Filter dict to contain only certain keys

Question

I ve got a dict that has a whole bunch of entries  I m only interested in a select few of them  Is there an easy way to prune all the other ones out

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Based on the accepted answer by delnan   What if one of your wanted keys aren t in the old dict  The delnan solution will throw a KeyError exception that you can catch  If that s not what you need maybe you want to    only include keys that excists both in the old dict and your set of wanted keys   old dict     name   Foobar    baz  42  wanted keys     name    age   new dict    k  old dict k  for k in set wanted keys   amp  set old dict keys       gt  gt  gt  new dict   name    Foobar    have a default value for keys that s not set in old dict   default   None new dict    k  old dict k  if k in old dict else default for k in wanted keys    gt  gt  gt  new dict   age   None   name    Foobar

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Slightly more elegant dict comprehension   foodict    k  v for k  v in mydict items   if k startswith  foo

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Here is another simple method using del in one liner   for key in e keys  del your dict key    e keys is the list of the keys to be excluded  It will update your dict rather than giving you a new one   If you want a new output dict  then make a copy of the dict before deleting   new dict   your dict copy              Making copy of dict  for key in e keys  del new dict key

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Constructing a new dict    dict you want     your key  old dict your key  for your key in your keys     Uses dictionary comprehension    If you use a version which lacks them  ie Python 2 6 and earlier   make it dict  your key  old dict your key   for       It s the same  though uglier   Note that this  unlike jnnnnn s version  has stable performance  depends only on number of your keys  for old dicts of any size  Both in terms of speed and memory  Since this is a generator expression  it processes one item at a time  and it doesn t looks through all items of old dict   Removing everything in-place   unwanted   set keys  - set your dict  for unwanted key in unwanted  del your dict unwanted key

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Here s an example in python 2 6    gt  gt  gt  a    1 1  2 2  3 3   gt  gt  gt  dict  key value  for key  value in a iteritems   if key    1   1  1    The filtering part is the if statement   This method is slower than delnan s answer if you only want to select a few of very many keys

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Another option   content   dict k1  foo   k2  nope   k3  bar   selection     k1    k3   filtered   filter lambda i  i 0  in selection  content items      But you get a list  Python 2  or an iterator  Python 3  returned by filter    not a dict

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Code 1   dict     key  key   10 for key in range 0  100    d1      for key  value in dict items        if key   2    0          d1 key    value   Code 2   dict     key  key   10 for key in range 0  100    d2    key  value for key  value in dict items   if key   2    0    Code 3   dict     key  key   10 for key in range 0  100    d3     key  dict key  for key in dict keys   if key   2    0    All pieced of code performance are measured with timeit using number 1000  and collected 1000 times for each piece of code      For python 3 6 the performance of three ways of filter dict keys almost the same  For python 2 7 code 3 is slightly faster

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You could use python-benedict  it s a dict subclass   Installation  pip install python-benedict  from benedict import benedict  dict you want   benedict your dict  subset keys   firstname    lastname    email      It s open-source on GitHub  https   github com fabiocaccamo python-benedict     Disclaimer  I m the author of this library

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Short form    s pop k  for k in list s keys    if k not in keep    As most of the answers suggest in order to maintain the conciseness we have to create a duplicate object be it a list or dict  This one creates a throw-away list but deletes the keys in original dict

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If we want to make a new dictionary with selected keys removed  we can make use of dictionary comprehension For example   d      a    1   b    2   c    3   x    key d key  for key in d keys   -   c    e      Python 3 y    key d key  for key in set d keys    -   c    e      Python 2     x is   a   1   b   2    y is   a   1   b   2

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Given your original dictionary orig and the set of entries that you re interested in keys   filtered   dict zip keys   orig k  for k in keys      which isn t as nice as delnan s answer  but should work in every Python version of interest  It is  however  fragile to each element of keys existing in your original dictionary

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This function will do the trick   def include keys dictionary  keys          Filters a dict by only including certain keys         key set   set keys   amp  set dictionary keys        return  key  dictionary key  for key in key set    Just like delnan s version  this one uses dictionary comprehension and has stable performance for large dictionaries  dependent only on the number of keys you permit  and not the total number of keys in the dictionary    And just like MyGGan s version  this one allows your list of keys to include keys that may not exist in the dictionary   And as a bonus  here s the inverse  where you can create a dictionary by excluding certain keys in the original   def exclude keys dictionary  keys          Filters a dict by excluding certain keys         key set   set dictionary keys    - set keys      return  key  dictionary key  for key in key set    Note that unlike delnan s version  the operation is not done in place  so the performance is related to the number of keys in the dictionary  However  the advantage of this is that the function will not modify the dictionary provided   Edit  Added a separate function for excluding certain keys from a dict

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You can do that with project function from my funcy library   from funcy import project small dict   project big dict  keys    Also take a look at select keys

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This one liner lambda should work   dictfilt   lambda x  y  dict    i x i   for i in x if i in set y       Here s an example   my dict     a  1  b  2  c  3  d  4  wanted keys     c   d      run it In  10   dictfilt my dict  wanted keys  Out 10     c   3   d   4    It s a basic list comprehension iterating over your dict keys  i in x  and outputs a list of tuple  key value  pairs if the key lives in your desired key list  y   A dict   wraps the whole thing to output as a dict object

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We can do simply with lambda function like this   gt  gt  gt  dict filter   lambda x  y  dict    i x i   for i in x if i in set y      gt  gt  gt  large dict     quot a quot  1  quot b quot  2  quot c quot  3  quot d quot  4   gt  gt  gt  new dict keys     quot c quot   quot d quot    gt  gt  gt  small dict dict filter large dict  new dict keys   gt  gt  gt  print small dict    c   3   d   4   gt  gt  gt

[python] Filter dict to contain only certain keys?

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