Iterating on a file doesn't work the second time

python file iteration
MYZ picture MYZ · Apr 21, 2012 · Viewed 111.7k times · Source

I have a problem with iterating on a file. Here's what I type on the interpreter and the result:

>>> f = open('baby1990.html', 'rU')
>>> 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!

>>> for line in f.readlines():
...  print(line)
... 
>>>

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?

Answer

Levon picture Levon · Apr 21, 2012

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.

Related questions

What is the perfect counterpart in Python for "while not EOF"

To read some text file, in C or Pascal, I always use the following snippets to read the data until EOF: while not eof do begin readline(a); do_something; end; Thus, I wonder how can I do this simple …

Read More
How do I check whether a file exists without exceptions?

How do I check if a file exists or not, without using the try statement?

Read More
How to read a file line-by-line into a list?

How do I read every line of a file in Python and store each line as an element in a list? I want to read the file line by line and append each line to the end of the list.

Read More