def getValue self           return self value if self isSet else self uberProperty method self obj       def clearValue self           del self value         self isSet   False      def uberProperty f           return UberProperty f   class Example UberObject         uberProperty     def x self           return datetime datetime now      1  I may be behind on whether this is still the case

User · Answer

I would prefer to use neither in most cases  The problem with properties is that they make the class less transparent  Especially  this is an issue if you were to raise an exception from a setter  For example  if you have an Account email property   class Account object        property     def email self           return self  email       email setter     def email self  value           if     not in value              raise ValueError  Invalid email address            self  email   value   then the user of the class does not expect that assigning a value to the property could cause an exception   a   Account   a email    badaddress  -- gt  ValueError  Invalid email address    As a result  the exception may go unhandled  and either propagate too high in the call chain to be handled properly  or result in a very unhelpful traceback being presented to the program user  which is sadly too common in the world of python and java    I would also avoid using getters and setters    because defining them for all properties in advance is very time consuming  makes the amount of code unnecessarily longer  which makes understanding and maintaining the code more difficult  if you were define them for properties only as needed  the interface of the class would change  hurting all users of the class   Instead of properties and getters setters I prefer doing the complex logic in well defined places such as in a validation method   class Account object               def validate self           if     not in self email              raise ValueError  Invalid email address      or a similiar Account save method   Note that I am not trying to say that there are no cases when properties are useful  only that you may be better off if you can make your classes simple and transparent enough that you don t need them

User · Answer

I feel like properties are about letting you get the overhead of writing getters and setters only when you actually need them   Java Programming culture strongly advise to never give access to properties  and instead  go through getters and setters  and only those which are actually needed  It s a bit verbose to always write these obvious pieces of code  and notice that 70  of the time they are never replaced by some non-trivial logic   In Python  people actually care for that kind of overhead  so that you can embrace the following practice     Do not use getters and setters at first  when if they not needed Use  property to implement them without changing the syntax of the rest of your code

User · Answer

Using properties lets you begin with normal attribute accesses and then back them up with getters and setters afterwards as necessary

User · Answer

In Python you don t use getters or setters or properties just for the fun of it  You first just use attributes and then later  only if needed  eventually migrate to a property without having to change the code using your classes   There is indeed a lot of code with extension  py that uses getters and setters and inheritance and pointless classes everywhere where e g  a simple tuple would do  but it s code from people writing in C   or Java using Python   That s not Python code

User · Answer

I am surprised that nobody has mentioned that properties are bound methods of a descriptor class  Adam Donohue and NeilenMarais get at exactly this idea in their posts -- that getters and setters are functions and can be used to    validate alter data duck type  coerce type to another type    This presents a smart way to hide implementation details and code cruft like regular expression  type casts  try    except blocks  assertions or computed values   In general doing CRUD on an object may often be fairly mundane but consider the example of data that will be persisted to a relational database   ORM s can hide implementation details of particular SQL vernaculars in the methods bound to fget  fset  fdel defined in a property class that will manage the awful if    elif    else ladders that are so ugly in OO code -- exposing the simple and elegant self variable   something and obviate the details for the developer using the ORM   If one thinks of properties only as some dreary vestige of a Bondage and Discipline language  i e  Java  they are missing the point of descriptors

User · Answer

I think both have their place  One issue with using  property is that it is hard to extend the behaviour of getters or setters in subclasses using standard class mechanisms  The problem is that the actual getter setter functions are hidden in the property   You can actually get hold of the functions  e g  with  class C object        p   1      property     def p self           return self  p      p setter     def p self  val           self  p   val   you can access the getter and setter functions as C p fget and C p fset  but you can t easily use the normal method inheritance  e g  super  facilities to extend them  After some digging into the intricacies of super  you can indeed use super in this way     Using super    class D C         Cannot use super D D  here to define the property       since D is not yet defined in this scope       property     def p self           return super D D  p fget self        p setter     def p self  val           print  Implement extra functionality here for D          super D D  p fset self  val     Using a direct reference to C class E C       p   C p       p setter     def p self  val           print  Implement extra functionality here for E          C p fset self  val    Using super   is  however  quite clunky  since the property has to be redefined  and you have to use the slightly counter-intuitive super cls cls  mechanism to get an unbound copy of p

User · Answer

Prefer properties  It s what they re there for   The reason is that all attributes are public in Python  Starting names with an underscore or two is just a warning that the given attribute is an implementation detail that may not stay the same in future versions of the code  It doesn t prevent you from actually getting or setting that attribute  Therefore  standard attribute access is the normal  Pythonic way of  well  accessing attributes   The advantage of properties is that they are syntactically identical to attribute access  so you can change from one to another without any changes to client code  You could even have one version of a class that uses properties  say  for code-by-contract or debugging  and one that doesn t for production  without changing the code that uses it  At the same time  you don t have to write getters and setters for everything just in case you might need to better control access later

User · Answer

The short answer is  properties wins hands down  Always   There is sometimes a need for getters and setters  but even then  I would  hide  them to the outside world  There are plenty of ways to do this in Python  getattr  setattr    getattribute    etc     but a very concise and clean one is   def set email self  value       if     not in value          raise Exception  This doesn t look like an email address        self  email   value  def get email self       return self  email  email   property get email  set email    Here s a brief article that introduces the topic of getters and setters in Python

User · Answer

Here is an excerpts from  Effective Python  90 Specific Ways to Write Better Python   Amazing book  I highly recommend it       Things to Remember        Define new class interfaces using simple public attributes and avoid   defining setter and getter methods         Use  property to define special behavior when attributes are   accessed on your objects  if necessary         Follow the rule of least surprise and avoid odd side effects in your    property methods         Ensure that  property methods are fast  for slow or complex   work   especially involving I O or causing side effects   use normal   methods instead       One advanced but common use of  property is transitioning what was   once a simple numerical attribute into an on-the-fly calculation  This   is extremely helpful because it lets you migrate all existing usage of   a class to have new behaviors without requiring any of the call sites   to be rewritten  which is especially important if there   s calling code   that you don   t control    property also provides an important stopgap   for improving interfaces over time       I especially like  property because it lets you make incremental   progress toward a better data model over time     property is a tool to   help you address problems you   ll come across in real-world code  Don   t   overuse it  When you find yourself repeatedly extending  property   methods  it   s probably time to refactor your class instead of further   paving over your code   s poor design         Use  property to give existing instance attributes   new functionality         Make incremental progress toward better data   models by using  property         Consider refactoring a class and all call   sites when you find yourself using  property too heavily

User · Answer

Both  property and traditional getters and setters have their advantages  It depends on your use case   Advantages of  property   You don t have to change the interface while changing the implementation of data access  When your project is small  you probably want to use direct attribute access to access a class member  For example  let s say you have an object foo of type Foo  which has a member num  Then you can simply get this member with num   foo num  As your project grows  you may feel like there needs to be some checks or debugs on the simple attribute access  Then you can do that with a  property within the class  The data access interface remains the same so that there is no need to modify client code   Cited from PEP-8      For simple public data attributes  it is best to expose just the attribute name  without complicated accessor mutator methods  Keep in mind that Python provides an easy path to future enhancement  should you find that a simple data attribute needs to grow functional behavior  In that case  use properties to hide functional implementation behind simple data attribute access syntax   Using  property for data access in Python is regarded as Pythonic    It can strengthen your self-identification as a Python  not Java  programmer  It can help your job interview if your interviewer thinks Java-style getters and setters are anti-patterns     Advantages of traditional getters and setters   Traditional getters and setters allow for more complicated data access than simple attribute access  For example  when you are setting a class member  sometimes you need a flag indicating where you would like to force this operation even if something doesn t look perfect  While it is not obvious how to augment a direct member access like foo num   num  You can easily augment your traditional setter with an additional force parameter   def Foo      def set num self  num  force False                Traditional getters and setters make it explicit that a class member access is through a method  This means    What you get as the result may not be the same as what is exactly stored within that class  Even if the access looks like a simple attribute access  the performance can vary greatly from that    Unless your class users expect a  property hiding behind every attribute access statement  making such things explicit can help minimize your class users surprises  As mentioned by  NeilenMarais and in this post  extending traditional getters and setters in subclasses is easier than extending properties  Traditional getters and setters have been widely used for a long time in different languages  If you have people from different backgrounds in your team  they look more familiar than  property  Also  as your project grows  if you may need to migrate from Python to another language that doesn t have  property  using traditional getters and setters would make the migration smoother    Caveats   Neither  property nor traditional getters and setters makes the class member private  even if you use double underscore before its name   class Foo      def   init   self           self   num   0       property     def num self           return self   num       num setter     def num self  num           self   num   num      def get num self           return self   num      def set num self  num           self   num   num  foo   Foo   print foo num             output  0 print foo get num         output  0 print foo  Foo  num       output  0

[python] Using @property versus getters and setters

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