How to delete items from a dictionary while iterating over it

Question

Is it legitimate to delete items from a dictionary in Python while iterating over it   For example   for k  v in mydict iteritems       if k    val       del mydict k    The idea is to remove elements that don t meet a certain condition from the dictionary  instead of creating a new dictionary that s a subset of the one being iterated over   Is this a good solution  Are there more elegant efficient ways

User · Accepted Answer

EDIT   This answer will not work for Python3 and will give a RuntimeError      RuntimeError  dictionary changed size during iteration    This happens because mydict keys   returns an iterator not a list  As pointed out in comments simply convert mydict keys   to a list by list mydict keys    and it should work     A simple test in the console shows you cannot modify a dictionary while iterating over it      gt  gt  gt  mydict     one   1   two   2   three   3   four   4   gt  gt  gt  for k  v in mydict iteritems           if k     two              del mydict k      ------------------------------------------------------------ Traceback  most recent call last     File   lt ipython console gt    line 1  in  lt module gt  RuntimeError  dictionary changed size during iteration   As stated in delnan s answer  deleting entries causes problems when the iterator tries to move onto the next entry  Instead  use the keys   method to get a list of the keys and work with that    gt  gt  gt  for k in mydict keys           if k     two              del mydict k       gt  gt  gt  mydict   four   4   three   3   one   1    If you need to delete based on the items value  use the items   method instead    gt  gt  gt  for k  v in mydict items            if v    3              del mydict k       gt  gt  gt  mydict   four   4   one   1

User · Answer

You could also do it in two steps   remove    k for k in mydict if k    val  for k in remove  del mydict k    My favorite approach is usually to just make a new dict     Python 2 7 and 3 x mydict     k v for k v in mydict items   if k  val     before Python 2 7 mydict   dict  k v  for k v in mydict iteritems   if k  val

User · Answer

You can use a dictionary comprehension    d    k d k  for k in d if d k     val

User · Answer

With python3  iterate on dic keys   will raise the dictionary size error  You can use this alternative way   Tested with python3  it works fine and the Error  dictionary changed size during iteration  is not raised   my dic     1 10  2 20  3 30     Is important here to cast because   keys    method returns a dict keys object  key list   list  my dic keys        Iterate on the list  for k in key list      print key list      print my dic      del  my dic k      print  my dic

User · Answer

There is a way that may be suitable if the items you want to delete are always at the  beginning  of the dict iteration  while mydict      key  value   next iter mydict items         if should delete key  value          del mydict key      else         break   The  beginning  is only guaranteed to be consistent for certain Python versions implementations  For example from What   s New In Python 3 7     the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec    This way avoids a copy of the dict that a lot of the other answers suggest  at least in Python 3

User · Answer

Iterate over a copy instead  such as the one returned by items     for k  v in list mydict items

User · Answer

You could first build a list of keys to delete  and then iterate over that list deleting them   dict     one    1   two    2   three    3   four    4  delete      for k v in dict items        if v 2    1          delete append k  for i in delete      del dict i

User · Answer

It s cleanest to use list mydict     gt  gt  gt  mydict     one   1   two   2   three   3   four   4   gt  gt  gt  for k in list mydict           if k     three               del mydict k        gt  gt  gt  mydict   four   4   two   2   one   1    This corresponds to a parallel structure for lists    gt  gt  gt  mylist     one    two    three    four    gt  gt  gt  for k in list mylist                                or mylist            if k     three               mylist remove k        gt  gt  gt  mylist   one    two    four     Both work in python2 and python3

User · Answer

You can t modify a collection while iterating it  That way lies madness - most notably  if you were allowed to delete and deleted the current item  the iterator would have to move on   1  and the next call to next would take you beyond that   2   so you d end up skipping one element  the one right behind the one you deleted   You have two options    Copy all keys  or values  or both  depending on what you need   then iterate over those  You can use  keys   et al for this  in Python 3  pass the resulting iterator to list   Could be highly wasteful space-wise though  Iterate over mydict as usual  saving the keys to delete in a seperate collection to delete  When you re done iterating mydict  delete all items in to delete from mydict  Saves some  depending on how many keys are deleted and how many stay  space over the first approach  but also requires a few more lines

User · Answer

I tried the above solutions in Python3 but this one seems to be the only one working for me when storing objects in a dict  Basically you make a copy of your dict   and iterate over that while deleting the entries in your original dictionary             tmpDict   realDict copy           for key  value in tmpDict items                if value                  del realDict key

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