A for loop reads a file from top to bottom, one line per repetition, and it never stops half way. Sometimes you want more control than that: read only the first line, or read the lines yourself and decide when to stop. That is what .readline() is for.

Reading the next line

.readline() reads one line from the file and gives it back as a string, newline included. Each time you use it, it moves on to the next line.

my_file = open("singers.txt", "r")

first = my_file.readline()
second = my_file.readline()
third = my_file.readline()

my_file.close()

print(first.strip())
print(second.strip())
print(third.strip())

With the singers.txt file from the previous page, this prints:

Kurt
Debbie
Freddie

The other eight names are never read. The program closed the file after three lines, and nothing forces it to read the rest.

That is exactly what you need when a file holds one value, such as a saved high score or a code: read the first line, close the file straight away, and use what you read.

Numbers in a file are text

A text file only holds text. A file containing the number 250 really contains the characters 2, 5 and 0, followed by a newline, so .readline() gives you the string "250\n". To do sums with it, turn it into a number with int(), the same as with anything you read with input().

score_file = open("high_score.txt", "r")
high_score = score_file.readline()
score_file.close()

high_score = int(high_score)
print("To beat the high score you need " + str(high_score + 1))

If high_score.txt holds 250, this prints To beat the high score you need 251.

int() does not mind the newline: it ignores spaces and newlines at the start and the end of the string, so int("250\n") is 250. It does mind anything else, so a line that holds a word, or a line that is completely empty, stops the program with a ValueError.

The end of the file

When there are no lines left, .readline() gives back an empty string, "": a string with nothing in it at all, not even a newline.

That is how a program can tell it has reached the end. A line in the middle of a file is never "". Even a blank line in the middle still has its newline, so it is "\n".

Reading a whole file with a while loop

Put .readline() and the empty string together and you can read a whole file with a condition controlled loop: keep reading while the line you just read is not empty.

my_file = open("singers.txt", "r")

line = my_file.readline()

while line != "":
    print(line.strip())
    line = my_file.readline()

my_file.close()

This prints all eleven names, the same as the for loop did. Look at where the two .readline() lines are:

• The first one is before the loop. It reads the first line, so that the condition has something to check the very first time.
• The second one is the last line inside the loop. It reads the next line, ready for the condition to check again.

That is the same shape as a counter in a while loop: set it up before the loop, check it in the condition, and change it at the end of the body. Leave out the second .readline() and line never changes, so the loop never ends.

If the file is empty, the very first .readline() gives back "", the condition is false straight away, and the body never runs at all.