In this topic students learn to read data from a text file: open it, read it line by line or one line at a time, and close it.

Learning objectives and goals

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

• Opens a text file in read mode and closes it again
• Reads every line of a file with a for loop
• Reads one line at a time with .readline(), and reads a whole file with a while loop that stops at the empty string
• Removes the newline from a line with .strip(), and converts a line to a number with int()

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 Writing files chapter shares it: on Google Slides. A PDF copy is kept on Dodona too.

Teacher notes

This is the first chapter where a program’s data comes from somewhere other than the code or the keyboard. The next chapter writes files; this one only reads them, so every file is opened with "r".

The chapter shows two ways of reading a whole file, on purpose. for line in my_file: is the short form and the one students should reach for; it is a count controlled loop, one repetition per line. The while line != "": loop over .readline() is the long form, and is condition controlled. It is the same relationship as for and while in the two iteration chapters: the long form makes every step visible (read before the loop, check, read again at the end of the body), and forgetting the last step gives an infinite loop, exactly as forgetting to change a counter does. The last make task asks for the long form, so students have to write that loop out.

The reading pages cannot run code that opens a file, so file-reading examples are shown with the file’s contents and their output instead of a Run button. Every exercise’s sandbox (the To sandbox button under the editor) does have its files, and the Modify exercise’s sandbox has singers.txt, the file the predict and investigate tasks use, so that is where students can run those examples for real.

Key points

• open("singers.txt", "r") needs the full file name, .txt included, and its result has to be stored in a variable to be of any use.
• Every line read from a file keeps its newline, so line is "Kurt\n", not "Kurt". print(line) therefore leaves a blank line after every line.
• .strip() removes the newline (and any spaces) from both ends. It returns a new string rather than changing the old one, like .lower().
• .readline() returns "" only at the end of the file. A blank line in the middle of a file is "\n".
• A file holds text. int() turns a line into a number and ignores the newline, so int("999\n") is 999; int("") and int("\n") are errors.

Errors and misconceptions

• Comparing a line that still has its newline. The second predict example checks if line == "John": and prints nothing, although John is in the file. This is the single most useful thing in the chapter, so it is worth letting students predict it wrong and then explaining it on the board, with repr if it helps: print(repr(line)) shows the \n.
• Opening a file without storing it. The investigate code opens the file and throws it away, then loops over a variable that does not exist. Students who read the first error message (FileNotFoundError, from the missing .txt) and stop there miss the NameError behind it.
• Quotes around a variable. In the Modify task, open("file_name", "r") looks for a file called file_name. The file name is in a variable, so it goes into open without quotes.
• Comparing text with a number. In the vault task, a line read from the file is text, so code == 999 without int() is always false. Students who compare it as text instead, code == "999", fail on the newline; the test file holding 0999 catches students who strip the line and compare text.
• Forgetting the second readline. In the pass counter, a loop that never reads the next line never reaches the empty string, and never ends.
• > instead of >=. A score of exactly 50 is a pass, and several of the test files contain one.