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?
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3 Answers
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.)
3 Comments
steveha
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.
Amber
@steveha - 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.steveha
My apologies. I'm not sure how I missed the
list() wrapper but I did. Sorry about that.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!']
