What is Python’s IndentationError: expected an indented block?

Unlike many other programming languages that use braces {} to define code structure, Python uses indentation. The IndentationError: expected an indented block is a very Python-specific error that occurs when these indentation rules are not followed.

This error typically appears when a required indentation is missing from a code block or when the indentation style is inconsistent.

1. Missing Indentation in a Code Block

This is the most common cause. Statements that initiate a new code block, such as if, for, def, and class, must be followed by indented code.

Incorrect Code:

def my_function():
print("This line is not indented.") # Requires indentation inside the def block

Corrected Code:

def my_function():
    print("This line is now indented.")

In Python, the standard is to use four spaces for one level of indentation. Indenting the print function after the def statement resolves the error.

2. Unnecessary Indentation

Conversely, adding indentation where it isn’t needed can also cause an IndentationError (specifically, an unexpected indent error).

Incorrect Code:

x = 10
    print(x) # Unnecessary indentation

Corrected Code:

x = 10
print(x)

Top-level code should always start at the far left, with no preceding spaces.

3. Mixing Tabs and Spaces

This is a tricky cause of IndentationError because it’s often invisible. Python treats tabs and spaces as different characters. If you indent one line with a tab and another with spaces, the interpreter sees them as different indentation levels and raises an error.

Potentially Incorrect Code (Visually Indistinguishable):

if True:
	print("This line is indented with a tab.")
    print("This line is indented with spaces.") # -> Raises IndentationError

Solution:

The best way to solve this is to configure your code editor to automatically convert tabs to spaces.

  • Visual Studio Code: In your settings.json, add the settings "editor.insertSpaces": true and "editor.tabSize": 4.
  • PyCharm: Go to Settings > Editor > Code Style > Python, uncheck Use tab character, and set Indent to 4.

To fix existing code, use your editor’s feature to “Convert Indentation to Spaces”.

Conclusion

IndentationError highlights the importance of indentation, which is fundamental to Python’s syntax. To avoid this error, it’s crucial to remember these rules:

  1. Be Consistent: Stick to one indentation style throughout your project (four spaces is the recommended standard).
  2. Configure Your Editor: Set up your editor to insert four spaces when you press the Tab key.
  3. Check Your Blocks: Always ensure that lines ending with a colon (:)—like if, for, and def—are followed by an indented code block.

Adopting proper indentation habits not only prevents IndentationError but is also the first step toward writing clean, readable code.

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