Mapping over values in a python dictionary

Question

Given a dictionary   k1  v1  k2  v2       I want to get   k1  f v1   k2  f v2        provided I pass a function f   Is there any such built in function  Or do I have to do  dict   k  f v   for  k  v  in my dictionary iteritems       Ideally I would just write  my dictionary map values f    or  my dictionary mutate values with f    That is  it doesn t matter to me if the original dictionary is mutated or a copy is created

User · Answer

While my original answer missed the point (by trying to solve this problem with the solution to Accessing key in factory of defaultdict), I have reworked it to propose an actual solution to the present question.

Here it is:

class walkableDict(dict):
  def walk(self, callback):
    try:
      for key in self:
        self[key] = callback(self[key])
    except TypeError:
      return False
    return True

Usage:

>>> d = walkableDict({ k1: v1, k2: v2 ... })
>>> d.walk(f)

The idea is to subclass the original dict to give it the desired functionality: "mapping" a function over all the values.

The plus point is that this dictionary can be used to store the original data as if it was a dict, while transforming any data on request with a callback.

Of course, feel free to name the class and the function the way you want (the name chosen in this answer is inspired by PHP's array_walk() function).

Note: Neither the try-except block nor the return statements are mandatory for the functionality, they are there to further mimic the behavior of the PHP's array_walk.

User · Answer

Due to PEP-0469 which renamed iteritems   to items   and PEP-3113 which removed Tuple parameter unpacking  in Python 3 x you should write Martijn Pieters  answer like this   my dictionary   dict map lambda item   item 0   f item 1     my dictionary items

User · Answer

There is no such function  the easiest way to do this is to use a dict comprehension   my dictionary    k  f v  for k  v in my dictionary items      In python 2 7  use the  iteritems   method instead of  items   to save memory  The dict comprehension syntax wasn t introduced until python 2 7   Note that there is no such method on lists either  you d have to use a list comprehension or the map   function   As such  you could use the map   function for processing your dict as well   my dictionary   dict map lambda kv   kv 0   f kv 1     my dictionary iteritems       but that s not that readable  really

User · Answer

These toolz are great for this kind of simple yet repetitive logic   http   toolz readthedocs org en latest api html toolz dicttoolz valmap  Gets you right where you want to be   import toolz def f x     return x 1  toolz valmap f  my list

User · Answer

Just came accross this use case  I implemented gens s answer  adding a recursive approach for handling values that are also dicts   def mutate dict in place f  d       for k  v in d iteritems            if isinstance v  dict               mutate dict in place f  v          else              d k    f v     Exemple handy usage def utf8 everywhere d       mutate dict in place           lambda value              value decode  utf-8               if isinstance value  bytes              else value                    d        my dict     a   b byte1    b     c   b byte2    d   b byte3    utf8 everywhere my dict  print my dict    This can be useful when dealing with json or yaml files that encode strings as bytes in Python 2

User · Answer

You can do this in-place  rather than create a new dict  which may be preferable for large dictionaries  if you do not need a copy    def mutate dict f d       for k  v in d iteritems            d k    f v   my dictionary     a  1   b  2  mutate dict lambda x  x 1  my dictionary    results in my dictionary containing     a   2   b   3

User · Answer

To avoid doing indexing from inside lambda  like   rval   dict map lambda kv    kv 0       join kv 1     rval iteritems       You can also do   rval   dict map lambda k v     k      join v    rval iteritems

[python] Mapping over values in a python dictionary

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