Can iterators be reset in Python

Question

Can I reset an iterator   generator in Python   I am using DictReader and would like to reset it to the beginning of the file

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For small files  you may consider using more itertools seekable - a third-party tool that offers resetting iterables   Demo  import csv  import more itertools as mit   filename    data iris csv  with open filename   r   as f      reader   csv DictReader f      iterable   mit seekable reader                       1     print next iterable                                  2     print next iterable       print next iterable        print   nReset iterable n--------------       iterable seek 0                                      3     print next iterable       print next iterable       print next iterable     Output    Sepal width    3 5    Petal width    0 2    Petal length    1 4    Sepal length    5 1    Species    Iris-setosa     Sepal width    3    Petal width    0 2    Petal length    1 4    Sepal length    4 9    Species    Iris-setosa     Sepal width    3 2    Petal width    0 2    Petal length    1 3    Sepal length    4 7    Species    Iris-setosa    Reset iterable --------------   Sepal width    3 5    Petal width    0 2    Petal length    1 4    Sepal length    5 1    Species    Iris-setosa     Sepal width    3    Petal width    0 2    Petal length    1 4    Sepal length    4 9    Species    Iris-setosa     Sepal width    3 2    Petal width    0 2    Petal length    1 3    Sepal length    4 7    Species    Iris-setosa     Here a DictReader is wrapped in a seekable object  1  and advanced  2    The seek   method is used to reset rewind the iterator to the 0th position  3    Note  memory consumption grows with iteration  so be wary applying this tool to  large files  as indicated in the docs

User · Answer

I m arriving at this same issue - while I like the tee   solution  I don t know how big my files are going to be and the memory warnings about consuming one first before the other are putting me off adopting that method    Instead  I m creating a pair of iterators using iter   statements  and using the first for my initial run-through  before switching to the second one for the final run    So  in the case of a dict-reader  if the reader is defined using   d   csv DictReader f  delimiter        I can create a pair of iterators from this  specification  - using   d1  d2   iter d   iter d    I can then run my 1st-pass code against d1  safe in the knowledge that the second iterator d2 has been defined from the same root specification    I ve not tested this exhaustively  but it appears to work with dummy data

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Problem  I ve had the same issue before  After analyzing my code  I realized that attempting to reset the iterator inside of loops slightly increases the time complexity and it also makes the code a bit ugly   Solution  Open the file and save the rows to a variable in memory     initialize list of rows rows         open the file and temporarily name it as  my file  with open  myfile csv    rb   as my file         set up the reader using the opened file     myfilereader   csv DictReader my file         loop through each row of the reader     for row in myfilereader            add the row to the list of rows         rows append row    Now you can loop through rows anywhere in your scope without dealing with an iterator

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Only if the underlying type provides a mechanism for doing so  e g  fp seek 0

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list generator    returns all remaining values for a generator and effectively resets it if it is not looped

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One possible option is to use itertools cycle    which will allow you to iterate indefinitely without any trick like  seek 0    iterDic   itertools cycle csv DictReader open  file csv

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Yes  if you use numpy nditer to build your iterator      gt  gt  gt  lst    1 2 3 4 5   gt  gt  gt  itr   numpy nditer  lst    gt  gt  gt  itr next   1  gt  gt  gt  itr next   2  gt  gt  gt  itr finished False  gt  gt  gt  itr reset    gt  gt  gt  itr next   1

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Return a newly created iterator at the last iteration during the  iter    call  class ResetIter     def   init   self  num       self num   num     self i   -1    def   iter   self       if self i    self num-1    here  return the new object       return self   class   self num       return self    def   next   self       if self i    self num-1        raise StopIteration      if self i  lt   self num-1        self i    1       return self i   reset iter   ResetRange 10  for i in reset iter    print i  end      print    for i in reset iter    print i  end      print    for i in reset iter    print i  end        Output   0 1 2 3 4 5 6 7 8 9  0 1 2 3 4 5 6 7 8 9  0 1 2 3 4 5 6 7 8 9

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This is perhaps orthogonal to the original question  but one could wrap the iterator in a function that returns the iterator   def get iter        return iterator   To reset  the iterator just call the function again   This is of course trivial if the function when the said function takes no arguments   In the case that the function requires some arguments  use functools partial to create a closure that can be passed instead of the original iterator   def get iter arg1  arg2      return iterator from functools import partial iter clos   partial get iter  a1  a2    This seems to avoid the caching that tee  n copies  or list  1 copy  would need to do

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For DictReader   f   open filename   rb   d   csv DictReader f  delimiter       f seek 0  d   init   f  delimiter        For DictWriter   f   open filename   rb    d   csv DictWriter f  fieldnames fields  delimiter       f seek 0  f truncate 0  d   init   f  fieldnames fields  delimiter      d writeheader   f flush

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While there is no iterator reset  the  itertools  module from python 2 6  and later  has some utilities that can help there  One of then is the  tee  which can make multiple copies of an iterator  and cache the results of the one running ahead  so that these results are used on the copies  I will seve your purposes    gt  gt  gt  def printiter n         for i in xrange n           print  iterating value  d    i         yield i   gt  gt  gt  from itertools import tee  gt  gt  gt  a  b   tee printiter 5   2   gt  gt  gt  list a  iterating value 0 iterating value 1 iterating value 2 iterating value 3 iterating value 4  0  1  2  3  4   gt  gt  gt  list b   0  1  2  3  4

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There s a bug in using  seek 0  as advocated by Alex Martelli and Wilduck above  namely that the next call to  next   will give you a dictionary of your header row in the form of  key1 key1  key2 key2         The work around is to follow file seek 0  with a call to reader next   to get rid of the header row   So your code would look something like this   f in   open  myfile csv   r   reader   csv DictReader f in   for record in reader      if some condition            reset reader to first row of data on 2nd line of file         f in seek 0          reader next           continue     do something record

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No  Python s iterator protocol is very simple  and only provides one single method   next   or   next       and no method to reset an iterator in general   The common pattern is to instead create a new iterator using the same procedure again   If you want to  save off  an iterator so that you can go back to its beginning  you may also fork the iterator by using itertools tee

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If you have a csv file named  blah csv  That looks like  a b c d 1 2 3 4 2 3 4 5 3 4 5 6   you know that you can open the file for reading  and create a DictReader with  blah   open  blah csv    r   reader  csv DictReader blah    Then  you will be able to get the next line with reader next    which should output    a  1  b  2  c  3  d  4    using it again will produce    a  2  b  3  c  4  d  5    However  at this point if you use blah seek 0   the next time you call reader next   you will get    a  1  b  2  c  3  d  4    again   This seems to be the functionality you re looking for  I m sure there are some tricks associated with this approach that I m not aware of however   Brian suggested simply creating another DictReader  This won t work if you re first reader is half way through reading the file  as your new reader will have unexpected keys and values from wherever you are in the file

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I see many answers suggesting itertools tee  but that s ignoring one crucial warning in the docs for it      This itertool may require significant   auxiliary storage  depending on how   much temporary data needs to be   stored   In general  if one iterator   uses most or all of the data before   another iterator starts  it is faster   to use list   instead of tee      Basically  tee is designed for those situation where two  or more  clones of one iterator  while  getting out of sync  with each other  don t do so by much -- rather  they say in the same  vicinity   a few items behind or ahead of each other    Not suitable for the OP s problem of  redo from the start    L   list DictReader       on the other hand is perfectly suitable  as long as the list of dicts can fit comfortably in memory   A new  iterator from the start   very lightweight and low-overhead  can be made at any time with iter L   and used in part or in whole without affecting new or existing ones  other access patterns are also easily available   As several answers rightly remarked  in the specific case of csv you can also  seek 0  the underlying file object  a rather special case    I m not sure that s documented and guaranteed  though it does currently work   it would probably be worth considering only for truly huge csv files  in which the list I recommmend as the general approach would have too large a memory footprint

[python] Can iterators be reset in Python?

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