Is it not possible to define multiple constructors in Python

Question

Possible Duplicate   What is a clean  pythonic way to have multiple constructors in Python    Is it not possible to define multiple constructors in Python  with different signatures   If not  what s the general way of getting around it  For example  let s say you wanted to define a class City  I d like to be able to say someCity   City   or someCity   City  quot Berlin quot    where the first just gives a default name value  and the second defines it

User · Accepted Answer

Unlike Java  you cannot define multiple constructors   However  you can define a default value if one is not passed   def   init   self  city  Berlin      self city   city

User · Answer

Jack M  is right  Do it this way   gt  gt  gt  class City          def   init   self  city None               self city   city         def   repr   self               if self city   return self city             return         gt  gt  gt  c   City  Berlin    gt  gt  gt  print c Berlin  gt  gt  gt  c   City    gt  gt  gt  print c   gt  gt  gt

User · Answer

For the example you gave  use default values   class City      def   init   self  name  Default City Name                          In general  you have two options   1  Do if-elif blocks based on the type   def   init   self  name       if isinstance name  str                   elif isinstance name  City                         2  Use duck typing --- that is  assume the user of your class is intelligent enough to use it correctly  This is typically the preferred option

User · Answer

The easiest way is through keyword arguments  class City      def   init   self  city None       pass  someCity   City city  quot Berlin quot    This is pretty basic stuff  Maybe look at the Python documentation

User · Answer

If your signatures differ only in the number of arguments  using default arguments is the right way to do it  If you want to be able to pass in different kinds of argument  I would try to avoid the isinstance-based approach mentioned in another answer  and instead use keyword arguments  If using just keyword arguments becomes unwieldy  you can combine it with classmethods  the bzrlib code likes this approach   This is just a silly example  but I hope you get the idea  class C object        def   init   self  fd             Assume fd is a file-like object          self fd   fd       classmethod     def fromfilename cls  name           return cls open name   rb       Now you can do  c   C fd    or  c   C fromfilename  a filename    Notice all those classmethods still go through the same   init    but using classmethods can be much more convenient than having to remember what combinations of keyword arguments to   init   work  isinstance is best avoided because Python s duck typing makes it hard to figure out what kind of object was actually passed in  For example  if you want to take either a filename or a file-like object you cannot use isinstance arg  file   because there are many file-like objects that do not subclass file  like the ones returned from urllib  or StringIO  or      It s usually a better idea to just have the caller tell you explicitly what kind of object was meant  by using different keyword arguments

[python] Is it not possible to define multiple constructors in Python?

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