[Python] The Terrifying Garbled Text! A Practical Guide to 120% Solving UnicodeDecodeError

When you start learning to program, one eerie error message you're almost guaranteed to encounter is `UnicodeDecodeError`. The first time I saw this error, my mind went blank, thinking, "Is this some kind of curse?" Especially when working with files, like trying to read a CSV or text file, this error suddenly appears and mercilessly steals our time.

Hello! I'm CopiCode, a former programming beginner who, with the help of AI, built two websites (buyonjapan.com, copicode.com) from scratch in just a month and a half.

I wrote this article for you, who, like me a few months ago, is struggling with `UnicodeDecodeError`. I'll explain my own experiences, the points where I got stuck, and how I used AI to solve them, all from the same beginner's perspective, using as little jargon as possible.

By the time you finish this article, you won't just be able to solve the error; you'll fundamentally understand "why garbled text happens" and will never fear this error again. I've prepared plenty of fully functional, copy-paste-ready code, so let's experience "making it work" together!

First Off, Why Does Garbled Text Happen? The Truth About "Encoding" for Beginners

Before we jump into solving the error, let me talk a little about the fundamentals. It might seem like a detour, but understanding this is the ultimate weapon to stay unfazed by any character encoding errors in the future.

To put it very simply, computers can't directly understand characters like "あ" or "A". All they understand are the numbers "0" and "1". Therefore, they need a correspondence table of characters and numbers that says, "When you see this number, display 'あ'," or "This number means 'A'." This "rulebook" is the true identity of encoding.

The problem is that there are several types of this "rulebook (encoding)."

• UTF-8: The current most standard rulebook that can cover almost all languages in the world. Websites and modern apps almost exclusively use this.
• Shift_JIS (S-JIS): An old, Japanese-specific rulebook that was standard in older versions of Windows.
• CP932: A Windows-specific rulebook that is a slight variation of Shift_JIS made by Microsoft. It's almost the same as Shift_JIS but differs in some symbols.

The `UnicodeDecodeError` is caused precisely by this "mismatch of rulebooks."

For example, what happens if someone writes a memo (file) with "こんにちは" using the "Shift_JIS" rulebook, and you try to read it using the "UTF-8" rulebook? Naturally, since the rules are different, it can't be read correctly, resulting in a meaningless string of characters (garbled text) or an error message saying, "I can't read with this rule!" (`UnicodeDecodeError`).

My Beginner's Story:
I was initially trying to read a CSV file created in Excel that I received from a client. No matter how many times I tried, I kept getting a `UnicodeDecodeError` and struggled for about half a day. The cause was that older versions of Excel saved CSV files in "Shift_JIS" (or more accurately, CP932). Python was trying to be helpful by reading it as "UTF-8," but that's what was causing the mismatch. When I finally realized this, I felt like collapsing to my knees.

In other words, there's only one thing we need to do: "Specify the correct rulebook (encoding) when reading the file." That's it.

[Solve by Copy-Pasting] How to Handle UnicodeDecodeError When Reading Files

Now, let's look at some concrete code solutions. The most common scenario is when using the `open()` function to open a file.

For instance, let's say you have a file named `test.txt` like the one below. The encoding in which this file is saved is the deciding factor.

Hello, World!
This is a Python test.

The Basics: Specify the `encoding` Argument

When opening a file in Python, you can specify which rulebook to use by passing an `encoding` argument to the `open()` function. If you don't specify this, your environment might automatically choose an unintended encoding (like UTF-8), leading to errors.

1. Reading with UTF-8 (The Most Basic)

Files downloaded from websites or created with modern text editors are almost always in UTF-8. Let's try this first.

# If 'test.txt' is saved in UTF-8
try:
    with open('test.txt', 'r', encoding='utf-8') as f:
        content = f.read()
        print("Successfully read with UTF-8!")
        print(content)
except FileNotFoundError:
    print("Error: 'test.txt' not found.")
except UnicodeDecodeError:
    print("Error: Could not decode with UTF-8. Please try other encodings.")

2. Reading with Shift_JIS (For older Windows files)

If UTF-8 doesn't work, the next thing to try is `shift_jis`. Shift_JIS is still actively used, especially for data provided by government agencies or CSV files exported from older systems.

# If 'test.txt' is saved in Shift_JIS
try:
    with open('test.txt', 'r', encoding='shift_jis') as f:
        content = f.read()
        print("Successfully read with Shift_JIS!")
        print(content)
except FileNotFoundError:
    print("Error: 'test.txt' not found.")
except UnicodeDecodeError:
    print("Error: Could not decode with Shift_JIS. Please try other encodings.")

3. Reading with CP932 (Effective for Excel CSV files, etc.)

If you still get an error with Shift_JIS, it's worth trying `cp932`, especially if the file was created with Windows Notepad or an older version of Excel. `cp932` is like a cousin of Shift_JIS and can correctly read files containing special characters (e.g., "①" or "～") that Shift_JIS can't handle. The CSV file that cost me half a day was solved with this.

# If 'test.txt' is saved in CP932 (Japanese Windows environment)
try:
    with open('test.txt', 'r', encoding='cp932') as f:
        content = f.read()
        print("Successfully read with CP932!")
        print(content)
except FileNotFoundError:
    print("Error: 'test.txt' not found.")
except UnicodeDecodeError:
    print("Error: Could not decode with CP932.")

[Advanced] The Ultimate Weapon When You Just Can't Figure Out the Encoding

"I've tried UTF-8, Shift_JIS, and CP932, but nothing works..."
Even in such a desperate situation, it's too early to give up. From here, I'll introduce more powerful techniques that even pros use.

Emergency Fix: Ignore or Replace Errors (Not Recommended)

The `open()` function has another useful argument: `errors`. This tells Python how to behave when it encounters a character it can't decode.

• `errors='ignore'`: Completely ignores and skips the undecodable characters.
• `errors='replace'`: Replaces the undecodable characters with a substitute like `?`.

[VERY IMPORTANT] These methods are not a fundamental solution. You risk data loss or garbled text. Use them only in emergencies when you just want to check the contents of a file or when you want to identify the source of the error.

Ignoring errors (`ignore`)

# Read by ignoring characters that cannot be read with UTF-8
# Note: The corresponding characters will be lost from the data
try:
    with open('test.txt', 'r', encoding='utf-8', errors='ignore') as f:
        content = f.read()
        print("Read by ignoring errors (potential for data loss)")
        print(content)
except FileNotFoundError:
    print("Error: 'test.txt' not found.")

Replacing errors (`replace`)

# Read by replacing characters that cannot be read with UTF-8 with "?"
# Note: The corresponding characters will become "?"
try:
    with open('test.txt', 'r', encoding='utf-8', errors='replace') as f:
        content = f.read()
        print("Read by replacing errors with '?' (potential for garbled text)")
        print(content)
except FileNotFoundError:
    print("Error: 'test.txt' not found.")

Your Strongest Ally! Auto-detect Encoding with the `chardet` Library

"I have no idea what the encoding is anymore!"
For you in this situation, your strongest ally is a library called `chardet`. It's a detective-like tool that analyzes the contents of a file and automatically guesses, "This file is probably written in XX (encoding)!"

This library is not included in Python by default, so you need to install it first. Run the following command in your terminal (Command Prompt or PowerShell on Windows).

pip install chardet

Once it's installed, try using the "magical, copy-paste-ready code" below. Just specify the file path, and it will automatically detect the encoding and use that result to open the file.

import chardet

# Enter the path of the file you want to investigate here
file_path = 'test.txt' 

try:
    # The key is to first read the file in "binary mode ('rb')"
    with open(file_path, 'rb') as f:
        raw_data = f.read()

    # Estimate the encoding with chardet
    result = chardet.detect(raw_data)
    encoding = result['encoding']
    confidence = result['confidence'] # Confidence of the guess (0.0 to 1.0)

    print(f"Estimated encoding: {encoding} (Confidence: {confidence * 100:.2f}%)")

    # If an encoding was detected, open the file with it
    if encoding:
        print("\n--- File Contents ---")
        # Now open in text mode ('r') using the detected encoding
        with open(file_path, 'r', encoding=encoding) as f:
            content = f.read()
            print(content)
    else:
        print("Could not estimate the encoding.")

except FileNotFoundError:
    print(f"Error: '{file_path}' not found.")
except Exception as e:
    print(f"An unexpected error occurred while reading the file: {e}")

Key points of this code:

• It first opens the file in `'rb'` (read binary) mode. This is to read the raw data (the sequence of numbers) before interpreting it as text.
• `chardet.detect()` analyzes that raw data and returns the result as a dictionary.
• You can get the estimated encoding name with `result['encoding']` and how certain the guess is (confidence) with `result['confidence']`.

My Beginner's Story:
I was shocked when an AI told me about the `chardet` library. "Something this convenient existed?!" There are surprisingly many situations where the encoding is unknown, like text files from overseas clients or HTML source code from web scraping. In such times, this code is a true lifesaver. It's now one of my "good luck charm" code snippets.

For Those Still Stuck: How to Effectively Ask AI (like ChatGPT) for Help

If you've tried all the methods so far and are still stuck, there might be another, more complex issue involved. In such cases, the fastest way forward is to rely on an AI (like ChatGPT or Gemini) instead of struggling alone.

However, to get an accurate answer from an AI, it's very important to know "how to ask questions effectively." I also wasted time in the beginning by asking poor questions and getting irrelevant answers.

Just by following the points below, the accuracy of the answers will dramatically improve.

Bad Question Example ❌

I'm getting garbled text in Python. Help me.

Good Question Example (Copy-Paste Template) ✅

By filling out this template with ① what you want to do, ② your code, ③ the full error message, ④ what you've tried, and ⑤ additional information, you make it much easier for the AI to identify the cause of the problem and provide a more accurate solution. The error message, even if it looks like a meaningless string of characters, is the biggest clue for the AI to pinpoint the cause. Always make sure to copy and paste the entire message.

Conclusion: Garbled Text Is No Longer Scary!

Congratulations on making it this far! It was a long journey, but you now have the weapons to defeat the formidable enemy known as `UnicodeDecodeError`.

Let's review today's adventure one last time.

1. The cause of the error is an "encoding mismatch": It happens because the "rulebook" used when the file was created is different from the "rulebook" Python is using to read it.
2. Try the basics first: Use `open()` and specify `encoding='utf-8'`, and if that doesn't work, try `'cp932'` or `'shift_jis'`. This solves about 80% of cases.
3. The ultimate weapon, `chardet`: When the encoding is completely unknown, using the `chardet` library to auto-detect it is the most powerful solution.
4. Make AI your partner: If you're still stuck, ask an AI for help, providing precise information (code, full error message, etc.).

In programming, errors are an unavoidable wall. However, each error is also a valuable experience point that will surely help you grow. `UnicodeDecodeError` is one of the first major hurdles many beginners face. By overcoming this wall, you have undoubtedly leveled up.

You will encounter many more errors in the future, but don't be afraid. Sometimes, rely on convenient tools like AI, and try to enjoy the process of solving problems itself. I'm cheering for you!