In python how do I cast a class object to a dict

Question

Let s say I ve got a simple class in python  class Wharrgarbl object       def   init   self  a  b  c  sum  version  old            self a   a         self b   b         self c   c         self sum   6         self version   version      def   int   self           return self sum   9000      def   what goes here   self           return   a   self a   b   self b   c   self c    I can cast it to an integer very easily   gt  gt  gt  w   Wharrgarbl  one    two    three   6   gt  gt  gt  int w  9006   Which is great  But  now I want to cast it to a dict in a similar fashion   gt  gt  gt  w   Wharrgarbl  one    two    three   6   gt  gt  gt  dict w    a    one    c    three    b    two     What do I need to define for this to work  I tried substituting both   dict   and dict for   what goes here    but dict w  resulted in a TypeError  Wharrgarbl object is not iterable in both cases  I don t think simply making the class iterable will solve the problem  I also attempted many googles with as many different wordings of  python cast object to dict  as I could think of but couldn t find anything relevant     Also  Notice how calling w   dict   won t do what I want because it s going to contain w version and w sum  I want to customize the cast to dict in the same way that I can customize the cast to int by using def int self    I know that I could just do something like this   gt  gt  gt  w   what goes here       a    one    c    three    b    two     But I am assuming there is a pythonic way to make dict w  work since it is the same type of thing as int w  or str w   If there isn t a more pythonic way  that s fine too  just figured I d ask  Oh  I guess since it matters  this is for python 2 7  but super bonus points for a 2 4 old and busted solution as well   There is another question Overloading   dict     on python class that is similar to this one but may be different enough to warrant this not being a duplicate  I believe that OP is asking how to cast all the data in his class objects as dictionaries  I m looking for a more customized approach in that I don t want everything in   dict   included in the dictionary returned by dict    Something like public vs private variables may suffice to explain what I m looking for  The objects will be storing some values used in calculations and such that I don t need want to show up in the resulting dictionaries   UPDATE  I ve chosen to go with the asdict route suggested but it was a tough choice selecting what I wanted to be the answer to the question  Both  RickTeachey and  jpmc26 provided the answer I m going to roll with but the former had more info and options and landed on the same result as well and was upvoted more so I went with it  Upvotes all around though and thanks for the help  I ve lurked long and hard on stackoverflow and I m trying to get my toes in the water more

User · Answer

Like many others  I would suggest implementing a to dict   function rather than  or in addition to  allowing casting to a dictionary  I think it makes it more obvious that the class supports that kind of functionality  You could easily implement such a method like this   def to dict self       class vars   vars MyClass     get any  default  attrs defined at the class level     inst vars   vars self     get any attrs defined on the instance  self      all vars   dict class vars      all vars update inst vars        filter out private attributes     public vars    k  v for k  v in all vars items   if not k startswith           return public vars

User · Answer

I think this will work for you     class A object       def   init   self  a  b  c  sum  version  old            self a   a         self b   b         self c   c         self sum   6         self version   version      def   int   self           return self sum   9000      def   iter   self           return self   dict   iteritems    a   A 1 2 3 4 5  print dict a    Output    a   1   c   3   b   2   sum   6   version   5

User · Answer

I am trying to write a class that is  both  a list or a dict  I want the programmer to be able to both  cast  this object to a list  dropping the keys  or dict  with the keys    Looking at the way Python currently does the dict   cast  It calls Mapping update   with the object that is passed  This is the code from the Python repo   def update self  other          kwds           D update  E     F  - gt  None   Update D from mapping iterable E and F          If E present and has a  keys   method  does      for k in E  D k    E k          If E present and lacks  keys   method  does      for  k  v  in E  D k    v         In either case  this is followed by  for k  v in F items    D k    v             if isinstance other  Mapping           for key in other              self key    other key      elif hasattr other   keys            for key in other keys                self key    other key      else          for key  value in other              self key    value     for key  value in kwds items            self key    value   The last subcase of the if statement  where it is iterating over other is the one most people have in mind  However  as you can see  it is also possible to have a keys   property  That  combined with a   getitem     should make it easy to have a subclass be properly casted to a dictionary   class Wharrgarbl object       def   init   self  a  b  c  sum  version  old            self a   a         self b   b         self c   c         self sum   6         self version   version      def   int   self           return self sum   9000      def   keys   self           return   a    b    c        def   getitem   self  key             have obj  a   - gt  obj a         return self   getattribute   key    Then this will work    gt  gt  gt  w   Wharrgarbl  one    two    three   6   gt  gt  gt  dict w    a    one    c    three    b    two

User · Answer

something like this would probably work  class MyClass      def   init   self x y z          self x   x        self y   y        self z   z     def   iter   self    overridding this to return tuples of  key value         return iter    x  self x    y  self y    z  self z     dict MyClass 5 6 7     because dict knows how to deal with tuples of  key value

User · Answer

It s hard to say without knowing the whole context of the problem  but I would not override   iter     I would implement   what goes here   on the class   as dict self      d       whatever you need         return d

User · Answer

There is no magic method that will do what you want. The answer is simply name it appropriately. asdict is a reasonable choice for a plain conversion to dict, inspired primarily by namedtuple. However, your method will obviously contain special logic that might not be immediately obvious from that name; you are returning only a subset of the class' state. If you can come up with with a slightly more verbose name that communicates the concepts clearly, all the better.

Other answers suggest using __iter__, but unless your object is truly iterable (represents a series of elements), this really makes little sense and constitutes an awkward abuse of the method. The fact that you want to filter out some of the class' state makes this approach even more dubious.

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There are at least five six ways  The preferred way depends on what your use case is  Option 1  Simply add an asdict   method  Based on the problem description I would very much consider the asdict way of doing things suggested by other answers  This is because it does not appear that your object is really much of a collection  class Wharrgarbl object                 def asdict self           return   a   self a   b   self b   c   self c   Using the other options below could be confusing for others unless it is very obvious exactly which object members would and would not be iterated or specified as key-value pairs  Option 1a  Inherit your class from  typing NamedTuple   or the mostly equivalent  collections namedtuple    and use the  asdict method provided for you  from typing import NamedTuple  class Wharrgarbl NamedTuple       a  str     b  str     c  str     sum  int   6     version  str    old   Using a named tuple is a very convenient way to add lots of functionality to your class with a minimum of effort  including an  asdict method  However  a limitation is that  as shown above  the NT will include all the members in its  asdict  If there are members you don t want to include in your dictionary  you ll need to modify the  asdict result  from typing import NamedTuple  class Wharrgarbl NamedTuple       a  str     b  str     c  str     sum  int   6     version  str    old       def  asdict self           d   super    asdict           del d  sum           del d  version           return d  Another limitation is that NT is read-only  This may or may not be desirable  Option 2  Implement   iter    Like this  for example  def   iter   self       yield  a   self a     yield  b   self b     yield  c   self c  Now you can just do  dict my object   This works because the dict   constructor accepts an iterable of  key  value  pairs to construct a dictionary  Before doing this  ask yourself the question whether iterating the object as a series of key value pairs in this manner- while convenient for creating a dict- might actually be surprising behavior in other contexts  E g   ask yourself the question  quot what should the behavior of list my object  be     quot  Additionally  note that accessing values directly using the get item obj  quot a quot   syntax will not work  and keyword argument unpacking won t work  For those  you d need to implement the mapping protocol  Option 3  Implement the mapping protocol  This allows access-by-key behavior  casting to a dict without using   iter    and also provides unpacking behavior     my obj   and keyword unpacking behavior if all the keys are strings  dict   my obj    The mapping protocol requires that you provide  at minimum  two methods together  keys   and   getitem    class MyKwargUnpackable      def keys self           return list  quot abc quot       def   getitem   self  key           return dict zip  quot abc quot    quot one two three quot  split     key   Now you can do things like   gt  gt  gt  m MyKwargUnpackable    gt  gt  gt  m  quot a quot    one   gt  gt  gt  dict m     cast to dict directly   a    one    b    two    c    three    gt  gt  gt  dict   m     unpack as kwargs   a    one    b    two    c    three    As mentioned above  if you are using a new enough version of python you can also unpack your mapping-protocol object into a dictionary comprehension like so  and in this case it is not required that your keys be strings    gt  gt  gt     m    a    one    b    two    c    three    Note that the mapping protocol takes precedence over the   iter   method when casting an object to a dict directly  without using kwarg unpacking  i e  dict m    So it is possible- and sometimes convenient- to cause the object to have different behavior when used as an iterable  e g   list m   vs  when cast to a dict  dict m    EMPHASIZED  Just because you CAN use the mapping protocol  does NOT mean that you SHOULD do so  Does it actually make sense for your object to be passed around as a set of key-value pairs  or as keyword arguments and values  Does accessing it by key- just like a dictionary- really make sense  If the answer to these questions is yes  it s probably a good idea to consider the next option  Option 4  Look into using the  collections abc  module  Inheriting your class from  collections abc Mapping or  collections abc MutableMapping signals to other users that  for all intents and purposes  your class is a mapping   and can be expected to behave that way  You can still cast your object to a dict just as you require  but there would probably be little reason to do so  Because of duck typing  bothering to cast your mapping object to a dict would just be an additional unnecessary step the majority of the time  This answer might also be helpful  As noted in the comments below  it s worth mentioning that doing this the abc way essentially turns your object class into a dict-like class  assuming you use MutableMapping and not the read-only Mapping base class   Everything you would be able to do with dict  you could do with your own class object  This may be  or may not be  desirable  Also consider looking at the numerical abcs in the numbers module  https   docs python org 3 library numbers html Since you re also casting your object to an int  it might make more sense to essentially turn your class into a full fledged int so that casting isn t necessary  Option 5  Look into using the dataclasses module  Python 3 7 only   which includes a convenient asdict   utility method  from dataclasses import dataclass  asdict  field  InitVar   dataclass class Wharrgarbl object       a  int     b  int     c  int     sum  InitVar int     note  InitVar will exclude this from the dict     version  InitVar str     quot old quot       def   post init   self  sum  version           self sum   6    this looks like an OP mistake          self version   str version   Now you can do this       gt  gt  gt  asdict Wharrgarbl 1 2 3 4  quot X quot          a   1   b   2   c   3   Option 6  Use typing TypedDict  which has been added in python 3 8  NOTE  option 6 is likely NOT what the OP  or other readers based on the title of this question  are looking for  See additional comments below  class Wharrgarbl TypedDict       a  str     b  str     c  str  Using this option  the resulting object is a dict  emphasis  it is not a Wharrgarbl   There is no reason at all to  quot cast quot  it to a dict  unless you are making a copy   And since the object is a dict  the initialization signature is identical to that of dict and as such it only accepts keyword arguments or another dictionary       gt  gt  gt  w   Wharrgarbl a 1 b 2 b 3       gt  gt  gt  w       a   1   b   2   c   3       gt  gt  gt  type w       lt class  dict  gt   Emphasized  the above  quot class quot  Wharrgarbl isn t actually a new class at all  It is simply syntactic sugar for creating typed dict objects with fields of different types for the type checker  As such this option can be pretty convenient for signaling to readers of your code  and also to a type checker such as mypy  that such a dict object is expected to have specific keys with specific value types  But this means you cannot  for example  add other methods  although you can try  class MyDict TypedDict       def my fancy method self           return  quot world changing result quot      but it won t work   gt  gt  gt  MyDict   my fancy method   Traceback  most recent call last    File  quot  lt stdin gt  quot   line 1  in  lt module gt  AttributeError   dict  object has no attribute  my fancy method       quot Mapping quot  has become the standard  quot name quot  of the dict-like duck type

[python] In python, how do I cast a class object to a dict

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