Reading and writing text files

What you will be able to do

After this lesson you can

• write text to a file and read it back,
• choose between the modes "r", "w" and "a",
• use with so that the file is always closed, and
• handle a file that does not exist.

Why files?

Everything that your program knows, in its variables, disappears when it ends. A list of marks that took an hour to type is gone. A file keeps data on the disk, so that it is there the next time, and so that another program can read it. Files are how programs remember.

On this site, your code runs inside your browser, with a small private disk of its own. Files that you make there work like real files, for the time that the page is open. On your own computer, they are real files in a folder.

Writing and reading

Predict first. How many characters will the text have? Then run it.

The three modes

At the end there are two more ways to write. f.writelines(...) writes a list of texts, but it does not add the new lines for you. And print("delta", file=f) uses the familiar print, which does add the new line. Both work. Choose the one that reads best.

Choose the mode for the job: "w" for a fresh report, "a" for a log that grows, and "r" (the default) only to read.

When the file is missing

Which is better? With a file, try and except is often the better style: between the moment you ask "does it exist?" and the moment you open it, something may change. Opening and handling the error is a single step.

Closing a file yourself

Common mistakes

• Forgetting \n with write. All the lines join into one long line.
• Opening with "w" a file that you wanted to add to. The old text is gone. Use "a".
• Reading a file that you opened for writing. An io.UnsupportedOperation error.
• Forgetting that a line ends with \n. Compare line.strip() and not line.
• Not using with. The file may stay open.
• Catching every error when you mean a missing file. Name FileNotFoundError.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Write and read lines. You write two small functions, and the checker makes a file and reads it.
2. Build. Open the problem Write or append? and the problem A safe read.
3. Stretch. Open the problem A file of numbers and add up the numbers of a file.

Remember these

• with open(name, mode) as f: opens a file and always closes it.
• "r" reads, "w" writes and empties, "a" adds to the end.
• write does not add a newline. A loop over a file gives lines that end with \n.
• A missing file is a FileNotFoundError. Handle it with try and except.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does open("a.txt", "w") do if a.txt already holds text?

2. What is \n, and does write add it for you?

3. Why is with better than open and close?