Text files

Why files?

• Variables vanish when a program ends. A file keeps data on disk.
• Files let you save results and read them back later.
• They also hold more data than you would type by hand.

Open, use, close

• open(name, mode) opens a file. The mode says what you will do.
• "r" reads, "w" writes a fresh file (replacing any old text), "a" adds to the end.
• Every file you open must be closed when you finish.

The with block

• with open(...) as f: opens the file and closes it for you.
• All the work goes in the indented block.
• This is the safest way — the file is never left open.

with open("greeting.txt", "w") as f:
    f.write("Hi there\n")

with open("greeting.txt", "r") as f:
    print(f.read())

Reading the lines

• f.read() returns the whole file as one string.
• for line in f: reads it one line at a time.
• Each line keeps its newline, so use line.strip() to remove it.

with open("fruit.txt", "w") as f:
    f.write("apple\npear\n")

with open("fruit.txt", "r") as f:
    for line in f:
        print(line.strip())

In Cambridge pseudocode

• The exam uses OPENFILE, WRITEFILE, READFILE, and CLOSEFILE.

OPENFILE "notes.txt" FOR WRITE
WRITEFILE "notes.txt", "Hello"
CLOSEFILE "notes.txt"

OPENFILE "notes.txt" FOR READ
READFILE "notes.txt", line
CLOSEFILE "notes.txt"
OUTPUT line

Now you try

• This playground keeps files between runs, so write the file first, then read it in the same task.
• Press Check answer to test your code.