Why Does Incorrect Python Indentation Lead to Errors?

Why Does Indentation Matter?

Python relies on indentation to demarcate code blocks, unlike other languages like Java or Javascript that use curly braces {} for this purpose. As a result, Python users must be vigilant about indentation, as even a single space can cause errors. Violations of indentation rules result in either an IndentationError or TabError exception.

Indentation Guidelines

The fundamental rule for Python indentation is: Each statement in a basic block must be indented by the same amount. While
PEP8 - the Python style guide - recommends using 4 spaces per indentation level, Python permits tabs as indentation. However, never use both tabs and spaces for indentation as doing so will almost certainly result in indentation errors.

IndentationError: unexpected indent

This error indicates that a statement is unnecessarily indented or does not conform to the indentation pattern of previous statements within the same block.

IndentationError: expected an indented block

Python anticipates an indented block after encountering certain compound statements (e.g., if or while) but fails to find one. This error may also occur with the SyntaxError: unexpected EOF.

IndentationError: unindent does not match any outer indentation level

This error is triggered when you remove an indent at an inappropriate location, causing inconsistencies with the indentation of other statements.

TabError: inconsistent use of tabs and spaces in indentation

Python prohibits mixing tabs and spaces for indentation. This taboo extends to statements that appear to be visually aligned but have inconsistent indentation due to mixed tabs and spaces. Python will either raise a TabError or an IndentationError when it detects such inconsistencies. To resolve this issue, convert all indentation to spaces using automated tools like autopep8 or re-indent the code manually.

SyntaxError Due to Indentation

Even SyntaxError exceptions can sometimes be traced back to indentation issues. For example, if a statement is mistakenly placed outside an intended block, Python may raise a SyntaxError. The solution is to carefully re-indent the code to reflect the intended structure.

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