无法使用readlines（）索引超出范围错误同时读取两行

Question

I have some text files contain 7 lines when I try to print line 3 and 4 using the following code, it prints line 3 then it gives me an index out of range error

for root, subFolders, files in os.walk(folder):
    for file in files:
        if file.endswith('Init.txt'):
            with open(os.path.join(root, file), 'r') as fRead:
                line_3 = fRead.readlines()[3]
                line_4 = fRead.readlines()[4]
                print line_3
                print line_4

However when I run either one of the following codes I get no errors

Code 1: Line 3 prints correctly

for root, subFolders, files in os.walk(folder):
    for file in files:
        if file.endswith('Init.txt'):
            with open(os.path.join(root, file), 'r') as fRead:
                line_3 = fRead.readlines()[3]
                print line_3

Code 2: Line 4 prints correctly

for root, subFolders, files in os.walk(folder):
    for file in files:
        if file.endswith('Init.txt'):
            with open(os.path.join(root, file), 'r') as fRead:
                line_4 = fRead.readlines()[4]
                print line_4

It seems as if I can't read both lines at the same time. This is so frustrating!

Answer 1

As stated in the documentation:

Help on built-in function readlines:

readlines(hint=-1, /) method of _io.TextIOWrapper instance Return a list of lines from the stream.

 hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint. 

Once you have consumed all lines, the next call to readlines will be empty.

Change your function to store the result in a temporary variable:

with open(os.path.join(root, file)) as fRead:
    lines = fRead.readlines()
    line_3 = lines[3]
    line_4 = lines[4]
    print line_3
    print line_4

Answer 2

The method readlines() reads all lines in a file until it hits the EOF (end of file). The "cursor" is then at the end of the file and a subsequent call to readlines() will not yield anything, because EOF is directly found.

Hence, after line_3 = fRead.readlines()[3] you have consumed the whole file but only stored the fourth (!) line of the file (if you start to count the lines at 1).

If you do

all_lines =  fRead.readlines()
line_3 = all_lines[3]
line_4 = all_lines[4]

you have read the file only once and saved every information you needed.