Why Python Indentation Error Keeps Happening Despite Correct Spacing

If you're getting Python indentation errors even when your code spacing looks perfectly correct, you're not alone. This frustrating issue has specific causes that aren't immediately obvious, but once you understand them, you can fix and prevent these errors permanently.

The Most Common Hidden Causes #

1. Mixed Spaces and Tabs (The Silent Killer) #

The number one reason why Python indentation error keeps happening despite correct spacing is invisible mixing of spaces and tabs. Your code might look perfectly aligned, but Python sees inconsistent indentation.

Problem Example:

def calculate_total(items):
    total = 0  # This line uses 4 spaces
    for item in items:  # This line uses 1 tab (looks the same but isn't)
        total += item
    return total

How to Detect: Most text editors can show whitespace characters. In VS Code, press Ctrl+Shift+P and search "Toggle Render Whitespace".

Solution:

def calculate_total(items):
    total = 0  # Always use 4 spaces
    for item in items:  # Always use 4 spaces
        total += item  # Always use 4 spaces
    return total

2. Inconsistent Indentation Levels #

Python expects consistent indentation within each code block, even if the spacing "looks right."

Problem Example:

if True:
    print("First line")  # 4 spaces
      print("Second line")  # 6 spaces - inconsistent!

Solution:

if True:
    print("First line")  # 4 spaces
    print("Second line")  # 4 spaces - consistent

3. Copy-Paste Indentation Issues #

When copying code from websites, PDFs, or different editors, invisible characters can cause indentation problems.

Quick Fix:

1. Select all your code
2. Remove all indentation (Shift+Tab repeatedly)
3. Re-indent properly using Tab or 4 spaces consistently

Step-by-Step Debugging Process #

1. Check Your Editor Settings #

Configure your text editor to:

• Show whitespace characters
• Convert tabs to spaces automatically
• Set tab width to 4 spaces
• Enable indentation guides

2. Use Python's Built-in Detection #

Run your code with verbose error reporting:

python -t your_script.py  # Warns about inconsistent tabs/spaces
python -tt your_script.py  # Errors on inconsistent tabs/spaces

3. The Nuclear Option (When All Else Fails) #

Create a fresh file and manually retype the problematic code section. This eliminates any hidden formatting issues.

Prevention Strategies #

1. Editor Configuration #

VS Code settings.json:

{
    "editor.insertSpaces": true,
    "editor.tabSize": 4,
    "editor.detectIndentation": false,
    "python.linting.enabled": true
}

2. Use a Formatter #

Install and configure a Python formatter like black or autopep8:

pip install black
black your_script.py

3. Consistent Development Environment #

Use the same editor and settings across all your Python projects to avoid indentation inconsistencies.

Quick Diagnostic Checklist #

When you encounter indentation errors:

• Enable whitespace visualization in your editor
• Check for mixed tabs and spaces
• Verify consistent indentation levels within blocks
• Run python -tt to catch tab/space mixing
• Check if code was copied from external sources
• Ensure your editor uses 4 spaces for tabs
• Consider using a Python formatter

Common Error Messages and Their Meanings #

"IndentationError: expected an indented block"

• Missing indentation after colon (:)
• Solution: Add proper indentation to the next line

"IndentationError: unindent does not match any outer indentation level"

• Mixed tabs and spaces, or inconsistent indentation levels
• Solution: Make all indentation consistent

"IndentationError: unexpected indent"

• Extra indentation where none is expected
• Solution: Remove unnecessary indentation

Summary #

Python indentation errors despite correct spacing typically stem from invisible formatting issues - mixed tabs and spaces, inconsistent indentation levels, or copy-paste artifacts. The solution involves configuring your editor properly, using consistent spacing (4 spaces is recommended), and leveraging Python's built-in detection tools. With proper setup and awareness of these common pitfalls, you can eliminate these frustrating errors permanently.