How to overload init method based on argument type

Question

Let s say I have a class that has a member called data which is a list     I want to be able to initialize the class with  for example  a filename  which contains data to initialize the list  or with an actual list   What s your technique for doing this   Do you just check the type by looking at   class     Is there some trick I might be missing   I m used to C   where overloading by argument type is easy

User · Answer

Excellent question  I ve tackled this problem as well  and while I agree that  factories   class-method constructors  are a good method  I would like to suggest another  which I ve also found very useful   Here s a sample  this is a read method and not a constructor  but the idea is the same    def read self  str None  filename None  addr 0           Read binary data and return a store object  The data         store is also saved in the interal  data  attribute           The data can either be taken from a string  str          argument  or a file  provide a filename  which will          be read in binary mode   If both are provided  the str          will be used  If neither is provided  an ArgumentError          is raised              if str is None          if filename is None              raise ArgumentError  Please supply a string or a filename            file   open filename   rb           str   file read           file close                     rest of code   The key idea is here is using Python s excellent support for named arguments to implement this  Now  if I want to read the data from a file  I say   obj read filename  blob txt     And to read it from a string  I say   obj read str   x34 x55     This way the user has just a single method to call  Handling it inside  as you saw  is not overly complex

User · Answer

Quick and dirty fix  class MyData      def   init   string None list None           if string is not None               do stuff         elif list is not None               do other stuff         else               make data empty   Then you can call it with  MyData astring  MyData None  alist  MyData

User · Answer

A better way would be to use isinstance and type conversion  If I m understanding you right  you want this   def   init    self  filename       if isinstance  filename  basestring             filename is a string     else            try to convert to a list         self path   list  filename

User · Answer

You probably want the isinstance builtin function   self data   data if isinstance data  list  else self parse data

User · Answer

Why don t you go even more pythonic    class AutoList  def   init   self  inp       try                            Assume an opened-file            self data   inp read       except AttributeError          try                        Assume an existent filename                with open inp   r   as fd                  self data   fd read           except              self data   inp        Who cares what that might be

User · Answer

My preferred solution is   class MyClass       data            init   self data None             do init stuff         if not data  return         self  data   list data    list   copies the list  instead of pointing to it    Then invoke it with either MyClass   or MyClass  1 2 3     Hope that helps  Happy Coding

User · Answer

A much neater way to get  alternate constructors  is to use classmethods  For instance    gt  gt  gt  class MyData          def   init   self  data                Initialize MyData from a sequence              self data   data                   classmethod         def fromfilename cls  filename                Initialize MyData from a file              data   open filename  readlines               return cls data                    classmethod         def fromdict cls  datadict                Initialize MyData from a dict s items              return cls datadict items          gt  gt  gt  MyData  1  2  3   data  1  2  3   gt  gt  gt  MyData fromfilename   tmp foobar   data   foo n    bar n    baz n    gt  gt  gt  MyData fromdict   spam    ham    data    spam    ham      The reason it s neater is that there is no doubt about what type is expected  and you aren t forced to guess at what the caller intended for you to do with the datatype it gave you  The problem with isinstance x  basestring  is that there is no way for the caller to tell you  for instance  that even though the type is not a basestring  you should treat it as a string  and not another sequence   And perhaps the caller would like to use the same type for different purposes  sometimes as a single item  and sometimes as a sequence of items  Being explicit takes all doubt away and leads to more robust and clearer code

User · Answer

You should use isinstance  isinstance          isinstance object  class-or-type-or-tuple  - gt  bool      Return whether an object is an instance of a class or of a subclass thereof      With a type as second argument  return whether that is the object s type      The form using a tuple  isinstance x   A  B         is a shortcut for     isinstance x  A  or isinstance x  B  or      etc

User · Answer

OK  great  I just tossed together this example with a tuple  not a filename  but that s easy  Thanks all   class MyData      def   init   self  data           self myList              if isinstance data  tuple               for i in data                  self myList append i          else              self myList   data      def GetData self           print self myList   a    1 2   b    2 3   c   MyData a   d   MyData b   c GetData    d GetData     1  2    2  3

User · Answer

with python3  you can use Implementing Multiple Dispatch with Function Annotations as Python Cookbook wrote   import time   class Date metaclass MultipleMeta       def   init   self  year int  month int  day int           self year   year         self month   month         self day   day      def   init   self           t   time localtime           self   init   t tm year  t tm mon  t tm mday    and it works like    gt  gt  gt  d   Date 2012  12  21   gt  gt  gt  d year 2012  gt  gt  gt  e   Date    gt  gt  gt  e year 2018

User · Answer

Excellent question  I ve tackled this problem as well  and while I agree that  factories   class-method constructors  are a good method  I would like to suggest another  which I ve also found very useful   Here s a sample  this is a read method and not a constructor  but the idea is the same    def read self  str None  filename None  addr 0           Read binary data and return a store object  The data         store is also saved in the interal  data  attribute           The data can either be taken from a string  str          argument  or a file  provide a filename  which will          be read in binary mode   If both are provided  the str          will be used  If neither is provided  an ArgumentError          is raised              if str is None          if filename is None              raise ArgumentError  Please supply a string or a filename            file   open filename   rb           str   file read           file close                     rest of code   The key idea is here is using Python s excellent support for named arguments to implement this  Now  if I want to read the data from a file  I say   obj read filename  blob txt     And to read it from a string  I say   obj read str   x34 x55     This way the user has just a single method to call  Handling it inside  as you saw  is not overly complex

[python] How to overload init method based on argument type?

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