Create a dictionary with list comprehension

Question

I like the Python list comprehension syntax   Can it be used to create dictionaries too  For example  by iterating over pairs of keys and values   mydict     k v  for  k v  in blah blah blah     doesn t work

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In fact  you don t even need to iterate over the iterable if it already comprehends some kind of mapping  the dict constructor doing it graciously for you    gt  gt  gt  ts     1  2    3  4    5  6    gt  gt  gt  dict ts   1  2  3  4  5  6   gt  gt  gt  gen     i  i 1  for i in range 1  6  2    gt  gt  gt  gen  lt generator object  lt genexpr gt  at 0xb7201c5c gt   gt  gt  gt  dict gen   1  2  3  4  5  6

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Try this   def get dic from two lists keys  values       return   keys i    values i  for i in range len keys       Assume we have two lists country and capital  country     India    Pakistan    China   capital     New Delhi    Islamabad    Beijing     Then create dictionary from the two lists   print get dic from two lists country  capital    The output is like this     Pakistan    Islamabad    China    Beijing    India    New Delhi

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Create a dictionary with list comprehension in Python I like the Python list comprehension syntax  Can it be used to create dictionaries too  For example  by iterating over pairs of keys and values  mydict     k v  for  k v  in blah blah blah    You re looking for the phrase  quot dict comprehension quot  - it s actually  mydict    k  v for k  v in iterable   Assuming blah blah blah is an iterable of two-tuples - you re so close  Let s create some  quot blahs quot  like that  blahs      blah0    blah      blah1    blah      blah2    blah      blah3    blah     Dict comprehension syntax  Now the syntax here is the mapping part  What makes this a dict comprehension instead of a set comprehension  which is what your pseudo-code approximates  is the colon    like below  mydict    k  v for k  v in blahs   And we see that it worked  and should retain insertion order as-of Python 3 7   gt  gt  gt  mydict   blah0    blah    blah1    blah    blah2    blah    blah3    blah    In Python 2 and up to 3 6  order was not guaranteed   gt  gt  gt  mydict   blah0    blah    blah1    blah    blah3    blah    blah2    blah    Adding a Filter  All comprehensions feature a mapping component and a filtering component that you can provide with arbitrary expressions  So you can add a filter part to the end   gt  gt  gt  mydict    k  v for k  v in blahs if not int k -1     2   gt  gt  gt  mydict   blah0    blah    blah2    blah    Here we are just testing for if the last character is divisible by 2 to filter out data before mapping the keys and values

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In Python 3 and Python 2 7   dictionary comprehensions look like the below   d    k v for k  v in iterable    For Python 2 6 or earlier  see fortran s answer

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Here is another example of dictionary creation using dict comprehension   What i am tring to do here is to create a alphabet dictionary where each pair  is the english letter and its corresponding position in english alphabet   gt  gt  gt  import string  gt  gt  gt  dict1    value   int key    1  for key  value in  enumerate list string ascii lowercase     gt  gt  gt  dict1   a   1   c   3   b   2   e   5   d   4   g   7   f   6   i   9   h   8    k   11   j   10   m   13   l   12   o   15   n   14   q   17   p   16   s    19   r   18   u   21   t   20   w   23   v   22   y   25   x   24   z   26   gt  gt  gt     Notice the use of enumerate here to get a list of alphabets and their indexes in the list and swapping the alphabets and indices to generate the key value pair for dictionary  Hope it gives a good idea of dictionary comp to you and encourages you to use it more often to make your code compact

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In Python 2 7  it goes like    gt  gt  gt  list1  list2     a    b    c     1 2 3   gt  gt  gt  dict  zip  list1  list2     a   1   c   3   b   2    Zip them

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Just to throw in another example  Imagine you have the following list   nums    4 2 2 1 3    and you want to turn it into a dict where the key is the index and value is the element in the list  You can do so with the following line of code    index nums index  for index in range 0 len nums

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gt  gt  gt   k  v  3 for  k  v  in zip string ascii lowercase  range 26     Python supports dict comprehensions  which allow you to express the creation of dictionaries at runtime using a similarly concise syntax  A dictionary comprehension takes the form  key  value for  key  value  in iterable   This syntax was introduced in Python 3 and backported as far as Python 2 7  so you should be able to use it regardless of which version of Python you have installed  A canonical example is taking two lists and creating a dictionary where the item at each position in the first list becomes a key and the item at the corresponding position in the second list becomes the value  The zip function used inside this comprehension returns an iterator of tuples  where each element in the tuple is taken from the same position in each of the input iterables  In the example above  the returned iterator contains the tuples     a     1       b     2   etc  Output    i   512   e   64   o   2744   h   343   l   1331   s   5832   b   1   w   10648   c   8   x   12167   y   13824   t   6859   p   3375   d   27   j   729   a   0   z   15625   f   125   q   4096   u   8000   n   2197   m   1728   r   4913   k   1000   g   216   v   9261

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Yes  it s possible  In python  Comprehension can be used in List  Set  Dictionary  etc  You can write it this way mydict    k v for  k v  in blah   Another detailed example of Dictionary Comprehension with the Conditional Statement and Loop  parents    father  mother               parents    parent 1 - P  quot mutation quot   if parent in two genes else 0 5 if parent in one gene else P  quot mutation quot   for parent in parents

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To add onto  fortran s answer  if you want to iterate over a list of keys key list as well as a list of values value list   d   dict  key  value  for  key  value  in zip key list  value list     or  d     key  value  for  key  value  in zip key list  value list

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Use a dict comprehension   key  value for  key  value  in iterable    Note  this is for Python 3 x  and 2 7 upwards   Formerly in Python 2 6 and earlier  the dict built-in could receive an iterable of key value pairs  so you can pass it a list comprehension or generator expression  For example  dict  key  func key   for key in keys   In simple cases you don t need a comprehension at all    But if you already have iterable s  of keys and or values  just call the dict built-in directly  1  consumed from any iterable yielding pairs of keys vals dict pairs   2   quot zip ped quot  from two separate iterables of keys vals dict zip list of keys  list of values

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Python version  gt   2 7  do the below  d    i  True for i in  1 2 3    Python version  lt  2 7 RIP  3 July 2010 - 31 December 2019   do the below  d   dict  i True  for i in  1 2 3

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You can create a new dict for each pair and merge it with the previous dict   reduce lambda p  q     p     q 0   q 1     bla bla bla        Obviously this approaches requires reduce from functools

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This code will create dictionary using list comprehension for multiple lists with different values that can be used for pd DataFrame      Multiple lists  model   A    B    C    D   launched  1983 1984 1984 1984  discontinued  1986  1985  1984  1986    Dictionary with list comprehension keys   model   launched   discontinued   vals  model  launched discontinued  data    key vals n  for n  key in enumerate keys     enumerate will pass n to vals to match each key with its list

[python] Create a dictionary with list comprehension

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