Indentation errors in Python

David Y.

The Problem

When I attempt to run my Python script, I receive the following error:

IndentationError: unindent does not match any outer indentation level

The Solution

Python uses indentation to group blocks of code. Top-level code in a Python script will start flush against the left margin, with each inner block being indented by an additional four spaces, per convention. If a block contains unevenly indented code (for example, two spaces before one line and four spaces before another), this confuses the interpreter and results in an IndentationError exception. Therefore, it is important to ensure that all statements in each block are prefaced with the same number of spaces.

However, Python code that appears to be correctly indented can also produce IdentationError exceptions. This usually arises from the use of both spaces and tabs for indentation, which Python disallows. The Python interpreter expects a single file to be indented with either spaces or tabs throughout, with spaces being officially preferred.

It should be possible to fix this error by replacing each tab with four spaces. This can be done automatically across whole files using the find-replace function in most text editors, or by using the following sed command on a Unix-based system:

sed 's/\t/ /g'
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