Named tuple and default values for optional keyword arguments

Question

I m trying to convert a longish hollow  data  class into a named tuple  My class currently looks like this   class Node object       def   init   self  val  left None  right None           self val   val         self left   left         self right   right   After conversion to namedtuple it looks like   from collections import namedtuple Node   namedtuple  Node    val left right     But there is a problem here  My original class allowed me to pass in just a value and took care of the default by using default values for the named keyword arguments  Something like   class BinaryTree object       def   init   self  val           self root   Node val    But this doesn t work in the case of my refactored named tuple since it expects me to pass all the fields  I can of course replace the occurrences of Node val  to Node val  None  None  but it isn t to my liking   So does there exist a good trick which can make my re-write successful without adding a lot of code complexity  metaprogramming  or should I just swallow the pill and go ahead with the  search and replace

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Here s a less flexible  but more concise version of Mark Lodato s wrapper  It takes the fields and defaults as a dictionary   import collections def namedtuple with defaults typename  fields dict       T   collections namedtuple typename      join fields dict keys         T   new     defaults     tuple fields dict values        return T   Example   In 1   fields     val   1   left   2   right  3   In 2   Node   namedtuple with defaults  Node   fields   In 3   Node   Out 3   Node val 1  left 2  right 3   In 4   Node 4 5 6  Out 4   Node val 4  left 5  right 6   In 5   Node val 10  Out 5   Node val 10  left 2  right 3

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In python3 7  there s a brand new defaults  keyword argument      defaults can be None or an iterable of default values  Since fields with a default value must come after any fields without a default  the defaults are applied to the rightmost parameters  For example  if the fieldnames are   x    y    z   and the defaults are  1  2   then x will be a required argument  y will default to 1  and z will default to 2    Example usage       python Python 3 7 0b1   heads 3 7 4d65430  Feb  1 2018  09 28 35    GCC 5 4 0 20160609  on linux Type  help    copyright    credits  or  license  for more information   gt  gt  gt  from collections import namedtuple  gt  gt  gt  nt   namedtuple  nt     a    b    c    defaults  1  2    gt  gt  gt  nt 0  nt a 0  b 1  c 2   gt  gt  gt  nt 0  3    nt a 0  b 3  c 2   gt  gt  gt  nt 0  c 3  nt a 0  b 1  c 3

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Since you are using namedtuple as a data class  you should be aware that python 3 7 will introduce a  dataclass decorator for this very purpose -- and of course it has default values    An example from the docs    dataclass class C      a  int          a  has no default value     b  int   0     assign a default value for  b    Much cleaner  readable and usable than hacking namedtuple  It is not hard to predict that usage of namedtuples will drop with the adoption of 3 7

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With typing NamedTuple in Python 3 6 1  you can provide both a default value and a type annotation to a NamedTuple field  Use typing Any if you only need the former   from typing import Any  NamedTuple   class Node NamedTuple       val  Any     left   Node    None     right   Node    None   Usage    gt  gt  gt  Node 1  Node val 1  left None  right None   gt  gt  gt  n   Node 1   gt  gt  gt  Node 2  left n  Node val 2  left Node val 1  left None  right None   right None    Also  in case you need both default values and optional mutability  Python 3 7 is going to have data classes  PEP 557  that can in some  many   cases replace namedtuples   Sidenote  one quirk of the current specification of annotations  expressions after   for parameters and variables and after - gt  for functions  in Python is that they are evaluated at definition time   So  since  class names become defined once the entire body of the class has been executed   the annotations for  Node  in the class fields above must be strings to avoid NameError   This kind of type hints is called  forward reference    1    2    and with PEP 563 Python 3 7  is going to have a   future   import  to be enabled by default in 4 0  that will allow to use forward references without quotes  postponing their evaluation     AFAICT only local variable annotations are not evaluated at runtime   source  PEP 526

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Python 3 7  Use the defaults parameter    gt  gt  gt  from collections import namedtuple  gt  gt  gt  fields     val    left    right    gt  gt  gt  Node   namedtuple  Node   fields  defaults  None     len fields    gt  gt  gt  Node   Node val None  left None  right None    Or better yet  use the new dataclasses library  which is much nicer than namedtuple    gt  gt  gt  from dataclasses import dataclass  gt  gt  gt  from typing import Any  gt  gt  gt   dataclass     class Node          val  Any   None         left   Node    None         right   Node    None  gt  gt  gt  Node   Node val None  left None  right None    Before Python 3 7  Set Node   new     defaults   to the default values    gt  gt  gt  from collections import namedtuple  gt  gt  gt  Node   namedtuple  Node    val left right    gt  gt  gt  Node   new     defaults      None     len Node  fields   gt  gt  gt  Node   Node val None  left None  right None    Before Python 2 6  Set Node   new   func defaults to the default values    gt  gt  gt  from collections import namedtuple  gt  gt  gt  Node   namedtuple  Node    val left right    gt  gt  gt  Node   new   func defaults    None     len Node  fields   gt  gt  gt  Node   Node val None  left None  right None    Order  In all versions of Python  if you set fewer default values than exist in the namedtuple  the defaults are applied to the rightmost parameters  This allows you to keep some arguments as required arguments    gt  gt  gt  Node   new     defaults      1 2   gt  gt  gt  Node   Traceback  most recent call last         TypeError    new     missing 1 required positional argument   val   gt  gt  gt  Node 3  Node val 3  left 1  right 2    Wrapper for Python 2 6 to 3 6  Here s a wrapper for you  which even lets you  optionally  set the default values to something other than None  This does not support required arguments   import collections def namedtuple with defaults typename  field names  default values          T   collections namedtuple typename  field names      T   new     defaults      None     len T  fields      if isinstance default values  collections Mapping           prototype   T   default values      else          prototype   T  default values      T   new     defaults     tuple prototype      return T   Example    gt  gt  gt  Node   namedtuple with defaults  Node    val left right    gt  gt  gt  Node   Node val None  left None  right None   gt  gt  gt  Node   namedtuple with defaults  Node    val left right    1  2  3    gt  gt  gt  Node   Node val 1  left 2  right 3   gt  gt  gt  Node   namedtuple with defaults  Node    val left right     right  7    gt  gt  gt  Node   Node val None  left None  right 7   gt  gt  gt  Node 4  Node val 4  left None  right 7

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Combining approaches of  Denis and  Mark   from collections import namedtuple import inspect  class Node namedtuple  Node    left right val           slots            def   new   cls   args    kwargs           args list   inspect getargspec super Node  cls    new    args len args  1           params    key  kwargs get key  for key in args list   kwargs keys            return super Node  cls    new   cls   args    params     That should support creating the tuple with positional arguments and also with mixed cases  Test cases    gt  gt  gt  print Node   Node left None  right None  val None    gt  gt  gt  print Node 1 2 3  Node left 1  right 2  val 3    gt  gt  gt  print Node 1  right 2  Node left 1  right 2  val None    gt  gt  gt  print Node 1  right 2  val 100  Node left 1  right 2  val 100    gt  gt  gt  print Node left 1  right 2  val 100  Node left 1  right 2  val 100    gt  gt  gt  print Node left 1  right 2  Node left 1  right 2  val None    but also support TypeError    gt  gt  gt  Node 1  left 2  TypeError    new     got multiple values for keyword argument  left

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Wrap it in a function   NodeT   namedtuple  Node    val left right    def Node val  left None  right None     return NodeT val  left  right

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A slightly extended example to initialize all missing arguments with None   from collections import namedtuple  class Node namedtuple  Node     value    left    right            slots            def   new   cls   args    kwargs             initialize missing kwargs with None         all kwargs    key  kwargs get key  for key in cls  fields          return super Node  cls    new   cls   args    all kwargs

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Short  simple  and doesn t lead people to use isinstance improperly   class Node namedtuple  Node     val    left    right           classmethod     def make cls  val  left None  right None           return cls val  left  right     Example x   Node make 3  x  replace right Node make 4

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Inspired by this answer to a different question  here is my proposed solution based on a metaclass and using super  to handle future subcalssing correctly    It is quite similar to justinfay s answer   from collections import namedtuple  NodeTuple   namedtuple  NodeTuple     val    left    right     class NodeMeta type       def   call   cls  val  left None  right None           return super NodeMeta  cls    call   val  left  right   class Node NodeTuple  metaclass NodeMeta         slots          Then    gt  gt  gt  Node 1  Node 2  Node 4    Node 3  None  Node 5     Node val 1  left Node val 2  left Node val 4  left None  right None   right None   right Node val 3  left None  right Node val 5  left None  right None

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I m not sure if there s an easy way with just the built-in namedtuple  There s a nice module called recordtype that has this functionality    gt  gt  gt  from recordtype import recordtype  gt  gt  gt  Node   recordtype  Node      val   None     left   None     right   None     gt  gt  gt  Node 3  Node val 3  left None  right None   gt  gt  gt  Node 3   L   Node val 3  left L  right None

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I find this version easier to read   from collections import namedtuple  def my tuple   kwargs       defaults              a   2 0           b   True           c    hello             default tuple   namedtuple  MY TUPLE       join defaults keys      defaults values        return default tuple  replace   kwargs    This is not as efficient as it requires creation of the object twice but you could change that by defining the default duple inside the module and just having the function do the replace line

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Here is a more compact version inspired by justinfay s answer   from collections import namedtuple from functools import partial  Node   namedtuple  Node     val left right    Node   new     partial Node   new    left None  right None

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You can also use this   import inspect  def namedtuple with defaults type  default value None    kwargs       args list   inspect getargspec type   new    args 1       params   dict   x  default value  for x in args list       params update kwargs       return type   params    This basically gives you the possibility to construct any named tuple with a default value and override just the parameters you need  for example   import collections  Point   collections namedtuple  Point     x    y    namedtuple with defaults Point   gt  gt  gt  Point x None  y None   namedtuple with defaults Point  x 1   gt  gt  gt  Point x 1  y None

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Another solution   import collections   def defaultargs func  defaults       def wrapper  args    kwargs           for key  value in  x for x in defaults len args    if len x     2               kwargs setdefault key  value          return func  args    kwargs      return wrapper   def namedtuple name  fields       NamedTuple   collections namedtuple name   x 0  for x in fields       NamedTuple   new     defaultargs NamedTuple   new      NamedTuple      fields      return NamedTuple   Usage    gt  gt  gt  Node   namedtuple  Node               val               left   None             right   None            main   Node   gt  gt  gt  Node 1  Node val 1  left None  right None    gt  gt  gt  Node 1  2  right 3  Node val 1  left 2  right 3

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Python 3 7  introduction of defaults param in namedtuple definition   Example as shown in the documentation    gt  gt  gt  Account   namedtuple  Account     type    balance    defaults  0    gt  gt  gt  Account  fields defaults   balance   0   gt  gt  gt  Account  premium   Account type  premium   balance 0    Read more here

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The answer by jterrace to use recordtype is great  but the author of the library recommends to use his namedlist project  which provides both mutable  namedlist  and immutable  namedtuple  implementations   from namedlist import namedtuple  gt  gt  gt  Node   namedtuple  Node     val     left   None     right   None     gt  gt  gt  Node 3  Node val 3  left None  right None   gt  gt  gt  Node 3   L   Node val 3  left L  right None

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Using the NamedTuple class from my Advanced Enum  aenum  library  and using the class syntax  this is quite simple   from aenum import NamedTuple  class Node NamedTuple       val   0     left   1   previous Node   None     right   2   next Node   None   The one potential drawback is the requirement for a   doc   string for any attribute with a default value  it s optional for simple attributes    In use it looks like    gt  gt  gt  Node   Traceback  most recent call last         TypeError  values not provided for field s   val   gt  gt  gt  Node 3  Node val 3  left None  right None    The advantages this has over justinfay s answer   from collections import namedtuple  class Node namedtuple  Node     value    left    right            slots            def   new   cls  value  left None  right None           return super Node  cls    new   cls  value  left  right    is simplicity  as well as  being metaclass based instead of exec based

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This is an example straight from the docs      Default values can be implemented by using  replace   to customize a   prototype instance    gt  gt  gt  Account   namedtuple  Account    owner balance transaction count    gt  gt  gt  default account   Account   lt owner name gt    0 0  0   gt  gt  gt  johns account   default account  replace owner  John    gt  gt  gt  janes account   default account  replace owner  Jane      So  the OP s example would be   from collections import namedtuple Node   namedtuple  Node    val left right   default node   Node None  None  None  example   default node  replace val  whut     However  I like some of the other answers given here better   I just wanted to add this for completeness

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I subclassed namedtuple and overrode the   new   method   from collections import namedtuple  class Node namedtuple  Node     value    left    right            slots            def   new   cls  value  left None  right None           return super Node  cls    new   cls  value  left  right    This preserves an intuitive type hierarchy  which the creation of a factory function disguised as a class does not

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Here s a short  simple generic answer with a nice syntax for a named tuple with default arguments   import collections  def dnamedtuple typename  field names    defaults       fields   sorted field names split    key lambda x  x in defaults      T   collections namedtuple typename      join fields       T   new     defaults     tuple defaults field  for field in fields -len defaults         return T   Usage   Test   dnamedtuple  Test    one two three   two 2  Test 1  3     Test one 1  three 3  two 2    Minified   def dnamedtuple tp  fs    df       fs   sorted fs split    key df   contains        T   collections namedtuple tp      join fs       T   new     defaults     tuple df i  for i in fs -len df         return T

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