Files Practice

These questions follow Files and Persistence. Work in a folder you can afford to make a mess in — several of these questions create files, and one of them is about destroying one.

Reading and writing

1. What do the three modes "r", "a", and "w" do? Which one can lose data, and how?
2. What is in notes.txt after this runs, exactly?

   with open("notes.txt", "w") as file:
       file.write("first")
       file.write("second")

3. Rewrite question 2 so the file contains two separate lines, then read the file back and print each line without a blank line between them.
4. What does this print the very first time it is run in a new folder, and why is that not an unusual case?

   with open("weights.txt", "r") as file:
       print(file.read())

Doing something useful

5. Handle the situation in question 4 so the program says something a person can act on and carries on with an empty list.
6. A file holds one number per line. Read it, total the numbers, and print how many entries there were.
7. A line is saved as Fifteen Dogs|14. Read the file and print each reminder as Fifteen Dogs — back in 14 days.
8. Challenge. Why is FILE_NAME = "reminders.txt" at the top of a program better than writing "reminders.txt" in four places? Give two reasons, one of them about a shared drive.

Answers

Answer 1

"r" reads an existing file. "a" appends to the end, creating the file if it does not exist. "w" writes — and empties the file first, which is how data gets lost: one letter’s difference between adding a record and deleting every record, with no warning and no undo.

Answer 2

One line: firstsecond. write adds exactly what you give it and nothing else, so without \n the two calls run together. Reading it back gives 'firstsecond'.

Answer 3

with open("notes.txt", "w") as file:
    file.write("first\n")
    file.write("second\n")
 
with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())

first
second

Without .strip() each printed line keeps its own newline and print adds another, so the output comes out double-spaced.

Answer 4

It stops with FileNotFoundError: [Errno 2] No such file or directory: 'weights.txt'. That is guaranteed to happen once for every person who ever uses your program, because the first run always predates the file. It is a certainty to design for, not an edge case.

Answer 5

try:
    with open("weights.txt", "r") as file:
        lines = file.readlines()
except FileNotFoundError:
    print("No entries yet — starting a new list.")
    lines = []
 
print(len(lines))

On a first run that prints the message and then 0. Setting lines = [] in the except block matters: the rest of the program can then carry on without a special case, because “no file” and “an empty file” now look the same to it.

Answer 6

total = 0.0
count = 0
 
with open("weights.txt", "r") as file:
    for line in file:
        total = total + float(line.strip())
        count = count + 1
 
print(f"{count} entries, {total:.1f} kg")

With 6.5, 3.0, and 11.2 in the file, that prints 3 entries, 20.7 kg. float() rather than int(), because weights have decimals — and .strip() first, because the newline is part of what was read.

Answer 7

with open("reminders.txt", "r") as file:
    for line in file:
        title, days = line.strip().split("|")
        print(f"{title} — back in {days} days")

.split("|") returns two strings and the two names on the left take one each. days stays as text here because it is only being printed; the moment you want to compare it to a number, int(days) is needed.

Answer 8

First, one place to change: when the file moves — into a data folder, or onto a shared drive where the whole department can reach it — you edit one line rather than hunting four. Second, four copies of a string is four chances at a typo, and a mistyped name in "a" mode does not fail loudly: it quietly creates a second file and writes there, so the data is not lost but it is not where anyone is looking either.