In this topic students learn to save data from a program into a text file, replacing the file with write mode or adding to it with append mode.

Learning objectives and goals

Be able to read, comprehend, trace, adapt and create Python code that:

• Opens a text file in write mode ("w") or append mode ("a") and closes it
• Writes text to a file, ending each line with "\n"
• Chooses between write and append mode depending on whether the old contents must be kept
• Uses a Boolean flag or a sentinel value to keep a loop writing until the user is finished

Slide deck

This chapter was written for Dodona from new material by the course author, after the rest of the course had been converted, so it has no deck of its own. The author’s Intermediate Python series has a File handling deck that covers both reading and writing files with different examples; the Reading files chapter shares it: on Google Slides. A PDF copy is kept on Dodona too.

Teacher notes

This chapter is the second half of file handling. The previous chapter covers reading a file; this one covers writing to one. The shape is the same in both (open, work with the file, close), and the new idea is the mode: "w" and "a" alongside the "r" students already know.

The chapter is built around one question students should ask before they open a file: when the program has finished, should the file still hold what it held before? If not, write mode; if so, append mode. The predict task is set up so that the answer to “did writing.txt change?” is not obvious: the program writes Bob to a file whose first line was already Bob, so a student who reads the output rather than the code thinks nothing happened, and misses that Sarah is gone.

Key points

• "w" creates the file if it does not exist and empties it if it does, at the moment it is opened.
• "a" creates the file if it does not exist and keeps it if it does; new text goes after the last character.
• write does not add a newline. Every line ends with "\n", written by hand, or the next piece of text carries on along the same line.
• write only takes text, so numbers go through str() first.
• close makes sure what was written has been saved. Close every file as soon as the program has finished with it.
• A Boolean variable holds True or False. Used as a flag, it keeps a loop going until something inside the loop sets it to False. Students met one in the 2D arrays chapter; this chapter uses it in every program from the investigate task on.

Errors and misconceptions

• Write mode where append was meant. The program runs without an error and the file holds only the last thing written. In the Modify tasks every test starts with people already in both files, so this shows up as missing lines rather than going unnoticed.
• Forgetting "\n". Names run together on one line. Students coming from print expect a new line for free. The investigate question on line 6 is the place to make this explicit.
• Opening the file inside the loop in write mode. In the results exercise the file is opened once, before the loop. A student who moves the open inside the loop with "w" wipes each student with the next, and ends up with only the last one.
• Deleting close. The investigate task asks what happens without it. The honest answer is “possibly nothing you can see”, because Python usually saves the data anyway when the variable is reused or the program ends. That is worth discussing: a bug that does not always show is still a bug.
• Only n ends the loop. In the starter program N, no and typing mistakes all count as “yes, another one”. The validation challenge exists to fix exactly that, and it is a good bridge to discussing why input should be checked at all.