Iterating on a file doesn t work the second time

Question

I have a problem with iterating on a file  Here s what I type on the interpreter and the result    gt  gt  gt  f   open  baby1990 html    rU    gt  gt  gt  for line in f readlines         print line             all the lines from the file appear here       When I try to iterate on the same open file again I get nothing    gt  gt  gt  for line in f readlines         print line        gt  gt  gt    There is no output at all  To solve this I have to close   the file then open it again for reading  Is that normal behavior

User · Accepted Answer

Yes  that is normal behavior  You basically read to the end of the file the first time  you can sort of picture it as reading a tape   so you can t read any more from it unless you reset it  by either using f seek 0  to reposition to the start of the file  or to close it and then open it again which will start from the beginning of the file   If you prefer you can use the with syntax instead which will automatically close the file for you   e g    with open  baby1990 html    rU   as f    for line in f       print line   once this block is finished executing  the file is automatically closed for you  so you could execute this block repeatedly without explicitly closing the file yourself and read the file this way over again

User · Answer

As the file object reads the file  it uses a pointer to keep track of where it is  If you read part of the file  then go back to it later it will pick up where you left off  If you read the whole file  and go back to the same file object  it will be like reading an empty file because the pointer is at the end of the file and there is nothing left to read  You can use file tell   to see where in the file the pointer is and file seek to set the pointer  For example    gt  gt  gt  file   open  myfile txt    gt  gt  gt  file tell   0  gt  gt  gt  file readline    one n   gt  gt  gt  file tell   4L  gt  gt  gt  file readline    2 n   gt  gt  gt  file tell   6L  gt  gt  gt  file seek 4   gt  gt  gt  file readline    2 n    Also  you should know that file readlines   reads the whole file and stores it as a list  That s useful to know because you can replace   for line in file readlines         do stuff file seek 0  for line in file readlines         do more stuff   with   lines   file readlines   for each line in lines       do stuff for each line in lines       do more stuff   You can also iterate over a file  one line at a time  without holding the whole file in memory  this can be very useful for very large files  by doing   for line in file       do stuff

User · Answer

Of course  That is normal and sane behaviour  Instead of closing and re-opening  you could rewind the file

User · Answer

The file object is a buffer  When you read from the buffer  that portion that you read is consumed  the read position is shifted forward   When you read through the entire file  the read position is at the end of the file  EOF   so it returns nothing because there is nothing left to read   If you have to reset the read position on a file object for some reason  you can do   f seek 0

[python] Iterating on a file doesn't work the second time

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