Defining private module functions in python

Question

According to http   www faqs org docs diveintopython fileinfo private html   Like most languages  Python has the concept of private elements   Private functions  which can t be called from outside their module   However  if I define two files   a py   num 1  and   b py import a print a   num  when i run b py it prints out 1 without giving any exception  Is diveintopython wrong  or did I misunderstand something  And is there some way to do define a module s function as private

User · Answer

Python has three modes via   private  public and protected  While importing a module only public mode is accessible  So private and protected modules cannot be called from outside of the module i e   when it is imported

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Python allows for private class members with the double underscore prefix   This technique doesn t work at a module level so I am thinking this is a mistake in Dive Into Python   Here is an example of private class functions   class foo        def bar self   pass     def   bar self   pass  f   foo   f bar       this call succeeds f   bar     this call fails

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embedded with closures or functions is one way   This is common in JS although not required for non-browser platforms or browser workers   In Python it seems a bit strange  but if something really needs to be hidden than that might be the way   More to the point using the python API and keeping things that require to be hidden in the C  or other language  is probably the best way   Failing that I would go for putting the code inside a function  calling that and having it return the items you want to export

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For methods   I am not sure if this exactly what you want  print thrice py def private method       def methodist string           if   name       quot   main   quot               method string      return methodist       private def private print3 string       print string   3   private print3  quot Hello  quot     output  Hello Hello Hello  other file py from print thrice import private print3 private print3  quot Hello From Another File   quot     no output  This is probably not a perfect solution  as you can still  quot see quot  and or  quot call quot  the method  Regardless  it doesn t execute

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In Python   privacy  depends on  consenting adults   levels of agreement - you can t force it  any more than you can in real life -    A single leading underscore means you re not supposed to access it  from the outside  -- two leading underscores  w o trailing underscores  carry the message even more forcefully    but  in the end  it still depends on social convention and consensus  Python s introspection is forceful enough that you can t handcuff every other programmer in the world to respect your wishes     Btw  though it s a closely held secret  much the same holds for C    with most compilers  a simple  define private public line before  includeing your  h file is all it takes for wily coders to make hash of your  privacy     -

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There may be confusion between class privates and module privates   A module private starts with one underscore Such a element is not copied along when using the from  lt module name gt  import   form of the import command   it is however imported if using the import  lt moudule name gt  syntax   see Ben Wilhelm s answer  Simply remove one underscore from the a   num of the question s example and it won t show in modules that import a py using the from a import   syntax   A class private starts with two underscores   aka dunder i e  d-ouble under-score   Such a variable has its name  mangled  to include the classname etc  It can still be accessed outside of the class logic  through the mangled name  Although the name mangling can serve as a mild prevention device against unauthorized access  its main purpose is to prevent possible name collisions with class members of the ancestor classes   See Alex Martelli s funny but accurate reference to consenting adults as he describes the convention used in regards to these variables    gt  gt  gt  class Foo object            bar   99        def PrintBar self              print self   bar       gt  gt  gt  myFoo   Foo    gt  gt  gt  myFoo   bar   direct attempt no go Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  AttributeError   Foo  object has no attribute    bar   gt  gt  gt  myFoo PrintBar      the class itself of course can access it 99  gt  gt  gt  dir Foo       yet can see it   PrintBar     Foo  bar      class        delattr        dict        doc         format        getattribute        hash        init        module        new        reduce        reduce ex        repr        setattr        sizeof        str         subclasshook        weakref      gt  gt  gt  myFoo  Foo  bar   and get to it by its mangled name     but I shouldn t     99  gt  gt  gt

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You can add an inner function   def public self  args      def private self root  data          if  self root    None             pass  do something with data   Something like that if you really need that level of privacy

User · Answer

This is an ancient question  but both module private  one underscore  and class-private  two underscores  mangled variables are now covered in the standard documentation   The Python Tutorial    Classes    Private Variables

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This question was not fully answered  since module privacy is not purely conventional  and since using import may or may not recognize module privacy  depending on how it is used   If you define private names in a module  those names will be imported into any script that uses the syntax   import module name   Thus  assuming you had correctly defined in your example the module private   num  in a py  like so     a py  num 1     you would be able to access it in b py with the module name symbol    b py import a     foo   a  num   1   To import only non-privates from a py  you must use the from syntax    b py from a import       foo    num   throws NameError  name   num  is not defined   For the sake of clarity  however  it is better to be explicit when importing names from modules  rather than importing them all with a        b py from a import name1  from a import name2

[python] Defining private module functions in python

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