Python Adding element to list while iterating

Question

I know that it is not allowed to remove elements while iterating a list  but is it allowed to add elements to a python list while iterating  Here is an example       for a in myarr        if somecond a             myarr append newObj      I have tried this in my code and it seems to work fine  however I don t know if it s because I am just lucky and that it will break at some point in the future   EDIT  I prefer not to copy the list since  myarr  is huge  and therefore it would be too slow  Also I need to check the appended objects with  somecond      EDIT  At some point  somecond a   will be false  so there can not be an infinite loop   EDIT  Someone asked about the  somecond    function  Each object in myarr has a size  and each time  somecond a   is true and a new object is appended to the list  the new object will have a size smaller than a   somecond    has an epsilon for how small objects can be and if they are too small it will return  false

User · Answer

Expanding S Lott s answer so that new items are processed as well   todo   myarr done      while todo      added          for a in todo          if somecond a               added append newObj        done extend todo      todo   added   The final list is in done

User · Answer

well  according to http   docs python org tutorial controlflow html      It is not safe to modify the sequence   being iterated over in the loop  this   can only happen for mutable sequence   types  such as lists   If you need to   modify the list you are iterating over    for example  to duplicate selected   items  you must iterate over a copy

User · Answer

Access your list elements directly by i  Then you can append to your list   for i in xrange len myarr        if somecond a i            myarr append newObj

User · Answer

In short  If you are absolutely sure all new objects fail somecond   check  then your code works fine  it just wastes some time iterating the newly added objects   Before giving a proper answer  you have to understand why it considers a bad idea to change list dict while iterating  When using for statement  Python tries to be clever  and returns a dynamically calculated item each time  Take list as example  python remembers a index  and each time it returns l index  to you  If you are changing l  the result l index  can be messy   NOTE  Here is a stackoverflow question to demonstrate this    The worst case for adding element while iterating is infinite loop  try or not if you can read a bug  the following in a python REPL   import random  l    0  for item in l      l append random randint 1  1000       print item   It will print numbers non-stop until memory is used up  or killed by system user   Understand the internal reason  let s discuss the solutions  Here are a few   1  make a copy of origin list  Iterating the origin list  and modify the copied one   result   l    for item in l      if somecond item           result append Obj      2  control when the loop ends  Instead of handling control to python  you decides how to iterate the list   length   len l  for index in range length       if somecond l index            l append Obj      Before iterating  calculate the list length  and only loop length times   3  store added objects in a new list  Instead of modifying the origin list  store new object in a new list and concatenate them afterward   added    Obj   for item in l if somecond item   l extend added

User · Answer

Alternate solution    reduce lambda x newObj   x   newObj  if somecond else x myarr myarr

User · Answer

make copy of your original list  iterate over it  see the modified code below  for a in myarr           if somecond a             myarr append newObj

User · Answer

You can do this   bonus rows      for a in myarr    if somecond a         bonus rows append newObj    myarr extend  bonus rows

User · Answer

You could use the islice from itertools to create an iterator over a smaller portion of the list   Then you can append entries to the list without impacting the items you re iterating over  islice myarr  0  len myarr -1   Even better  you don t even have to iterate over all the elements   You can increment a step size

User · Answer

I had a similar problem today  I had a list of items that needed checking  if the objects passed the check  they were added to a result list  If they didn t pass  I changed them a bit and if they might still work  size   0 after the change   I d add them on to the back of the list for rechecking   I went for a solution like  items       what I want to check     result      while items      recheck items          for item in items          if check item               result append item          else              item   change item     Note that this always lowers the integer size                                       so no danger of an infinite loop             if item size    gt  0                  recheck items append item      items   recheck items    Let the loop restart with these  if any   My list is effectively a queue  should probably have used some sort of queue  But my lists are small  like 10 items  and this works too

User · Answer

You can use an index and a while loop instead of a for loop if you want the loop to also loop over the elements that is added to the list during the loop   i   0 while i  lt  len myarr       a   myarr i       i   i   1      if somecond a           myarr append newObj

User · Answer

Why don t you just do it the idiomatic C way  This ought to be bullet-proof  but it won t be fast  I m pretty sure indexing into a list in Python walks the linked list  so this is a  Shlemiel the Painter  algorithm  But I tend not to worry about optimization until it becomes clear that a particular section of code is really a problem  First make it work  then worry about making it fast  if necessary   If you want to iterate over all the elements   i   0   while i  lt  len some list       more elements   do something with some list i       some list extend more elements      i    1     If you only want to iterate over the elements that were originally in the list   i   0   original len   len some list    while i  lt  original len      more elements   do something with some list i       some list extend more elements      i    1

[python] Python: Adding element to list while iterating

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