Common File Handling Mistakes

You've covered file handling from every angle. Here are the mistakes that show up most often — and how to fix them.

1. Not closing the file — or forgetting with open()

f = open("soldiers.txt", "r")
content = f.read()
# forgot f.close()

An unclosed file stays locked. Other programs can't access it. On some systems, data doesn't get written until the file is closed.

with open("soldiers.txt", "r", encoding="utf-8") as f:
    content = f.read()
# closed automatically — always use this pattern

2. "w" mode — silent overwrite

with open("roster.txt", "w", encoding="utf-8") as f:
    f.write("Raven\n")

If roster.txt already exists with a hundred entries — they're gone. No warning. No confirmation. No undo.

Use "w" only when you intend to replace the file completely. Use "a" when you want to keep existing data.

3. FileNotFoundError — wrong path or missing file

with open("data/soldiers.txt", "r") as f:
    content = f.read()

Output → FileNotFoundError: [Errno 2] No such file or directory

Python looks for files relative to where the script is run — not where the script is saved. Check the path, check the filename, check the extension.

try:
    with open("soldiers.txt", "r", encoding="utf-8") as f:
        content = f.read()
except FileNotFoundError:
    print("File not found. Check the path.")

4. Missing \n — everything on one line

soldiers = ["Raven", "Wolf", "Ghost"]
with open("roster.txt", "w", encoding="utf-8") as f:
    for soldier in soldiers:
        f.write(soldier)

Output in file → RavenWolfGhost

write() doesn't add newlines. You add them yourself:

f.write(soldier + "\n")

5. CSV values are strings — forgetting to convert

import csv

with open("soldiers.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["score"] + 10)

Output → TypeError: can only concatenate str (not "int") to str

Every value read from a CSV file is a string. Convert explicitly:

print(int(row["score"]) + 10)

6. Missing newline="" when writing CSV

with open("output.csv", "w", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

On Windows, this produces blank lines between every row in the CSV.

with open("output.csv", "w", encoding="utf-8", newline="") as f:
    writer = csv.writer(f)
    writer.writerows(rows)

Always pass newline="" when writing CSV files.

7. Reading a file twice without reopening

with open("soldiers.txt", "r", encoding="utf-8") as f:
    first  = f.read()
    second = f.read()

print(len(first))   # full content
print(len(second))  # 0 — empty

After read(), the file pointer is at the end. A second read returns nothing.

Open the file again if you need to read it twice — or store the content in a variable and reuse it.

What you should take away

• Always use with open() — it closes the file automatically
• "w" overwrites silently — use "a" to preserve existing data
• Wrap reads in try/except FileNotFoundError when the file might not exist
• Add \n manually — write() doesn't do it for you
• CSV values are strings — convert with int(), float(), bool()
• Pass newline="" when writing CSV files
• A file can only be read once per open — store content in a variable to reuse it