Question

#1
input_file = 'my-textfile.txt'
current_file = open(input_file)
print current_file.readline()
print current_file.readline()

#2
input_file = 'my-textfile.txt'
print open(input_file).readline()
print open(input_file).readline()

Why is it that #1 works fine and displays the first and second line, but #2 prints 2 copies of the first line and doesn’t print the same as #1 ?

Answer 1

When you call open you are opening the file anew and starting from the first line. Every time you call readline on an already open file it moves its internal “pointer” to the start of the next line. However, if you re-open the file the “pointer” is also re-initialized – and when you call readline it reads the first line again.

Imagine that open returned a file object that looked like this:

class File(object):
    """Instances of this class are returned by `open` (pretend)"""

    def __init__(self, filesystem_handle):
        """Called when the file object is initialized by `open`"""

        print "Starting up a new file instance for {file} pointing at position 0.".format(...)

        self.position = 0
        self.handle = filesystem_handle


    def readline(self):
        """Read a line. Terribly naive. Do not use at home"

        i = self.position
        c = None
        line = ""
        while c != "\n":
            c = self.handle.read_a_byte()
            line += c

        print "Read line from {p} to {end} ({i} + {p})".format(...)

        self.position += i
        return line

When you ran your first example you would get something like the following output:

Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)
Read line from 80 to 160 (80 + 80)

While the output of your second example would look something like this:

Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)
Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)