Use cases for the setdefault dict method

Question

The addition of collections defaultdict in Python 2 5 greatly reduced the need for dict s setdefault method  This question is for our collective education    What is setdefault still useful for  today in Python 2 6 2 7  What popular use cases of setdefault were superseded with collections defaultdict

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The different use case for setdefault   is when you don t want to overwrite the value of an already set key  defaultdict overwrites  while setdefault   does not  For nested dictionaries it is more often the case that you want to set a default only if the key is not set yet  because you don t want to remove the present sub dictionary  This is when you use setdefault     Example with defaultdict    gt  gt  gt  from collection import defaultdict    gt  gt  gt  foo   defaultdict    gt  gt  gt  foo  a     4  gt  gt  gt  foo  a     2  gt  gt  gt  print foo  defaultdict None    a   2     setdefault doesn t overwrite    gt  gt  gt  bar   dict    gt  gt  gt  bar setdefault  a   4   gt  gt  gt  bar setdefault  a   2   gt  gt  gt  print bar    a   4

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Edit  Very wrong  The setdefault would always trigger long computation  Python being eager   Expanding on Tuttle s answer  For me the best use case is cache mechanism  Instead of   if x not in memo     memo x  long computation x  return memo x    which consumes 3 lines and 2 or 3 lookups  I would happily write    return memo setdefault x  long computation x

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Another use case that I don t think was mentioned above  Sometimes you keep a cache dict of objects by their id where primary instance is in the cache and you want to set cache when missing   return self objects by id setdefault obj id  obj    That s useful when you always want to keep a single instance per distinct id no matter how you obtain an obj each time  For example when object attributes get updated in memory and saving to storage is deferred

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I like the answer given here   http   stupidpythonideas blogspot com 2013 08 defaultdict-vs-setdefault html  In short  the decision  in non-performance-critical apps  should be made on the basis of how you want to handle lookup of empty keys downstream  viz  KeyError versus default value

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I use setdefault   when I want a default value in an OrderedDict  There isn t a standard Python collection that does both  but there are ways to implement such a collection

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As most answers state setdefault or defaultdict would let you set a default value when a key doesn t exist  However  I would like to point out a small caveat with regard to the use cases of setdefault  When the Python interpreter executes setdefaultit will always evaluate the second argument to the function even if the key exists in the dictionary  For example   In  d    1 5  2 6   In  d Out   1  5  2  6   In  d setdefault 2  0  Out  6  In  d setdefault 2  print  test    test Out  6   As you can see  print was also executed even though 2 already existed in the dictionary  This becomes particularly important if you are planning to use setdefault for example for an optimization like memoization  If you add a recursive function call as the second argument to setdefault  you wouldn t get any performance out of it as Python would always be calling the function recursively   Since memoization was mentioned  a better alternative is to use functools lru cache decorator if you consider enhancing a function with memoization  lru cache handles the caching requirements for a recursive function better

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As Muhammad said  there are situations in which you only sometimes wish to set a default value  A great example of this is a data structure which is first populated  then queried   Consider a trie  When adding a word  if a subnode is needed but not present  it must be created to extend the trie  When querying for the presence of a word  a missing subnode indicates that the word is not present and it should not be created   A defaultdict cannot do this  Instead  a regular dict with the get and setdefault methods must be used

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One drawback of defaultdict over dict  dict setdefault  is that a defaultdict object creates a new item EVERYTIME non existing key is given  eg with     print   Also the defaultdict class is generally way less common then the dict class  its more difficult to serialize it IME   P S  IMO functions methods not meant to mutate an object  should not mutate an object

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I rewrote the accepted answer and facile it for the newbies    break it down and understand it intuitively  new      for  key  value  in data      if key not in new          new key         this is core of setdefault equals to new setdefault key              new key  append value      else          new key  append value      easy with setdefault new      for  key  value  in data      group   new setdefault key        it is new key           group append value       even simpler with defaultdict new   defaultdict list  for  key  value  in data      new key  append value    all keys have a default value of empty list      Additionally I categorized the methods as reference   dict methods 11                  views    keys    values    items                 add    update   setdefault                 remove    pop    popitem   clear                 retrieve    get                  copy    copy   fromkeys

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In addition to what have been suggested  setdefault might be useful in situations where you don t want to modify a value that has been already set  For example  when you have duplicate numbers and you want to treat them as one group  In this case  if you encounter a repeated duplicate key which has been already set  you won t update the value of that key  You will keep the first encountered value  As if you are iterating updating the repeated keys once only  Here s a code example of recording the index for the keys elements of a sorted list  nums    2 2 2 2 2  d      for idx  num in enumerate sorted nums          This will be updated with the value index of the of the last repeated key       d num    idx   Result  sorted indices    4  4  4  4  4        In the case of setdefault  all encountered repeated keys won t update the key        However  only the first encountered key s index will be set      d setdefault num idx    Result  sorted indices    0  0  0  0  0   sorted indices    d i  for i in nums

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I commonly use setdefault for keyword argument dicts  such as in this function   def notify self  level   pargs    kwargs       kwargs setdefault  persist   level  gt   DANGER      self   defcon set level    kwargs      try          kwargs setdefault  name   self client player entity   name      except pytibia PlayerEntityNotFound          pass     return  notify level   pargs    kwargs    It s great for tweaking arguments in wrappers around functions that take keyword arguments

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You could say defaultdict is useful for settings defaults before filling the dict and setdefault is useful for setting defaults while or after filling the dict    Probably the most common use case  Grouping items  in unsorted data  else use itertools groupby     really verbose new      for  key  value  in data      if key in new          new key  append  value       else          new key     value      easy with setdefault new      for  key  value  in data      group   new setdefault key        key might exist already     group append  value       even simpler with defaultdict  from collections import defaultdict new   defaultdict list  for  key  value  in data      new key  append  value     all keys have a default already   Sometimes you want to make sure that specific keys exist after creating a dict  defaultdict doesn t work in this case  because it only creates keys on explicit access  Think you use something HTTP-ish with many headers -- some are optional  but you want defaults for them   headers   parse headers  msg     parse the message  get a dict   now add all the optional headers for headername  defaultvalue in optional headers      headers setdefault  headername  defaultvalue

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I use setdefault frequently when  get this  setting a default       in a dictionary  somewhat commonly the os environ dictionary     Set the venv dir if it isn t already overridden  os environ setdefault  VENV DIR     my default path     Less succinctly  this looks like this     Set the venv dir if it isn t already overridden  if  VENV DIR  not in os environ      os environ  VENV DIR       my default path     It s worth noting that you can also use the resulting variable   venv dir   os environ setdefault  VENV DIR     my default path     But that s less necessary than it was before defaultdicts existed

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Theoretically speaking  setdefault would still be handy if you sometimes want to set a default and sometimes not  In real life  I haven t come across such a use case   However  an interesting use case comes up from the standard library  Python 2 6   threadinglocal py     gt  gt  gt  mydata   local    gt  gt  gt  mydata   dict     number   42   gt  gt  gt  mydata   dict   setdefault  widgets           gt  gt  gt  mydata widgets      I would say that using   dict   setdefault is a pretty useful case   Edit  As it happens  this is the only example in the standard library and it is in a comment  So may be it is not enough of a case to justify the existence of setdefault  Still  here is an explanation   Objects store their attributes in the   dict   attribute  As it happens  the   dict   attribute is writeable at any time after the object creation  It is also a dictionary not a defaultdict  It is not sensible for objects in the general case to have   dict   as a defaultdict because that would make each object having all legal identifiers as attributes  So I can t foresee any change to Python objects getting rid of   dict   setdefault  apart from deleting it altogether if it was deemed not useful

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Here are some examples of setdefault to show its usefulness       d        To add a key- gt value pair  do the following  d setdefault key      append value     To retrieve a list of the values for a key list of values   d key     To remove a key- gt value pair is still easy  if   you don t mind leaving empty lists behind when   the last value for a given key is removed  d key  remove value     Despite the empty lists  it s still possible to    test for the existance of values easily  if d has key key  and d key       pass   d has some values for key    Note  Each value can exist multiple times      e      print e e setdefault  Cars       append  Toyota   print e e setdefault  Motorcycles       append  Yamaha   print e e setdefault  Airplanes       append  Boeing   print e e setdefault  Cars       append  Honda   print e e setdefault  Cars       append  BMW   print e e setdefault  Cars       append  Toyota   print e    NOTE  now e  Cars        Toyota    Honda    BMW    Toyota   e  Cars   remove  Toyota   print e   NOTE  it s still true that   Toyota  in e  Cars

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One very important use-case I just stumbled across   dict setdefault   is great for multi-threaded code when you only want a single canonical object  as opposed to multiple objects that happen to be equal    For example  the  Int Flag Enum in Python 3 6 0 has a bug  if multiple threads are competing for a composite  Int Flag member  there may end up being more than one   from enum import IntFlag  auto import threading  class TestFlag IntFlag       one   auto       two   auto       three   auto       four   auto       five   auto       six   auto       seven   auto       eight   auto        def   eq   self  other           return self is other      def   hash   self           return hash self value   seen   set    class cycle enum threading Thread       def run self           for i in range 256               seen add TestFlag i    threads      for i in range 8       threads append cycle enum     for t in threads      t start    for t in threads      t join    len seen    272   should be 256    The solution is to use setdefault   as the last step of saving the computed composite member -- if another has already been saved then it is used instead of the new one  guaranteeing unique Enum members

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defaultdict is great when the default value is static  like a new list  but not so much if it s dynamic   For example  I need a dictionary to map strings to unique ints  defaultdict int  will always use 0 for the default value  Likewise  defaultdict intGen    always produces 1   Instead  I used a regular dict   nextID   intGen   myDict      for lots of complicated stuff       stuff that generates unpredictable  possibly already seen str     strID   myDict setdefault myStr  nextID      Note that dict get key  nextID    is insufficient because I need to be able to refer to these values later as well   intGen is a tiny class I build that automatically increments an int and returns its value   class intGen      def   init   self           self i   0      def   call   self           self i    1     return self i   If someone has a way to do this with defaultdict I d love to see it

[python] Use cases for the 'setdefault' dict method

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