Remove an item from a dictionary when its key is unknown

Question

What is the best way to remove an item from a dictionary by value  i e  when the item s key is unknown   Here s a simple approach   for key  item in some dict items        if item is item to remove          del some dict key    Are there better ways   Is there anything wrong with mutating  deleting items  from the dictionary while iterating it

User · Answer

This is how I would do it   for key in some dict keys        if some dict key     item to remove          some dict pop key          break

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y   username   admin   machine    a   b   c    if  c  in y  machine     del y  machine   y  machine   index  c

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c is the new dictionary  and a is your original dictionary    z   w   are the keys you want to remove from a  c    key a key  for key in a keys   -   z    w      Also check  https   www safaribooksonline com library view python-cookbook-3rd 9781449357337 ch01 html

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items   returns a list  and it is that list you are iterating  so mutating the dict in the loop doesn t matter here  If you were using iteritems   instead  mutating the dict in the loop would be problematic  and likewise for viewitems   in Python 2 7   I can t think of a better way to remove items from a dict by value

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I d build a list of keys that need removing  then remove them  It s simple  efficient and avoids any problem about simultaneously iterating over and mutating the dict   keys to remove    key for key  value in some dict iteritems                     if value    value to remove  for key in keys to remove      del some dict key

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Be aware that you re currently testing for object identity  is only returns True if both operands are represented by the same object in memory - this is not always the case with two object that compare equal with      If you are doing this on purpose  then you could rewrite your code as  some dict    key  value for key  value in some dict items                 if value is not value to remove    But this may not do what you want    gt  gt  gt  some dict    1   Hello   2   Goodbye   3   You say yes   4   I say no    gt  gt  gt  value to remove    You say yes   gt  gt  gt  some dict    key  value for key  value in some dict items   if value is not value to remove   gt  gt  gt  some dict  1   Hello   2   Goodbye   3   You say yes   4   I say no    gt  gt  gt  some dict    key  value for key  value in some dict items   if value    value to remove   gt  gt  gt  some dict  1   Hello   2   Goodbye   4   I say no     So you probably want    instead of is not

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The dict pop key   default   method allows you to remove items when you know the key  It returns the value at the key if it removes the item otherwise it returns what is passed as default  See the docs    Example    gt  gt  gt  dic     a  1   b  2   gt  gt  gt  dic   a   1   b   2   gt  gt  gt  dic pop  c   0  0  gt  gt  gt  dic pop  a   0  1  gt  gt  gt  dic   b   2

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A simple comparison between del and pop     import timeit code       results     A   1   B   2   C   3  del results  A   del results  B       print timeit timeit code  number 100000  code       results     A   1   B   2   C   3  results pop  A   results pop  B       print timeit timeit code  number 100000    result    0 0329667857143 0 0451040902256   So  del is faster than pop

User · Answer

There is nothing wrong with deleting items from the dictionary while iterating  as you ve proposed   Be careful about multiple threads using the same dictionary at the same time  which may result in a KeyError or other problems   Of course  see the docs at http   docs python org library stdtypes html typesmapping

User · Answer

a     name    your name   class   4  if  name  in a  del a  name

[python] Remove an item from a dictionary when its key is unknown

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