Take the content of a list and append it to another list

Question

I am trying to understand if it makes sense to take the content of a list and append it to another list   I have the first list created through a loop function  that will get specific lines out of a file and will save them in a list   Then a second list is used to save these lines  and start a new cycle over another file   My idea was to get the list once that the for cycle is done  dump it into the second list  then start a new cycle  dump the content of the first list again into the second but appending it  so the second list will be the sum of all the smaller list files created in my loop  The list has to be appended only if certain conditions met    It looks like something similar to this     This is done for each log in my directory  i have a loop running for logs in mydir       for line in mylog              if the conditions are met         list1 append line       for item in list1          if  string  in item   if somewhere in the list1 i have a match for a string             list2 append list1    append every line in list1 to list2             del list1       delete the content of the list1             break         else              del list1       delete the list content and start all over   Does this makes sense or should I go for a different route   I need something efficient that would not take up too many cycles  since the list of logs is long and each text file is pretty big  so I thought that the lists would fit the purpose

User · Answer

Using the map   and reduce   built-in functions  def file to list file         stuff to parse file to a list      return list  files       list of files      L   map file to list  files   flat L   reduce lambda x y x y  L    Minimal  for looping  and elegant coding pattern

User · Answer

To recap on the previous answers  If you have a list with  0 1 2  and another one with  3 4 5  and you want to merge them  so it becomes  0 1 2 3 4 5   you can either use chaining or extending and should know the differences to use it wisely for your needs  Extending a list Using the list classes extend method  you can do a copy of the elements from one list onto another  However this will cause extra memory usage  which should be fine in most cases  but might cause problems if you want to be memory efficient  a    0 1 2  b    3 4 5  a extend b   gt  gt  0 1 2 3 4 5    Chaining a list Contrary you can use itertools chain to wire many lists  which will return a so called iterator that can be used to iterate over the lists  This is more memory efficient as it is not copying elements over but just pointing to the next list  import itertools a    0 1 2  b    3 4 5  c   itertools chain a  b     Make an iterator that returns elements from the first iterable until it is exhausted  then proceeds to the next iterable  until all of the iterables are exhausted  Used for treating consecutive sequences as a single sequence

User · Answer

That seems fairly reasonable for what you re trying to do   A slightly shorter version which leans on Python to do more of the heavy lifting might be   for logs in mydir       for line in mylog              if the conditions are met         list1 append line       if any True for line in list1 if  string  in line           list2 extend list1      del list1             The  True for line in list1 if  string  in line  iterates over list and emits True whenever a match is found  any   uses short-circuit evaluation to return True as soon as the first True element is found  list2 extend   appends the contents of list1 to the end

User · Answer

Take a look at itertools chain for a fast way to treat many small lists as a single big list  or at least as a single big iterable  without copying the smaller lists    gt  gt  gt  import itertools  gt  gt  gt  p     a    b    c    gt  gt  gt  q     d    e    f    gt  gt  gt  r     g    h    i    gt  gt  gt  for x in itertools chain p  q  r           print x upper

User · Answer

You probably want  list2 extend list1    instead of  list2 append list1    Here s the difference    gt  gt  gt  a   range 5   gt  gt  gt  b   range 3   gt  gt  gt  c   range 2   gt  gt  gt  b append a   gt  gt  gt  b  0  1  2   0  1  2  3  4    gt  gt  gt  c extend a   gt  gt  gt  c  0  1  0  1  2  3  4    Since list extend   accepts an arbitrary iterable  you can also replace  for line in mylog      list1 append line    by  list1 extend mylog

User · Answer

You can also combine two lists  say a b  using the     operator  For example   a    1 2 3 4  b    4 5 6 7  c   a   b  Output   gt  gt  gt  c  1  2  3  4  4  5  6  7

User · Answer

You can simply concatnate two lists  e g  list1    0  1  list2    2  3  list3   list1   list2  print list3   gt  gt   0  1  2  3

User · Answer

If we have list like below    list     2 2 3 4    two ways to copy it into another list   1   x    list     x     x append list  same  print  length is     format len x    for i in x      print i     length is 1  2  2  3  4     2   x    l for l in list  print  length is     format len x    for i in x      print i     length is 4 2 2 3 4

[python] Take the content of a list and append it to another list

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