How to set default value to all keys of a dict object in python

Question

I know you can use setdefault key  value  to set default value for a given key  but is there a way to set default values of all keys to some value after creating a dict    Put it another way  I want the dict to return the specified default value for every key I didn t yet set

User · Answer

In case you actually mean what you seem to ask, I'll provide this alternative answer.

You say you want the dict to return a specified value, you do not say you want to set that value at the same time, like defaultdict does. This will do so:

class DictWithDefault(dict):
    def __init__(self, default, **kwargs):
        self.default = default
        super(DictWithDefault, self).__init__(**kwargs)

    def __getitem__(self, key):
        if key in self:
            return super(DictWithDefault, self).__getitem__(key)
        return self.default

Use like this:

d = DictWIthDefault(99, x=5, y=3)
print d["x"]   # 5
print d[42]    # 99
42 in d        # False
d[42] = 3
42 in d        # True

Alternatively, you can use a standard dict like this:

d = {3: 9, 4: 2}
default = 99
print d.get(3, default)  # 9
print d.get(42, default) # 99

User · Answer

You can use the following class  Just change zero to any default value you like  The solution was tested in Python 2 7   class cDefaultDict dict         dictionary that returns zero for missing keys       keys with zero values are not stored      def   missing   self key           return 0      def   setitem   self  key  value           if value  0              if key in self     returns zero anyway  so no need to store it                 del self key          else              dict   setitem   self  key  value

User · Answer

You can replace your old dictionary with a defaultdict    gt  gt  gt  from collections import defaultdict  gt  gt  gt  d     foo   123   bar   456   gt  gt  gt  d  baz   Traceback  most recent call last     File   lt stdin gt    line 1  in  lt module gt  KeyError   baz   gt  gt  gt  d   defaultdict lambda  -1  d   gt  gt  gt  d  baz   -1   The  trick  here is that a defaultdict can be initialized with another dict  This means  that you preserve the existing values in your normal dict    gt  gt  gt  d  foo   123

User · Answer

defaultdict can do something like that for you   Example    gt  gt  gt  from collections import defaultdict  gt  gt  gt  d   defaultdict list   gt  gt  gt  d defaultdict  lt class  list  gt        gt  gt  gt  d  new   append 10   gt  gt  gt  d defaultdict  lt class  list  gt     new    10

User · Answer

Is this what you want    gt  gt  gt  d   a  1  b  2  c  3   gt  gt  gt  default val 99  gt  gt  gt  for k in d          d k  default val           gt  gt  gt  d   a   99   b   99   c   99   gt  gt  gt     gt  gt  gt  d   a  1  b  2  c  3   gt  gt  gt  from collections import defaultdict  gt  gt  gt  d defaultdict lambda 99 d   gt  gt  gt  d defaultdict  lt function  lt lambda gt  at 0x03D21630 gt     a   1   c   3   b   2    gt  gt  gt  d 3  99

User · Answer

Not after creating it  no  But you could use a defaultdict in the first place  which sets default values when you initialize it

User · Answer

Use defaultdict  from collections import defaultdict a       a   defaultdict lambda 0 a  a  anything       gt  0   This is very useful for case like this where default values for every key is set as 0   results     pre-access      count   4   pass count   2   no-access      count   55   pass count   19  for k v in results iteritems      a  count      v  count     a  pass count      v  pass count

[python] How to set default value to all keys of a dict object in python?

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