Python Dictionary Comprehension

Question

Is it possible to create a dictionary comprehension in Python  for the keys    Without list comprehensions  you can use something like this   l      for n in range 1  11       l append n    We can shorten this to a list comprehension  l    n for n in range 1  11     However  say I want to set a dictionary s keys to the same value  I can do   d      for n in range 1  11        d n    True   same value for each   I ve tried this   d      d i for i in range 1  11     True   However  I get a SyntaxError on the for   In addition  I don t need this part  but just wondering   can you set a dictionary s keys to a bunch of different values  like this   d      for n in range 1  11       d n    n   Is this possible with a dictionary comprehension   d      d i for i in range 1  11      x for x in range 1  11     This also raises a SyntaxError on the for

User · Answer

Consider this example of counting the occurrence of words in a list using dictionary comprehension

my_list = ['hello', 'hi', 'hello', 'today', 'morning', 'again', 'hello']
my_dict = {k:my_list.count(k) for k in my_list}
print(my_dict)

And the result is

{'again': 1, 'hi': 1, 'hello': 3, 'today': 1, 'morning': 1}

User · Answer

Use dict   on a list of tuples  this solution will allow you to have arbitrary values in each list  so long as they are the same length  i s   range 1  11  x s   range 1  11    x s   range 11  1  -1    Also works d   dict   i s index   x s index     for index in range len i s

User · Answer

you can t hash a list like that  try this instead  it uses tuples  d tuple  i for i in range 1 11       True

User · Answer

There are dictionary comprehensions in Python 2 7   but they don t work quite the way you re trying   Like a list comprehension  they create a new dictionary  you can t use them to add keys to an existing dictionary   Also  you have to specify the keys and values  although of course you can specify a dummy value if you like    gt  gt  gt  d    n  n  2 for n in range 5    gt  gt  gt  print d  0  0  1  1  2  4  3  9  4  16    If you want to set them all to True    gt  gt  gt  d    n  True for n in range 5    gt  gt  gt  print d  0  True  1  True  2  True  3  True  4  True    What you seem to be asking for is a way to set multiple keys at once on an existing dictionary   There s no direct shortcut for that   You can either loop like you already showed  or you could use a dictionary comprehension to create a new dict with the new values  and then do oldDict update newDict  to merge the new values into the old dict

User · Answer

You can use the dict fromkeys class method       gt  gt  gt  dict fromkeys range 5   True   0  True  1  True  2  True  3  True  4  True    This is the fastest way to create a dictionary where all the keys map to the same value   But do not use this with mutable objects   d   dict fromkeys range 5          0      1      2      3      4      d 1  append 2     0   2   1   2   2   2   3   2   4   2         If you don t actually need to initialize all the keys  a defaultdict might be useful as well   from collections import defaultdict d   defaultdict True      To answer the second part  a dict-comprehension is just what you need    k  k for k in range 10     You probably shouldn t do this but you could also create a subclass of dict which works somewhat like a defaultdict if you override   missing      gt  gt  gt  class KeyDict dict          def   missing   self  key              self key    key    Maybe add this also            return key       gt  gt  gt  d   KeyDict    gt  gt  gt  d 1  1  gt  gt  gt  d 2  2  gt  gt  gt  d 3  3  gt  gt  gt  print d

User · Answer

I really like the  mgilson comment  since if you have a two iterables  one that corresponds to the keys and the other the values  you can also do the following   keys     a    b    c   values    1  2  3  d   dict zip keys  values     giving      d     a   1   b   2   c   3

User · Answer

gt  gt  gt   i i for i in range 1  11    1  1  2  2  3  3  4  4  5  5  6  6  7  7  8  8  9  9  10  10

User · Answer

The main purpose of a list comprehension is to create a new list based on another one without changing or destroying the original list  Instead of writing l      for n in range 1  11       l append n   or l    n for n in range 1  11    you should write only l   range 1  11   In the two top code blocks you re creating a new list  iterating through it and just returning each element  It s just an expensive way of creating a list copy  To get a new dictionary with all keys set to the same value based on another dict  do this  old dict     a   1   c   3   b   2  new dict     key  your value here  for key in old dict keys     You re receiving a SyntaxError because when you write d      d i for i in range 1  11     True  you re basically saying   quot Set my key  i for i in range 1  11   to True quot  and  quot i for i in range 1  11  quot  is not a valid key  it s just a syntax error  If dicts supported lists as keys  you would do something like d  i for i in range 1  11      True  and not d i for i in range 1  11     True  but lists are not hashable  so you can t use them as dict keys

[python] Python Dictionary Comprehension

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