How to Resolve Python "IndentationError: unindent does not match any outer indentation level"

Python is unique in its reliance on indentation to define code blocks (loops, functions, conditional statements, etc.), unlike many other languages that use curly braces or keywords. The IndentationError: unindent does not match any outer indentation level is a common syntax error indicating that Python has encountered inconsistent indentation, specifically when a line is "unindented" to a level that doesn't align logically with any previous block structure. This almost always happens due to mixing tabs and spaces for indentation within the same block.

This guide explains the importance of indentation in Python and provides clear steps to fix this error.

Understanding Indentation in Python

In Python, whitespace at the beginning of a line is not just for readability; it's part of the syntax. It defines code structure. Lines of code belonging to the same block (e.g., inside an if statement, a for loop, or a def function) must have the same level of indentation. When indentation decreases ("unindent"), Python expects it to align perfectly with the start of a preceding block structure.

# Correct Indentation Example
def greet(name):
    print("Starting greeting...") # Indented, belongs to the function block
    if name:
        print(f"Hello, {name}!") # Further indented, belongs to the if block
    else:
        print("Hello there!")   # Same indentation as the previous print, also in the if block (implicitly part of the 'else' concept here relative to 'if')
    print("Greeting finished.") # Same indentation as first print, still in the function block

print("Function defined.") # Not indented, outside the function block

The Cause: Mixing Tabs and Spaces

The unindent does not match any outer indentation level error arises almost exclusively when you mix tab characters and space characters for indentation within the same logical block of code.

While your editor might display tabs and spaces similarly (often making a tab look like 4 or 8 spaces), Python treats them as distinct characters. If one line in a block is indented with, say, one tab, and the next line is indented with four spaces, Python sees these as different indentation levels. When it later encounters an "unindent", it might not find a previous line that started at exactly the same indentation level because the mix of tabs and spaces created inconsistent starting positions.

def check_value(x):
    print("Checking...") # e.g., 4 spaces
    if x > 10:
# The next line might cause the error if indented differently:
        print("Greater than 10") # e.g., 1 tab (might look like 4 spaces but isn't)
    print("Check complete.") # ⛔️ Error here! Unindented using 4 spaces,
                             # which doesn't match the apparent level of the 'if'
                             # because the previous line used a tab.
                             # Python expects indentation consistent with
                             # either the 'print("Checking...")' level (4 spaces)
                             # or the function definition level (0 indentation).
                             # The mixed indentation broke the block structure.
try:
  check_value(15)
except IndentationError as e:
  print(f"Error: {e}")

Solution 1: Ensure Consistent Indentation (Manual Fix)

The fundamental solution is to choose one style (either only tabs OR only spaces) for indentation throughout your project, or at least within any single code block, and stick to it. The widely accepted standard (PEP 8) recommends using 4 spaces per indentation level.

To fix manually:

1. Identify the block of code mentioned in the error message.
2. Visually inspect the beginning of each line within that block in your editor. Most editors can be configured to show whitespace characters (tabs often as → or >>, spaces as ·).
3. Delete the leading whitespace from the inconsistent lines.
4. Re-indent those lines using only spaces (press the spacebar 4 times per level) or only tabs (press the Tab key once per level), ensuring all lines in the block match perfectly.

Solution 2: Use IDE Features to Fix Indentation

Most modern code editors and IDEs have built-in tools to detect and fix inconsistent indentation automatically.

Visual Studio Code (VS Code)

1. Open the Command Palette: Ctrl+Shift+P (Windows/Linux) or Cmd+Shift+P (macOS).
2. Type Convert Indentation to Spaces or Convert Indentation to Tabs.
3. Select the desired command. This will apply the conversion to the current file.
4. Save the file.

note

Tip: Configure VS Code to automatically insert spaces when you press Tab (Editor: Insert Spaces) and set the tab size (Editor: Tab Size, usually to 4). You can also enable "editor.renderWhitespace": "all" in settings to visually see tabs and spaces.

Sublime Text

1. Go to the menu: View -> Indentation.
2. Select either Convert Indentation to Spaces or Convert Indentation to Tabs.
3. Ensure the settings under View -> Indentation (like Indent Using Spaces and Tab Width: 4) match your preferred style.

PyCharm

1. Select the problematic code block or the entire file (Ctrl+A / Cmd+A).
2. Go to the menu: Code -> Reformat Code. PyCharm will typically fix indentation issues based on your project's code style settings (which usually default to PEP 8 spaces).
3. Check your code style settings under File -> Settings -> Editor -> Code Style -> Python -> Tabs and Indents.

VIM

1. Ensure your settings prefer spaces or tabs (e.g., set expandtab, tabstop=4, shiftwidth=4 in your .vimrc).
2. Go to normal mode (Esc).
3. To re-indent the entire file: gg=G.
4. To re-indent a specific block, visually select it (V, move cursor) and press =.

IDLE

1. Select the region of code (or Ctrl+A for all).
2. Go to the menu: Format.
3. Choose Untabify Region (to convert tabs to spaces - recommended) or Tabify Region (to convert spaces to tabs).
4. Confirm the number of spaces per tab if prompted (usually 4 or 8).

Proactive Tip: Configure Your Editor

The best long-term solution is to configure your code editor to automatically enforce your chosen indentation style (preferably 4 spaces per level) and potentially convert tabs to spaces on save. This prevents mixing them in the first place.

Diagnosing with tabnanny

Python includes a built-in module called tabnanny specifically designed to check source code for ambiguous indentation. You can run it from your terminal:

python -m tabnanny your_problem_file.py
# Or for Python 3 specifically:
python3 -m tabnanny your_problem_file.py

If tabnanny finds inconsistent mixing of tabs and spaces, it will print the filename and line number where the ambiguity is detected, helping you pinpoint the exact location to fix.

PEP 8 Style Guide Recommendation

The official Python style guide, PEP 8, strongly recommends using spaces for indentation, specifically 4 spaces per indentation level. While Python runs code indented purely with tabs, using spaces is the predominant convention in the Python community and avoids potential issues with tab widths varying across different editors or systems. Use tabs only if you're contributing to a project that already consistently uses them.

Conclusion

The IndentationError: unindent does not match any outer indentation level is Python's way of saying your code's structure is ambiguous because you've mixed tab characters and space characters within the same indentation block.

The fix involves ensuring consistency:

1. Choose one style: Either exclusively tabs or exclusively spaces (4 spaces recommended by PEP 8).
2. Apply consistently: Ensure all lines within a given block (if, for, def, class, etc.) use the same indentation method and level.
3. Use IDE tools: Leverage features like "Convert Indentation to Spaces/Tabs" or code reformatting to fix existing files.
4. Configure your editor: Set up your editor to automatically use your preferred style (e.g., insert spaces for tabs) to prevent future errors.
5. Use tabnanny if needed to locate problematic lines.

Consistent indentation is fundamental to Python syntax – mastering it resolves this common error.