Does Python have private variables in classes

Question

I m coming from the Java world and reading Bruce Eckels  Python 3 Patterns  Recipes and Idioms    While reading about classes  it goes on to say that in Python there is no need to declare instance variables  You just use them in the constructor  and boom  they are there    So for example   class Simple      def   init   self  s           print  inside the simple constructor           self s   s      def show self           print self s       def showMsg self  msg           print msg        self show      If that   s true  then any object of class Simple can just change the value of variable s outside of the class    For example    if   name         main         x   Simple  constructor argument       x s    test15    this changes the value     x show       x showMsg  A message     In Java  we have been taught about public private protected variables   Those keywords make sense because at times you want variables in a class to which no one outside the class has access to    Why is that not required in Python

User · Answer

Python has limited support for private identifiers, through a feature that automatically prepends the class name to any identifiers starting with two underscores. This is transparent to the programmer, for the most part, but the net effect is that any variables named this way can be used as private variables.

See here for more on that.

In general, Python's implementation of object orientation is a bit primitive compared to other languages. But I enjoy this, actually. It's a very conceptually simple implementation and fits well with the dynamic style of the language.

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There is a variation of private variables in the underscore convention   In  5   class Test object               def   private method self                   return  Boo              def public method self                   return self   private method                 In  6   x   Test    In  7   x public method   Out 7    Boo   In  8   x   private method   --------------------------------------------------------------------------- AttributeError                            Traceback  most recent call last   lt ipython-input-8-fa17ce05d8bc gt  in  lt module gt    ---- gt  1 x   private method    AttributeError   Test  object has no attribute    private method    There are some subtle differences  but for the sake of programming pattern ideological purity  its good enough   There are examples out there of  private decorators that more closely implement the concept  but YMMV  Arguably one could also write a class defintion that uses meta

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In java  we have been taught about public private protected variables    Why is that not required in python    For the same reason  it s not required in Java   You re free to use -- or not use private and protected   As a Python and Java programmer  I ve found that private and protected are very  very important design concepts   But as a practical matter  in tens of thousands of lines of Java and Python  I ve never actually used private or protected   Why not   Here s my question  protected from whom      Other programmers on my team   They have the source   What does protected mean when they can change it   Other programmers on other teams   They work for the same company   They can -- with a phone call -- get the source   Clients   It s work-for-hire programming  generally    The clients  generally  own the code   So  who -- precisely -- am I protecting it from

User · Answer

It s cultural   In Python  you don t write to other classes  instance or class variables   In Java  nothing prevents you from doing the same if you really want to - after all  you can always edit the source of the class itself to achieve the same effect  Python drops that pretence of security and encourages programmers to be responsible  In practice  this works very nicely   If you want to emulate private variables for some reason  you can always use the    prefix from PEP 8  Python mangles the names of variables like   foo so that they re not easily visible to code outside the class that contains them  although you can get around it if you re determined enough  just like you can get around Java s protections if you work at it    By the same convention  the   prefix means stay away even if you re not technically prevented from doing so  You don t play around with another class s variables that look like   foo or  bar

User · Answer

As mentioned earlier  you can indicate that a variable or method is private by prefixing it with an underscore  If you don t feel like this is enough  you can always use the property decorator  Here s an example   class Foo       def   init   self  bar           self  bar   bar       property     def bar self              Getter for   bar              return self  bar   This way  someone or something that references bar is actually referencing the return value of the bar function rather than the variable itself  and therefore it can be accessed but not changed  However  if someone really wanted to  they could simply use  bar and assign a new value to it  There is no surefire way to prevent someone from accessing variables and methods that you wish to hide  as has been said repeatedly  However  using property is the clearest message you can send that a variable is not to be edited  property can also be used for more complex getter setter deleter access paths  as explained here  https   docs python org 3 library functions html property

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Sorry guys for  resurrecting  the thread  but  I hope this will help someone   In Python3 if you just want to  encapsulate  the class attributes  like in Java  you can just do the same thing like this   class Simple      def   init   self  str           print  inside the simple constructor           self   s   str      def show self           print self   s       def showMsg self  msg           print msg        self show      To instantiate this do   ss   Simple  lol   ss show     Note that  print ss   s  will throw an error   In practice  Python3 will obfuscate the global attribute name  Turning this like a  private  attribute  like in Java  The attribute s name is still global  but in an inaccessible way  like a private attribute in other languages    But don t be afraid of it  It doesn t matter  It does the job too

User · Answer

The only time I ever use private variables is when I need to do other things when writing to or reading from the variable and as such I need to force the use of a setter and or getter   Again this goes to culture  as already stated  I ve been working on projects where reading and writing other classes variables was free-for-all  When one implementation became deprecated it took a lot longer to identify all code paths that used that function  When use of setters and getters was forced  a debug statement could easily be written to identify that the deprecated method had been called and the code path that calls it   When you are on a project where anyone can write an extension  notifying users about deprecated methods that are to disappear in a few releases hence is vital to keep module breakage at a minimum upon upgrades   So my answer is  if you and your colleagues maintain a simple code set then protecting class variables is not always necessary  If you are writing an extensible system then it becomes imperative when changes to the core is made that needs to be caught by all extensions using the code

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As correctly mentioned by many of the comments above  let s not forget the main goal of Access Modifiers  To help users of code understand what is supposed to change and what is supposed not to  When you see a private field you don t mess around with it  So it s mostly syntactic sugar which is easily achieved in Python by the   and

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About sources  to change the access rights and thus bypass language encapsulation like java or C      You don t always have the sources and EVEN if you do  the sources are managed by a system that only allows certain programmers to access a source  in a professional context   Often  every programmer is responsible for certain classes and therefore knows what he can and cannot do  The source manager also locks the sources being modified and of course  manages the access rights of programmers  So i trust more in software than in human  by experience  So convention is good but MULTIPLE protections are better  like access management  real private variable    sources management

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private and protected concepts are very important  But python - just a tool for prototyping and rapid development with restricted resources available for development  that is why some of protection levels are not so strict followed in python  You can use      in class member  it works properly  but looks not good enough - each access to such field contains these characters    Also  you can noticed that python OOP concept is not perfect  smaltalk or ruby much closer to pure OOP concept  Even C  or Java are closer    Python is very good tool  But it is simplified OOP language  Syntactically and conceptually simplified  The main goal of python existence is to bring to developers possibility to write easy readable code with high abstraction level in a very fast manner

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Private variables in python is more or less a hack  the interpreter intentionally renames the variable   class A      def   init   self           self   var   123     def printVar self           print self   var   Now  if you try to access   var outside the class definition  it will fail     gt  gt  gt x   A     gt  gt  gt x   var   this will return error   A has no attribute   var     gt  gt  gt x printVar     this gives back 123   But you can easily get away with this     gt  gt  gt x   dict     this will show everything that is contained in object x                  which in this case is something like    A  var    123     gt  gt  gt x  A  var   456   you now know the masked name of private variables   gt  gt  gt x printVar     this gives back 456   You probably know that methods in OOP are invoked like this  x printVar     gt  A printVar x   if A printVar   can access some field in x  this field can also be accessed outside A printVar     after all  functions are created for reusability  there is no special power given to the statements inside   The game is different when there is a compiler involved  privacy is a compiler level concept   It know about class definition with access control modifiers so it can error out if the rules are not being followed at compile time

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Python does not have any private variables like C   or Java does  You could access any member variable at any time if wanted  too  However  you don t need private variables in Python  because in Python it is not bad to expose your classes member variables  If you have the need to encapsulate a member variable  you can do this by using  quot  property quot  later on without breaking existing client code  In python the single underscore  quot   quot  is used to indicate  that a method or variable is not considered as part of the public api of a class and that this part of the api could change between different versions  You can use these methods variables  but your code could break  if you use a newer version of this class  The double underscore  quot    quot   does not mean a  quot private variable quot   You use it to define variables which are  quot class local quot  and which can not be easily overidden by subclasses  It mangles the variables name  For example  class A object       def   init   self           self   foobar   None   will be automatically mangled to self  A  foobar  class B A       def   init   self           self   foobar   1   will be automatically mangled to self  B  foobar  self   foobar s name is automatically mangled to self  A  foobar in class A  In class B it is mangled to self  B  foobar  So every subclass can define its own variable   foobar without overriding its parents variable s   But nothing prevents you from accessing variables beginning with double underscores  However  name-mangling prevents you from calling this variables  methods incidentally  I strongly recommend to watch Raymond Hettingers talk  quot Pythons class development toolkit quot  from Pycon 2013  should be available on Youtube   which gives a good example why and how you should use  property and  quot    quot -instance variables  If you have exposed public variables and you have the need to encapsulate them  then you can use  property  Therefore you can start with the simplest solution possible  You can leave member variables public unless you have a concrete reason to not do so  Here is an example  class Distance      def   init   self  meter           self meter   meter   d   Distance 1 0  print d meter    prints 1 0  class Distance      def   init   self  meter             Customer request  Distances must be stored in millimeters            Public available internals must be changed            This would break client code in C              This is why you never expose public variables in C   or Java            However  this is python          self millimeter   meter   1000        In python we have  property to the rescue       property     def meter self           return self millimeter  0 001       meter setter     def meter self  value           self millimeter   meter   1000  d   Distance 1 0  print d meter    prints 1 0

[python] Does Python have “private” variables in classes?

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