How to return a value from init in Python

Question

I have a class with an   init   function    How can I return an integer value from this function when an object is created   I wrote a program  where   init   does command line parsing and I need to have some value set  Is it OK set it in global variable and use it in other member functions  If so how to do that  So far  I declared a variable outside class  and setting it one function doesn t reflect in other function

User · Answer

Sample Usage of the matter in question can be like:

class SampleObject(object):

    def __new__(cls, item):
        if cls.IsValid(item):
            return super(SampleObject, cls).__new__(cls)
        else:
            return None

    def __init__(self, item):
        self.InitData(item) #large amount of data and very complex calculations

...

ValidObjects = []
for i in data:
    item = SampleObject(i)
    if item:             # in case the i data is valid for the sample object
        ValidObjects.append(item)

I do not have enough reputation so I can not write a comment, it is crazy! I wish I could post it as a comment to weronika

User · Answer

Well  if you don t care about the object instance anymore     you can just replace it   class MuaHaHa    def   init   self  ret       self ret  print MuaHaHa  foo     foo

User · Answer

The   init   method  like other methods and functions returns None by default in the absence of a return statement  so you can write it like either of these   class Foo      def   init   self           self value 42  class Bar      def   init   self           self value 42         return None   But  of course  adding the return None doesn t buy you anything   I m not sure what you are after  but you might be interested in one of these   class Foo      def   init   self           self value 42     def   str   self           return str self value   f Foo   print f value print f   prints   42 42

User · Answer

init   return none value solved perfectly  class Solve  def   init   self w d       self value w     self unit d def   str   self       return str  my speed is   str self value      str self unit   ob Solve 21  kmh   print  ob    output  my speed is 21 kmh

User · Answer

From the documentation of   init        As a special constraint on constructors  no value may be returned  doing so will cause a TypeError to be raised at runtime    As a proof  this code   class Foo object       def   init   self           return 2  f   Foo     Gives this error   Traceback  most recent call last     File  test init py   line 5  in  lt module gt      f   Foo   TypeError    init     should return None  not  int

User · Answer

You can just set it to a class variable and read it from the main program   class Foo      def   init   self            Do your stuff here         self returncode   42 bar   Foo   baz   bar returncode

User · Answer

init   doesn t return anything and should always return None

User · Answer

Why would you want to do that   If you want to return some other object when a class is called  then use the   new     method   class MyClass object       def   init   self           print  never called in this case      def   new   cls           return 42  obj   MyClass   print obj

User · Answer

Just wanted to add  you can return classes in   init     property def failureException self       class MyCustomException AssertionError           def   init   self    args    kwargs                   Your code here                 return super     init    args    kwargs       MyCustomException   name     AssertionError   name       return MyCustomException   The above method helps you implement a specific action upon an Exception in your test

User · Answer

init   is required to return None  You cannot  or at least shouldn t  return something else   Try making whatever you want to return an instance variable  or function     gt  gt  gt  class Foo          def   init   self               return 42       gt  gt  gt  foo   Foo   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  TypeError    init     should return None

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