Python JSON serialize a Decimal object

Question

I have a Decimal  3 9   as part of an object  and wish to encode this to a JSON string which should look like   x   3 9   I don t care about precision on the client side  so a float is fine   Is there a good way to serialize this  JSONDecoder doesn t accept Decimal objects  and converting to a float beforehand yields   x   3 8999999999999999  which is wrong  and will be a big waste of bandwidth

User · Accepted Answer

How about subclassing json JSONEncoder  class DecimalEncoder json JSONEncoder       def default self  o           if isinstance o  decimal Decimal                 wanted a simple yield str o  in the next line                but that would mean a yield on the line with super                     which wouldn t work  see my comment below   so                return  str o  for o in  o           return super DecimalEncoder  self  default o   Then use it like so  json dumps   x   decimal Decimal  5 5     cls DecimalEncoder

User · Answer

Based on stdOrgnlDave answer I have defined this wrapper that it can be called with optional kinds so the encoder will work only for certain kinds inside your projects  I believe the work should be done inside your code and not to use this  default  encoder since  it is better explicit than implicit   but I understand using this will save some of your time   -   import time import json import decimal from uuid import UUID from datetime import datetime  def JSONEncoder newdefault kind   uuid    datetime    time    decimal                 JSON Encoder newdfeault is a wrapper capable of encoding several kinds     Use it anywhere on your code to make the full system to work with this defaults          JSONEncoder newdefault      for everything         JSONEncoder newdefault   decimal       only for Decimal             JSONEncoder olddefault   json JSONEncoder default      def JSONEncoder wrapped self  o                       json JSONEncoder default   JSONEncoder newdefault                     if   uuid  in kind  and isinstance o  uuid UUID               return str o          if   datetime  in kind  and isinstance o  datetime               return str o          if   time  in kind  and isinstance o  time struct time               return datetime fromtimestamp time mktime o           if   decimal  in kind  and isinstance o  decimal Decimal               return str o          return JSONEncoder olddefault self  o      json JSONEncoder default   JSONEncoder wrapped    Example if   name         main         JSONEncoder newdefault

User · Answer

For Django users   Recently came across TypeError  Decimal  2337 00   is not JSON serializable while JSON encoding i e  json dumps data   Solution     converts Decimal  Datetime  UUIDs to str for Encoding from django core serializers json import DjangoJSONEncoder    json dumps response data  cls DjangoJSONEncoder    But  now the Decimal value will be a string  now we can explicitly set the decimal float value parser when decoding data  using parse float option in json loads   import decimal   data   json loads data  parse float decimal Decimal    default is float num str

User · Answer

I tried switching from simplejson to builtin json for GAE 2 7  and had issues with the decimal  If default returned str o  there were quotes  because  iterencode calls  iterencode on the results of default   and float o  would remove trailing 0   If default returns an object of a class that inherits from float  or anything that calls repr without additional formatting  and has a custom   repr   method  it seems to work like I want it to   import json from decimal import Decimal  class fakefloat float       def   init   self  value           self  value   value     def   repr   self           return str self  value   def defaultencode o       if isinstance o  Decimal             Subclass float with custom repr          return fakefloat o      raise TypeError repr o      is not JSON serializable    json dumps  10 20   10 20   Decimal  10 20     default defaultencode    10 2   10 20   10 20

User · Answer

From the JSON Standard Document  as linked in json org      JSON is agnostic about the semantics of numbers  In any programming language  there can be a variety of   number types of various capacities and complements  fixed or floating  binary or decimal  That can make   interchange between different programming languages difficult  JSON instead offers only the representation of   numbers that humans use  a sequence of digits  All programming languages know how to make sense of digit   sequences even if they disagree on internal representations  That is enough to allow interchange    So it s actually accurate to represent Decimals as numbers  rather than strings  in JSON  Bellow lies a possible solution to the problem   Define a custom JSON encoder   import json   class CustomJsonEncoder json JSONEncoder        def default self  obj           if isinstance obj  Decimal               return float obj          return super CustomJsonEncoder  self  default obj    Then use it when serializing your data   json dumps data  cls CustomJsonEncoder    As noted from comments on the other answers  older versions of python might mess up the representation when converting to float  but that s not the case anymore   To get the decimal back in Python   Decimal str value     This solution is hinted in Python 3 0 documentation on decimals      To create a Decimal from a float  first convert it to a string

User · Answer

If you want to pass a dictionary containing decimals to the requests library  using the json keyword argument   you simply need to install simplejson     pip3 install simplejson       python3  gt  gt  gt  import requests  gt  gt  gt  from decimal import Decimal  gt  gt  gt    This won t error out   gt  gt  gt  requests post  https   www google com   json   foo   Decimal  1 23       The reason of the problem is that requests uses simplejson only if it is present  and falls back to the built-in json if it is not installed

User · Answer

You can create a custom JSON encoder as per your requirement   import json from datetime import datetime  date from time import time  struct time  mktime import decimal  class CustomJSONEncoder json JSONEncoder       def default self  o           if isinstance o  datetime               return str o          if isinstance o  date               return str o          if isinstance o  decimal Decimal               return float o          if isinstance o  struct time               return datetime fromtimestamp mktime o             Any other serializer if needed         return super CustomJSONEncoder  self  default o    The Decoder can be called like this   import json from decimal import Decimal json dumps   x   Decimal  3 9     cls CustomJSONEncoder    and the output will be    gt  gt    x   3 9

User · Answer

I would like to let everyone know that I tried Michal Marczyk s answer on my web server that was running Python 2 6 5 and it worked fine  However  I upgraded to Python 2 7 and it stopped working  I tried to think of some sort of way to encode Decimal objects and this is what I came up with  import decimal  class DecimalEncoder json JSONEncoder       def default self  o           if isinstance o  decimal Decimal               return str o          return super DecimalEncoder  self  default o   Note that this will convert the decimal to its string representation  e g    quot 1 2300 quot   to a  not lose significant digits and b  prevent rounding errors  This should hopefully help anyone who is having problems with Python 2 7  I tested it and it seems to work fine  If anyone notices any bugs in my solution or comes up with a better way  please let me know

User · Answer

The native option is missing so I ll add it for the next guy gall that looks for it   Starting on Django 1 7 x there is a built-in DjangoJSONEncoder that you can get it from django core serializers json   import json from django core serializers json import DjangoJSONEncoder from django forms models import model to dict  model instance   YourModel object first   model dict   model to dict model instance   json dumps model dict  cls DjangoJSONEncoder    Presto

User · Answer

My   02   I extend a bunch of the JSON encoder since I am serializing tons of data for my web server  Here s some nice code  Note that it s easily extendable to pretty much any data format you feel like and will reproduce 3 9 as  thing   3 9  JSONEncoder olddefault   json JSONEncoder default def JSONEncoder newdefault self  o       if isinstance o  UUID   return str o      if isinstance o  datetime   return str o      if isinstance o  time struct time   return datetime fromtimestamp time mktime o       if isinstance o  decimal Decimal   return str o      return JSONEncoder olddefault self  o  json JSONEncoder default   JSONEncoder newdefault   Makes my life so much easier

User · Answer

this can be done by adding       elif isinstance o  decimal Decimal           yield str o    in  Lib json encoder py JSONEncoder  iterencode  but I was hoping for a better solution

User · Answer

For anybody that wants a quick solution here is how I removed Decimal from my queries in Django total development cost var   process assumption objects values  total development cost   aggregate sum dev   Sum  total development cost   output field FloatField     total development cost var   list total development cost var values      Step 1  use   output field FloatField   in you r query Step 2  use list eg list total development cost var values     Hope it helps

User · Answer

For those who don t want to use a third-party library     An issue with Elias Zamaria s answer is that it converts to float  which can run into problems   For example    gt  gt  gt  json dumps   x   Decimal  0 0000001     cls DecimalEncoder     x   1e-07    gt  gt  gt  json dumps   x   Decimal  100000000000 01734     cls DecimalEncoder     x   100000000000 01733     The JSONEncoder encode   method lets you return the literal json content  unlike JSONEncoder default    which has you return a json compatible type  like float  that then gets encoded in the normal way   The problem with encode   is that it  normally  only works at the top level   But it s still usable  with a little extra work  python 3 x    import json from collections abc import Mapping  Iterable from decimal import Decimal  class DecimalEncoder json JSONEncoder       def encode self  obj           if isinstance obj  Mapping               return            join f  self encode k     self encode v    for  k  v  in obj items                  if isinstance obj  Iterable  and  not isinstance obj  str                return            join map self encode  obj                 if isinstance obj  Decimal               return f  obj normalize   f      using normalize   gets rid of trailing 0s  using   f  prevents scientific notation         return super   encode obj    Which gives you    gt  gt  gt  json dumps   x   Decimal  0 0000001     cls DecimalEncoder     x   0 0000001    gt  gt  gt  json dumps   x   Decimal  100000000000 01734     cls DecimalEncoder     x   100000000000 01734

User · Answer

This is what I have  extracted from our class  class CommonJSONEncoder json JSONEncoder                Common JSON Encoder     json dumps myString  cls CommonJSONEncoder               def default self  obj            if isinstance obj  decimal Decimal               return   type decimal    str obj    class CommonJSONDecoder json JSONDecoder                Common JSON Encoder     json loads myString  cls CommonJSONEncoder                classmethod     def object hook cls  obj           for key in obj              if isinstance key  six string types                   if  type decimal      key                      try                          return decimal Decimal obj key                       except                          pass      def   init   self    kwargs           kwargs  object hook     self object hook         super CommonJSONDecoder  self    init     kwargs    Which passes unittest   def test encode and decode decimal self       obj   Decimal  1 11       result   json dumps obj  cls CommonJSONEncoder      self assertTrue  type decimal   in result      new obj   json loads result  cls CommonJSONDecoder      self assertEqual new obj  obj       obj     test   Decimal  1 11        result   json dumps obj  cls CommonJSONEncoder      self assertTrue  type decimal   in result      new obj   json loads result  cls CommonJSONDecoder      self assertEqual new obj  obj       obj     test     abc   Decimal  1 11         result   json dumps obj  cls CommonJSONEncoder      self assertTrue  type decimal   in result      new obj   json loads result  cls CommonJSONDecoder      self assertEqual new obj  obj

User · Answer

3 9 can not be exactly represented in IEEE floats  it will always come as 3 8999999999999999  e g  try  print repr 3 9   you can read more about it here     http   en wikipedia org wiki Floating point http   docs sun com source 806-3568 ncg goldberg html    So if you don t want float  only option you have to send it as string  and to allow automatic conversion of decimal objects to JSON  do something like this   import decimal from django utils import simplejson  def json encode decimal obj       if isinstance obj  decimal Decimal           return str obj      raise TypeError repr obj      is not JSON serializable    d   decimal Decimal  3 5   print simplejson dumps  d   default json encode decimal

User · Answer

Simplejson 2 1 and higher has native support for Decimal type    gt  gt  gt  json dumps Decimal  3 9    use decimal True   3 9    Note that use decimal is True by default   def dumps obj  skipkeys False  ensure ascii True  check circular True      allow nan True  cls None  indent None  separators None      encoding  utf-8   default None  use decimal True      namedtuple as object True  tuple as array True      bigint as string False  sort keys False  item sort key None      for json False  ignore nan False    kw     So    gt  gt  gt  json dumps Decimal  3 9     3 9    Hopefully  this feature will be included in standard library

User · Answer

In my Flask app  Which uses python 2 7 11  flask alchemy with  db decimal  types   and Flask Marshmallow   for  instant  serializer and deserializer   i had this error  every time i did a GET or POST  The serializer and deserializer  failed to convert  Decimal types into any JSON identifiable format    I did a   pip install simplejson   then  Just by adding  import simplejson as json   the serializer and deserializer starts to purr again  I did nothing else    DEciamls are displayed as  234 00  float format

[python] Python JSON serialize a Decimal object

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