Python Log File Analyzer Example

A log file analyzer is a good beginner project because it combines several useful Python skills in one script.

In this example, you will read a log file, look for important lines such as ERROR, WARNING, and INFO, count them, and print a simple summary.

This page focuses on building one practical script. If you want a separate lesson on file handling, see how to read a file in Python or how to read a file line by line in Python.

Quick working version #

If you want a simple version first, start with this:

from collections import Counter

log_file = "app.log"
counts = Counter()

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()
        if "ERROR" in line:
            counts["ERROR"] += 1
        elif "WARNING" in line:
            counts["WARNING"] += 1
        elif "INFO" in line:
            counts["INFO"] += 1

print("Log summary")
for level, total in counts.items():
    print(f"{level}: {total}")

This version:

opens the file
reads it one line at a time
checks each line for common log levels
counts matches
prints the totals

Next, let’s build it step by step so the code is easier to understand.

What this example builds #

This script will:

Read a text log file line by line
Look for common log levels such as ERROR, WARNING, and INFO
Count how many times each level appears
Print a simple report at the end

What a log file is #

A log file is a text file created by a program.

It usually stores messages about what the program did, such as:

normal activity
warnings
errors

For example, a program might write messages like:

INFO: User logged in
WARNING: Disk space is low
ERROR: Could not connect to database

Skills used in this project #

This example uses several basic Python skills:

Opening a file with open()
Looping through lines with a for loop
Cleaning text with strip()
Checking text with the in operator
Counting values with a dictionary or Counter
Printing readable output

If dictionaries are new to you, see Python dictionaries explained. If loops are new, see Python for loops explained.

Sample log file input #

Create a file named app.log in the same folder as your Python script.

Put this sample text inside it:

INFO: Application started
INFO: User login successful
WARNING: Disk space is getting low
ERROR: Could not connect to server
INFO: Retrying connection
ERROR: Connection failed again

This file contains:

several INFO lines
one WARNING line
two ERROR lines

Build the analyzer step by step #

1. Open the file safely #

Use with open(...) to open the file.

log_file = "app.log"

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        print(line)

Why this works:

open() opens the file for reading
"r" means read mode
encoding="utf-8" is a safe choice for text files
with closes the file automatically when finished

2. Loop over each line #

A file can be looped through directly.

log_file = "app.log"

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        print(repr(line))

Using repr(line) helps you see hidden characters like \n.

3. Remove extra newline characters with `strip()` #

Each line usually ends with a newline character. You can remove it with strip().

log_file = "app.log"

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()
        print(line)

This makes the line cleaner before you check its content.

4. Check whether a line contains `ERROR`, `WARNING`, or `INFO` #

Now check each cleaned line with the in operator.

log_file = "app.log"

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()

        if "ERROR" in line:
            print("Found an error line")
        elif "WARNING" in line:
            print("Found a warning line")
        elif "INFO" in line:
            print("Found an info line")

Why use elif here?

Because one line should only be counted once. Using elif stops Python after the first matching condition.

5. Increase the correct count #

Now let’s count each type of line.

This version uses Counter, which is a convenient counting tool.

from collections import Counter

log_file = "app.log"
counts = Counter()

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()

        if "ERROR" in line:
            counts["ERROR"] += 1
        elif "WARNING" in line:
            counts["WARNING"] += 1
        elif "INFO" in line:
            counts["INFO"] += 1

print(counts)

After reading the sample log file, counts will contain:

Counter({'INFO': 3, 'ERROR': 2, 'WARNING': 1})

6. Print the final totals #

Here is the full script:

from collections import Counter

log_file = "app.log"
counts = Counter()
total_lines = 0

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        total_lines += 1
        line = line.strip()

        if "ERROR" in line:
            counts["ERROR"] += 1
        elif "WARNING" in line:
            counts["WARNING"] += 1
        elif "INFO" in line:
            counts["INFO"] += 1

print("Log summary")
print(f"Total lines: {total_lines}")

for level in ["ERROR", "WARNING", "INFO"]:
    print(f"{level}: {counts[level]}")

Expected output #

With the sample app.log file shown earlier, the output will be:

Log summary
Total lines: 6
ERROR: 2
WARNING: 1
INFO: 3

This output is simple and easy to compare with the input file.

Useful beginner improvements #

Once the basic version works, you can try small improvements.

Count all lines, not only matched lines #

The full example already shows total_lines += 1.

This helps you compare:

total lines in the file
matched log level lines

Store matching lines in separate lists #

You can save the actual lines, not just the totals.

error_lines = []
warning_lines = []

with open("app.log", "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()

        if "ERROR" in line:
            error_lines.append(line)
        elif "WARNING" in line:
            warning_lines.append(line)

This is useful if you want to print or save the problem lines later.

Find the first error line #

You can stop as soon as the first error is found.

first_error = None

with open("app.log", "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()
        if "ERROR" in line:
            first_error = line
            break

print(first_error)

Ignore letter case with `upper()` #

Some files may use error, Error, or ERROR.

To handle that, convert each line to one case first:

from collections import Counter

counts = Counter()

with open("app.log", "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()
        upper_line = line.upper()

        if "ERROR" in upper_line:
            counts["ERROR"] += 1
        elif "WARNING" in upper_line:
            counts["WARNING"] += 1
        elif "INFO" in upper_line:
            counts["INFO"] += 1

print(counts)

Ask the user for the file name with `input()` #

You can make the script more flexible:

from collections import Counter

log_file = input("Enter the log file name: ")
counts = Counter()

with open(log_file, "r", encoding="utf-8") as file:
    for line in file:
        line = line.strip()

        if "ERROR" in line:
            counts["ERROR"] += 1
        elif "WARNING" in line:
            counts["WARNING"] += 1
        elif "INFO" in line:
            counts["INFO"] += 1

print("Log summary")
for level in ["ERROR", "WARNING", "INFO"]:
    print(f"{level}: {counts[level]}")

This page is a full example project.

It is not a complete lesson on every file or string method used here.

For deeper explanations, use these related pages:

Common mistakes #

Here are some common problems beginners run into with this kind of script.

The log file name is wrong #

If the file does not exist, Python raises a FileNotFoundError.

Example:

with open("wrong_name.log", "r", encoding="utf-8") as file:
    pass

If that happens, check:

the spelling of the file name
that the file is in the correct folder
your current working directory

The path points to a folder instead of a file #

Make sure log_file contains a file path, not just a folder name.

The log levels use different letter case #

If your file contains error instead of ERROR, your checks may fail.

Fix this by converting the line first:

upper_line = line.upper()

You forget to strip newline characters #

Without strip(), the line may contain extra spaces or \n.

That can make debugging harder.

You use `if` for every check instead of `elif` #

If you use separate if statements, one line could be counted more than once in some cases.

Using elif makes the checks exclusive.

The file contains unexpected text format #

Some log files may have extra spaces, timestamps, or different wording.

If needed, print each line while testing so you can see the real format.

FAQ #

Do I need a special library to analyze a log file? #

No. A basic analyzer can use only built-in Python tools such as open(), loops, strings, and dictionaries.

Should I read the whole file at once? #

For beginners, reading line by line is usually better. It uses less memory and is easier to understand.

What if my log file uses lowercase words like error? #

Convert each line to one case first, such as line.upper(), then check for ERROR.

Can this example search for dates or usernames too? #

Yes. You can check each line for other patterns and store extra counts or matching lines.