Using "readlines()" twice in a row [duplicate]

Question

I'm trying to do something like this:

Lines = file.readlines()
# do something
Lines = file.readlines()  

but the second time Lines is empty. Is that normal?

Via

Answer 1

You need to reset the file pointer using

file.seek(0)

before using

file.readlines()

again.

Lotic

Or else store a copy of the lines. – Clean

Answer 2

Yes, because .readlines() advances the file pointer to the end of the file.

Why not just store a copy of the lines in a variable?

file_lines = file.readlines()
Lines = list(file_lines)
# do something that modifies Lines
Lines = list(file_lines)

It'd be far more efficient than hitting the disk twice. (Note that the list() call is necessary to create a copy of the list so that modifications to Lines won't affect file_lines.)

Schizo

Your technique here sets Lines to the same Python object as file_lines. Anything that mutates one would mutate the other for they are the same object. To really be safe, you need to do lines = file_lines[:] or in some other way make an actual copy. By the way, the PEP 8 standard suggests using lower-case for variable names, and reserving mixed-case for classes, so I suggest changing Lines to just lines. – Helpless

@Helpless - I already added list() calls. As for the variable name, I was simply re-using the OP's name to make it easier for them to read. I agree with you that lower-case is generally better. – Schizo

My apologies. I'm not sure how I missed the list() wrapper but I did. Sorry about that. – Helpless

Answer 3

In order to not have to reset every time by using seek method again and again, use the readlines method, but you must store it in variable like this example below:

%%writefile test.txt
this is a test file!
#open it
op_file = open('test.txt')
#read the file
re_file = op_file.readlines()
re_file
#output
['this is a test file!']
# the output still the same
re_file
['this is a test file!']

Wilford