Serializing class instance to JSON

Question

I am trying to create a JSON string representation of a class instance and having difficulty  Let s say the class is built like this   class testclass      value1    a      value2    b    A call to the json dumps is made like this   t   testclass   json dumps t    It is failing and telling me that the testclass is not JSON serializable   TypeError   lt   main   testclass object at 0x000000000227A400 gt  is not JSON serializable   I have also tried using the pickle module    t   testclass   print pickle dumps t  pickle HIGHEST PROTOCOL     And it gives class instance information but not a serialized content of the class instance   b  x80 x03c  main   ntestclass nq x00  x81q x01 q x02b     What am I doing wrong

User · Answer

There s another really simple and elegant approach that can be applied here which is to just subclass  dict  since it is serializable by default  from json import dumps  class Response dict       def   init   self  status code  body           super     init                status code   status code              body   body            r   Response   dumps r

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There are some good answers on how to get started on doing this  But there are some things to keep in mind    What if the instance is nested inside a large data structure  What if also want the class name  What if you want to deserialize the instance  What if you re using   slots   instead of   dict    What if you just don t want to do it yourself    json-tricks is a library  that I made and others contributed to  which has been able to do this for quite a while  For example   class MyTestCls      def   init   self    kwargs           for k  v in kwargs items                setattr self  k  v   cls instance   MyTestCls s  ub   dct   7   7    json   dumps cls instance  indent 4  instance   loads json    You ll get your instance back  Here the json looks like this            instance type                json tricks test class            MyTestCls              attributes              s    ub            dct                  7   7                     If you like to make your own solution  you might look at the source of json-tricks so as not to forget some special cases  like   slots      It also does other types like numpy arrays  datetimes  complex numbers  it also allows for comments

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I just do   data json dumps myobject   dict      This is not the full answer  and if you have some sort of complicated object class you certainly will not get everything  However I use this for some of my simple objects    One that it works really well on is the  options  class that you get from the OptionParser module   Here it is along with the JSON request itself      def executeJson self  url  options           data json dumps options   dict            if options verbose              print data         headers     Content-type    application json    Accept    text plain           return requests post url  data  headers headers

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The basic problem is that the JSON encoder json dumps   only knows how to serialize a limited set of object types by default  all built-in types   List here  https   docs python org 3 3 library json html encoders-and-decoders  One good solution would be to make your class inherit from JSONEncoder and then implement the JSONEncoder default   function  and make that function emit the correct JSON for your class   A simple solution would be to call json dumps   on the    dict   member of that instance   That is a standard Python dict and if your class is simple it will be JSON serializable   class Foo object       def   init   self           self x   1         self y   2  foo   Foo   s   json dumps foo    raises TypeError with  is not JSON serializable   s   json dumps foo   dict      s set to    x  1   y  2    The above approach is discussed in this blog posting    nbsp  nbsp  nbsp  nbsp Serializing arbitrary Python objects to JSON using   dict

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Here are two simple functions for serialization of any non-sophisticated classes  nothing fancy as explained before   I use this for configuration type stuff because I can add new members to the classes with no code adjustments   import json  class SimpleClass      def   init   self  a None  b None  c None           self a   a         self b   b         self c   c  def serialize json instance None  path None       dt          dt update vars instance        with open path   w   as file          json dump dt  file   def deserialize json cls None  path None       def read json  path           with open  path   r   as file              return json load file       data   read json path       instance   object   new   cls       for key  value in data items            setattr instance  key  value       return instance    Usage  Create class and serialize under Windows file system  write settings   SimpleClass a 1  b 2  c 3  serialize json write settings  r c  temp test json      Read back and rehydrate  read settings   deserialize json SimpleClass  r c  temp test json      results are the same  print vars write settings   print vars read settings      output      c   3   b   2   a   1      c   3   b   2   a   1

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Using jsonpickle  import jsonpickle  object   YourClass   json object   jsonpickle encode object

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Use arbitrary  extensible object  and then serialize it to JSON  import json  class Object object       pass  response   Object   response debug      response result   Object      Any manipulations with the object  response debug append  quot Debug string here quot   response result body    quot 404 Not Found quot  response result code   404    Proper JSON output  with nice formatting  print json dumps response  indent 4  default lambda x  x   dict

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Python3 x  The best aproach I could reach with my knowledge was this  Note that this code treat set   too  This approach is generic just needing the extension of class  in the second example   Note that I m just doing it to files  but it s easy to modify the behavior to your taste   However this is a CoDec   With a little more work you can construct your class in other ways  I assume a default constructor to instance it  then I update the class dict   import json import collections   class JsonClassSerializable json JSONEncoder        REGISTERED CLASS           def register ctype           JsonClassSerializable REGISTERED CLASS ctype   name      ctype      def default self  obj           if isinstance obj  collections Set               return dict  set object list obj           if isinstance obj  JsonClassSerializable               jclass                  jclass  name     type obj    name               jclass  dict     obj   dict               return dict  class object jclass          else              return json JSONEncoder default self  obj       def json to class self  dct           if   set object  in dct              return set dct   set object            elif   class object  in dct              cclass   dct   class object               cclass name   cclass  name               if cclass name not in self REGISTERED CLASS                  raise RuntimeError                       Class    not registered in JSON Parser                       format cclass  name                                  instance   self REGISTERED CLASS cclass name                instance   dict     cclass  dict               return instance         return dct      def encode  self  file           with open file   w   as outfile              json dump                  self   dict    outfile                  cls JsonClassSerializable                  indent 4                  sort keys True                    def decode  self  file           try              with open file   r   as infile                  self   dict     json load                      infile                      object hook self json to class                           except FileNotFoundError              print  Persistence load failed                           do not exists  format file                        class C JsonClassSerializable        def   init   self           self mill    s    JsonClassSerializable register C    class B JsonClassSerializable        def   init   self           self a   1230         self c   C     JsonClassSerializable register B    class A JsonClassSerializable        def   init   self           self a   1         self b    1  2          self c   B    JsonClassSerializable register A   A   encode   test   b   A   b decode   test   print b a  print b b  print b c a      Edit  With some more of research I found a way to generalize without the need of the SUPERCLASS register method call  using a metaclass  import json import collections  REGISTERED CLASS       class MetaSerializable type        def   call   cls   args    kwargs           if cls   name   not in REGISTERED CLASS              REGISTERED CLASS cls   name      cls         return super MetaSerializable  cls    call    args    kwargs    class JsonClassSerializable json JSONEncoder  metaclass MetaSerializable        def default self  obj           if isinstance obj  collections Set               return dict  set object list obj           if isinstance obj  JsonClassSerializable               jclass                  jclass  name     type obj    name               jclass  dict     obj   dict               return dict  class object jclass          else              return json JSONEncoder default self  obj       def json to class self  dct           if   set object  in dct              return set dct   set object            elif   class object  in dct              cclass   dct   class object               cclass name   cclass  name               if cclass name not in REGISTERED CLASS                  raise RuntimeError                       Class    not registered in JSON Parser                       format cclass  name                                  instance   REGISTERED CLASS cclass name                instance   dict     cclass  dict               return instance         return dct      def encode  self  file           with open file   w   as outfile              json dump                  self   dict    outfile                  cls JsonClassSerializable                  indent 4                  sort keys True                    def decode  self  file           try              with open file   r   as infile                  self   dict     json load                      infile                      object hook self json to class                           except FileNotFoundError              print  Persistence load failed                           do not exists  format file                        class C JsonClassSerializable        def   init   self           self mill    s    class B JsonClassSerializable        def   init   self           self a   1230         self c   C     class A JsonClassSerializable        def   init   self           self a   1         self b    1  2          self c   B     A   encode   test   b   A   b decode   test   print b a    1 print b b     1  2  print b c a    1230 print b c c mill    s

User · Answer

You could use jsonic for to serialize pretty much anything to JSON  https   github com OrrBin Jsonic Example  class TestClass  def   init   self       self x   1     self y   2  instance   TestClass   s   serialize instance     instance s set to    quot x quot  1   quot y quot  2  d   deserialize s    d is a new class instance of TestClass  pythonic has some nice features like declaring class attributes transient and type safe deserialization   A few years late with the answer  but i think it might help others

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I believe instead of inheritance as suggested in accepted answer  it s better to use polymorphism  Otherwise you have to have a big if else statement to customize encoding of every object  That means create a generic default encoder for JSON as   def jsonDefEncoder obj      if hasattr obj   jsonEnc          return obj jsonEnc      else   some default behavior       return obj   dict     and then have a jsonEnc   function in each class you want to serialize  e g   class A object      def   init   self lengthInFeet         self lengthInFeet lengthInFeet    def jsonEnc self         return   lengthInMeters   lengthInFeet   0 3     each foot is 0 3 meter   Then you call json dumps classInstance default jsonDefEncoder

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You can specify the default named parameter in the json dumps   function   json dumps obj  default lambda x  x   dict      Explanation   Form the docs  2 7  3 6      default obj    is a function that should return a serializable version of obj or raise TypeError  The default simply raises TypeError     Works on Python 2 7 and Python 3 x   Note  In this case you need instance variables and not class variables  as the example in the question tries to do   I am assuming the asker meant class instance to be an object of a class   I learned this first from  phihag s answer here  Found it to be the simplest and cleanest way to do the job

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JSON is not really meant for serializing arbitrary Python objects   It s great for serializing dict objects  but the pickle module is really what you should be using in general   Output from pickle is not really human-readable  but it should unpickle just fine   If you insist on using JSON  you could check out the jsonpickle module  which is an interesting hybrid approach   https   github com jsonpickle jsonpickle

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An approach which I have been using in my Flask app to serialize Class instance to JSON response  Github project for reference from json import JSONEncoder import json from typing import List  class ResponseEncoder JSONEncoder       def default self  o           return o   dict    class ListResponse      def   init   self  data  List           self data   data         self count   len data   class A      def   init   self  message  str           self message   message  class B      def   init   self  record  A           self record   record  class C      def   init   self  data  B           self data   data  Now create an instance of A  B  C then encode  data a   A  Test Data   data b   B data a  data c   C data b   response   ResponseEncoder   encode data c  json response   json loads response   Output        quot data quot              quot record quot                  quot message quot    quot Test Data quot                     For list type response records     One    Two    Three   list response   ListResponse records  response   ResponseEncoder   encode list response  json response   json loads response   Output        quot data quot              quot One quot            quot Two quot            quot Three quot              quot count quot   3

User · Answer

There s one way that works great for me that you can try out   json dumps   can take an optional parameter default where you can specify a custom serializer function for unknown types  which in my case looks like   def serialize obj          JSON serializer for objects not serializable by default json code         if isinstance obj  date           serial   obj isoformat           return serial      if isinstance obj  time           serial   obj isoformat           return serial      return obj   dict     First two ifs are for date and time serialization and then there is a obj   dict   returned for any other object   the final call looks like   json dumps myObj  default serialize    It s especially good when you are serializing a collection and you don t want to call   dict   explicitly for every object  Here it s done for you automatically   So far worked so good for me  looking forward for your thoughts

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This can be easily handled with pydantic  as it already has this functionality built-in  Option 1  normal way from pydantic import BaseModel  class testclass BaseModel       value1  str    quot a quot      value2  str    quot b quot   test   testclass     gt  gt  gt  print test json indent 4          quot value1 quot    quot a quot        quot value2 quot    quot b quot     Option 2  using pydantic s dataclass import json from pydantic dataclasses import dataclass from pydantic json import pydantic encoder   dataclass class testclass      value1  str    quot a quot      value2  str    quot b quot   test   testclass    gt  gt  gt  print json dumps test  indent 4  default pydantic encoder          quot value1 quot    quot a quot        quot value2 quot    quot b quot

[python] Serializing class instance to JSON

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