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

Common mistakes

• Use with open(...) as f: so the file closes itself.
• "w" erases the file first; "a" adds to the end.

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.