Delete an element from a dictionary

Question

Is there a way to delete an item from a dictionary in Python   Additionally  how can I delete an item from a dictionary to return a copy  i e   not modifying the original

User · Answer

Here a top level design approach   def eraseElement d k       if isinstance d  dict           if k in d              d pop k              print d          else              print  Cannot find matching key       else          print  Not able to delete     exp     A  34   B  55   C  87  eraseElement exp   C     I m passing the dictionary and the key I want into my function  validates if it s a dictionary and if the key is okay  and if both exist  removes the value from the dictionary and prints out the left-overs   Output    B   55   A   34   Hope that helps

User · Answer

I think your solution is best way to do it  But if you want another solution  you can create a new dictionary with using the keys from old dictionary without including your specified key  like this    gt  gt  gt  a  0   zero   1   one   2   two   3   three    gt  gt  gt   i a i  for i in a if i  0   1   one   2   two   3   three

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gt  gt  gt  def delete key dict  key           del dict key          return dict       gt  gt  gt  test dict     one   1   two    2   gt  gt  gt  print delete key test dict   two     one   1   gt  gt  gt    this doesn t do any error handling  it assumes the key is in the dict  you might want to check that first and raise if its not

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There re a lot of nice answers  but I want to emphasize one thing   You can use both dict pop   method and a more generic del statement to remove items from a dictionary  They both mutate the original dictionary  so you need to make a copy  see details below    And both of them will raise a KeyError if the key you re providing to them is not present in the dictionary   key to remove    c  d     a   1   b   2  del d key to remove     Raises  KeyError   c     and  key to remove    c  d     a   1   b   2  d pop key to remove     Raises  KeyError   c     You have to take care of this   by capturing the exception   key to remove    c  d     a   1   b   2  try      del d key to remove  except KeyError as ex      print  No such key    s     ex message    and  key to remove    c  d     a   1   b   2  try      d pop key to remove  except KeyError as ex      print  No such key    s     ex message    by performing a check   key to remove    c  d     a   1   b   2  if key to remove in d      del d key to remove    and  key to remove    c  d     a   1   b   2  if key to remove in d      d pop key to remove    but with pop   there s also a much more concise way - provide the default return value   key to remove    c  d     a   1   b   2  d pop key to remove  None     No  KeyError  here   Unless you use pop   to get the value of a key being removed you may provide anything  not necessary None  Though it might be that using del with in check is slightly faster due to pop   being a function with its own complications causing overhead  Usually it s not the case  so pop   with default value is good enough     As for the main question  you ll have to make a copy of your dictionary  to save the original dictionary and have a new one without the key being removed   Some other people here suggest making a full  deep  copy with copy deepcopy    which might be an overkill  a  normal   shallow  copy  using copy copy   or dict copy    might be enough  The dictionary keeps a reference to the object as a value for a key  So when you remove a key from a dictionary this reference is removed  not the object being referenced  The object itself may be removed later automatically by the garbage collector  if there re no other references for it in the memory  Making a deep copy requires more calculations compared to shallow copy  so it decreases code performance by making the copy  wasting memory and providing more work to the GC  sometimes shallow copy is enough   However  if you have mutable objects as dictionary values and plan to modify them later in the returned dictionary without the key  you have to make a deep copy   With shallow copy   def get dict wo key dictionary  key          Returns a   shallow   copy of the dictionary without a key          dict   dictionary copy        dict pop key  None      return  dict   d     a    1  2  3    b   2   c   3  key to remove    c   new d   get dict wo key d  key to remove  print d       a    1  2  3    b   2   c   3  print new d       a    1  2  3    b   2  new d  a   append 100  print d       a    1  2  3  100    b   2   c   3  print new d       a    1  2  3  100    b   2  new d  b     2222 print d       a    1  2  3  100    b   2   c   3  print new d       a    1  2  3  100    b   2222    With deep copy   from copy import deepcopy   def get dict wo key dictionary  key          Returns a   deep   copy of the dictionary without a key          dict   deepcopy dictionary       dict pop key  None      return  dict   d     a    1  2  3    b   2   c   3  key to remove    c   new d   get dict wo key d  key to remove  print d       a    1  2  3    b   2   c   3  print new d       a    1  2  3    b   2  new d  a   append 100  print d       a    1  2  3    b   2   c   3  print new d       a    1  2  3  100    b   2  new d  b     2222 print d       a    1  2  3    b   2   c   3  print new d       a    1  2  3  100    b   2222

User · Answer

The del statement removes an element  del d key   Note that this mutates the existing dictionary  so the contents of the dictionary changes for anybody else who has a reference to the same instance  To return a new dictionary  make a copy of the dictionary  def removekey d  key       r   dict d      del r key      return r  The dict   constructor makes a shallow copy  To make a deep copy  see the copy module   Note that making a copy for every dict del assignment etc  means you re going from constant time to linear time  and also using linear space  For small dicts  this is not a problem  But if you re planning to make lots of copies of large dicts  you probably want a different data structure  like a HAMT  as described in this answer

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how can I delete an item from a dictionary to return a copy  i e   not modifying the original     A dict is the wrong data structure to use for this   Sure  copying the dict and popping from the copy works  and so does building a new dict with a comprehension  but all that copying takes time   you ve replaced a constant-time operation with a linear-time one  And all those copies alive at once take space   linear space per copy   Other data structures  like hash array mapped tries  are designed for exactly this kind of use case  adding or removing an element returns a copy in logarithmic time  sharing most of its storage with the original 1  Of course there are some downsides  Performance is logarithmic rather than constant  although with a large base  usually 32-128   And  while you can make the non-mutating API identical to dict  the  mutating  API is obviously different  And  most of all  there s no HAMT batteries included with Python 2  The pyrsistent library is a pretty solid implementation of HAMT-based dict-replacements  and various other types  for Python  It even has a nifty evolver API for porting existing mutating code to persistent code as smoothly as possible  But if you want to be explicit about returning copies rather than mutating  you just use it like this    gt  gt  gt  from pyrsistent import m  gt  gt  gt  d1   m a 1  b 2   gt  gt  gt  d2   d1 set  c   3   gt  gt  gt  d3   d1 remove  a    gt  gt  gt  d1 pmap   a   1   b   2    gt  gt  gt  d2 pmap   c   3   a   1   b   2    gt  gt  gt  d3 pmap   b   2     That d3   d1 remove  a   is exactly what the question is asking for   If you ve got mutable data structures like dict and list embedded in the pmap  you ll still have aliasing issues   you can only fix that by going immutable all the way down  embedding pmaps and pvectors     1  HAMTs have also become popular in languages like Scala  Clojure  Haskell because they play very nicely with lock-free programming and software transactional memory  but neither of those is very relevant in Python   2  In fact  there is an HAMT in the stdlib  used in the implementation of contextvars  The earlier withdrawn PEP explains why  But this is a hidden implementation detail of the library  not a public collection type

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Here s another variation using list comprehension   original d     a   None   b    Some   d   dict  k v  for k  v in original d iteritems   if v    result should be   b    Some     The approach is based on an answer from this post  Efficient way to remove keys with empty strings from a dict

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pop mutates the dictionary     gt  gt  gt  lol     hello    gdbye     gt  gt  gt  lol pop  hello         gdbye    gt  gt  gt  lol           If you want to keep the original you could just copy it

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species     HI     1    1215 671  0 41600000000000004      10    919 351  0 0012      1025    1025 722  0 0791      11    918 129  0 0009199999999999999      12    917 181  0 000723      1215    1215 671  0 41600000000000004      13    916 429  0 0005769999999999999      14    915 824  0 000468      15    915 329  0 00038500000000000003     CII     1036    1036 3367  0 11900000000000001    1334    1334 532  0 129      The following code will make a copy of dict species and delete items which are not in trans HI  trans HI   1025   1215   for transition in species  HI   copy   keys        if transition not in trans HI          species  HI   pop transition

User · Answer

The del statement is what you re looking for  If you have a dictionary named foo with a key called  bar   you can delete  bar  from foo like this   del foo  bar     Note that this permanently modifies the dictionary being operated on  If you want to keep the original dictionary  you ll have to create a copy beforehand    gt  gt  gt  foo     bar    baz    gt  gt  gt  fu   dict foo   gt  gt  gt  del foo  bar    gt  gt  gt  print foo     gt  gt  gt  print fu   bar    baz     The dict call makes a shallow copy  If you want a deep copy  use copy deepcopy   Here s a method you can copy  amp  paste  for your convenience   def minus key key  dictionary       shallow copy   dict dictionary      del shallow copy key      return shallow copy

User · Answer

No  there is no other way than  def dictMinus dct  val      copy   dct copy      del copy val     return copy   However  often creating copies of only slightly altered dictionaries is probably not a good idea because it will result in comparatively large memory demands  It is usually better to log the old dictionary if even necessary  and then modify it

User · Answer

Below code snippet will help you definitely  I have added comments in each line which will help you in understanding the code   def execute       dic     a  1  b  2     dic2   remove key from dict dic   b        print dict2                a   1     print dict                 a  1  b  2   def remove key from dict dictionary to use  key to delete      copy of dict   dict dictionary to use        creating clone copy of the dictionary    if key to delete in copy of dict             checking given key is present in the dictionary        del copy of dict  key to delete          deleting the key from the dictionary     return copy of dict                          returning the final dictionary   or you can also use dict pop    d     a   1   b   2   res   d pop  c      No  KeyError  here print  res          this line will not execute   or the better approach is  res   d pop  c    key not found   print  res      key not found print  d          a   1   b   2   res   d pop  b    key not found   print  res      2 print  d          a   1

User · Answer

d    1  2   2   3  5  7  del d 5  print  d      d   Result  d    1  2   2   3

User · Answer

mutate remove with a default ret val   body pop  key   5    no mutation with a default ret val   body get  key   5

User · Answer

Simply call del d  key     However  in production  it is always a good practice to check if  key  exists in d   if  key  in d      del d  key

[python] Delete an element from a dictionary

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