Logging B: Saving Logs to a File and Recording Important Actions

What will we learn?#

• How to write logs to a file, and not only to the screen.
• Writing modes: add to the end of the file, or start over.
• How to record an error with all the details (Traceback).
• What to record: which actions are important and at which level.

Why do we need this?#

Remember the topic on the computer, the operating system and the network? What is on the screen and in memory disappears when the program ends. A file is kept on Storage. A log on the screen helps only whoever is sitting in front of it now. A log in a file can be opened tomorrow, searched, and used to understand what happened during the night.

On real servers, logs are the main tool for investigating faults. “Get me the logs from 3 a.m.” is a routine sentence in a development team.

Explanation from the basics#

Logging to a file#

You add the filename parameter to basicConfig. From that moment, the records are written to the file instead of to the screen. The file is created automatically if it does not exist, in the current folder (the Current Working Directory, like any relative path).

Writing modes (filemode)#

logging.exception — an error with all the details#

logging.error("Payment failed") records only the message. Inside an except block, you can use logging.exception("..."): it records at the ERROR level and also attaches the full Traceback, so the file shows exactly on which line and in which function the error happened. As if you saw the crash yourself, without the program crashing.

What is worth recording?#

Syntax#

import logging

logging.basicConfig(
    filename="app.log",          # write to this file (relative to the working directory)
    filemode="a",                # "a" = append (default), "w" = overwrite each run
    level=logging.INFO,
    format="%(asctime)s %(levelname)-8s %(message)s",
    encoding="utf-8",            # safe for any characters in messages
)

try:
    risky()
except SomeError:
    logging.exception("What we were trying to do")   # ERROR + full traceback

First code example#

week-2/file_logger.py:

import logging

logging.basicConfig(
    filename="app.log",
    level=logging.INFO,
    format="%(asctime)s %(levelname)-8s %(message)s",
    encoding="utf-8",
)

logging.info("Program started")
logging.warning("Low disk space")
logging.info("Program finished")

print("Done. Check app.log")

On the screen:

Done. Check app.log

In the file app.log (after two runs):

2026-10-06 10:02:41,203 INFO     Program started
2026-10-06 10:02:41,203 WARNING  Low disk space
2026-10-06 10:02:41,204 INFO     Program finished
2026-10-06 10:03:15,881 INFO     Program started
2026-10-06 10:03:15,881 WARNING  Low disk space
2026-10-06 10:03:15,882 INFO     Program finished

Breaking down the code line by line#

To read the file: open it in VS Code, or from the Terminal with cat app.log. In PowerShell you can also follow it in real time: Get-Content app.log -Wait. On Mac/Linux: tail -f app.log.

Another example#

A full log for a program that processes payments, with logging.exception. week-2/payments.py:

import logging

logging.basicConfig(
    filename="payments.log",
    filemode="w",
    level=logging.INFO,
    format="%(asctime)s %(levelname)-8s %(message)s",
    encoding="utf-8",
)

def charge(customer, amount_text):
    amount = float(amount_text)
    if amount <= 0:
        raise ValueError("amount must be positive, got " + str(amount))
    logging.info("Charged %s: %.2f NIS", customer, amount)
    return amount

payments = [("dana", "120"), ("avi", "abc"), ("noa", "-5"), ("yossi", "80.5")]

logging.info("Batch started: %s payments", len(payments))
total = 0
failed = 0

for customer, amount_text in payments:
    try:
        total = total + charge(customer, amount_text)
    except ValueError:
        failed = failed + 1
        logging.exception("Payment failed for %s (input: %r)", customer, amount_text)

logging.info("Batch finished: total %.2f NIS, %s failed", total, failed)
print("Batch done. Total:", total, "| Failed:", failed)

On the screen:

Batch done. Total: 200.5 | Failed: 2

In payments.log:

2026-10-06 10:20:01,010 INFO     Batch started: 4 payments
2026-10-06 10:20:01,010 INFO     Charged dana: 120.00 NIS
2026-10-06 10:20:01,011 ERROR    Payment failed for avi (input: 'abc')
Traceback (most recent call last):
  File "payments.py", line 28, in <module>
    total = total + charge(customer, amount_text)
  File "payments.py", line 13, in charge
    amount = float(amount_text)
ValueError: could not convert string to float: 'abc'
2026-10-06 10:20:01,012 ERROR    Payment failed for noa (input: '-5')
Traceback (most recent call last):
  File "payments.py", line 28, in <module>
    total = total + charge(customer, amount_text)
  File "payments.py", line 15, in charge
    raise ValueError("amount must be positive, got " + str(amount))
ValueError: amount must be positive, got -5.0
2026-10-06 10:20:01,012 INFO     Charged yossi: 80.50 NIS
2026-10-06 10:20:01,012 INFO     Batch finished: total 200.50 NIS, 2 failed

Breakdown:

• filemode="w" — here we chose to overwrite on every run, so that in practice there is only the last log.
• %.2f — a decimal number with 2 digits after the point (120.00). Suitable for money amounts.
• %r — shows the value with quotes ('abc'). That way you clearly see what was received, including extra spaces or an empty string.
• logging.exception(...) — an ERROR record, with the full Traceback below it. Both errors are ValueError, but for different reasons. The file shows exactly which line threw each one: float() (line 13) or our raise (line 15).
• The program did not crash: it continues to the next payment, and all the information for investigating is saved.
• print summarizes for the user in one line. The log tells the full story to the developer.

Common mistake#

Mistake 1: “the log is not written / I can’t find the file”. The file is created in the Current Working Directory. If you ran from another folder, the file is there. (Remember the topic on the computer and the operating system!)

Mistake 2: logging.error inside except when you want to investigate.

except ValueError:
    logging.error("Payment failed")

It says it failed, but not why and not where. Inside except, logging.exception gives the full picture with no effort. (If the error is expected and ordinary, like wrong input from a user, and you do not need a Traceback, you can use logging.warning with the error’s message: logging.warning("bad input: %s", error).)

Mistake 3: filemode="w" on a real server. Every restart of the server deletes the history. Exactly after a crash, when you need the log most, it is deleted. On servers: only "a".

Mistake 4: thinking that logging.exception handles the error. It only records it. The decision what to do (continue, try again, pass it on with raise) is still yours.

Practice#

The exercises use only what was taught in this lesson and the lessons before it. Try on your own first, and only then open the solution.

Exercise 1: A log to a file#

Set up a log that is saved to the file shop.log, at level INFO, with time, level and message, and that is not deleted between runs. Record in it:

• INFO when order number 17 is created;
• WARNING when only 2 units are left in stock.

import logging

logging.basicConfig(
    filename="shop.log",
    filemode="a",
    level=logging.INFO,
    format="%(asctime)s %(levelname)-8s %(message)s",
    encoding="utf-8",
)

logging.info("order 17 created")
logging.warning("low stock: 2 left")

Exercise 2: append versus write#

1. What is the difference between filemode="a" and filemode="w"?
2. A server crashed during the night and was restarted automatically. The log was set up with "w". What is the problem?

Exercise 3: logging.exception#

Write a loop that charges a list of payments. Each amount is text; an amount that is not a number is recorded in the log with all the error’s details, and the loop continues. At the end the total amount is printed.

payments = ["120", "abc", "80"]

Expected output:

Total charged: 200

import logging

logging.basicConfig(filename="payments.log", filemode="a", level=logging.INFO, encoding="utf-8")

payments = ["120", "abc", "80"]
total = 0
for amount_text in payments:
    try:
        total = total + int(amount_text)
        logging.info("charged %s", amount_text)
    except ValueError:
        logging.exception("invalid amount: %s", amount_text)

print("Total charged:", total)

Total charged: 200

Exercise 4: What to record and what not#

For each item — record it in the log or not? Explain.

1. The number of the order that failed.
2. The password the user typed, when the login failed.
3. The username that tried to log in.
4. The full credit card number.
5. How long a slow action took.

Exercise 5: A login journal#

You have a list of login attempts. Each attempt is a username and a password (two parallel lists). The correct password for all the users in the exercise is "1234". Record in the log security.log:

• INFO for every successful login;
• WARNING for every wrong password (without recording the password!).

At the end, print how many logins succeeded and how many failed.

users = ["dana", "avi", "dana", "noa"]
passwords = ["1234", "0000", "1111", "1234"]

Expected output:

Successful: 2, failed: 2

import logging

CORRECT_PASSWORD = "1234"

logging.basicConfig(
    filename="security.log",
    filemode="a",
    level=logging.INFO,
    format="%(asctime)s %(levelname)-8s %(message)s",
    encoding="utf-8",
)

users = ["dana", "avi", "dana", "noa"]
passwords = ["1234", "0000", "1111", "1234"]

successful = 0
failed = 0
for i in range(len(users)):
    if passwords[i] == CORRECT_PASSWORD:
        successful = successful + 1
        logging.info("login ok: %s", users[i])
    else:
        failed = failed + 1
        logging.warning("wrong password for %s", users[i])

logging.info("summary: %s successful, %s failed", successful, failed)
print("Successful: " + str(successful) + ", failed: " + str(failed))

Successful: 2, failed: 2

Summary#

• basicConfig(filename="app.log") writes logs to a file on Storage, and they are kept after the program ends.
• filemode="a" (the default) appends. "w" overwrites. On servers, always append.
• logging.exception("...") inside except records an ERROR with the full Traceback, without the program crashing.
• Record start/end, changes to data, security and errors. Do not record passwords and sensitive information.
• The file is created in the Current Working Directory. tail -f / Get-Content -Wait follow it in real time.

Official documentation#

• Logging HOWTO — Logging to a file
• logging.basicConfig()
• logging.exception()