2to3

2to3 is a Python program that automates the conversion of Python 2 source code to Python 3. It is part of the Python standard library, distributed with the lib2to3 package, and has been available since the mid-2000s. The tool analyzes Python 2 code and applies a collection of transformation rules to produce Python 3 compatible output. It can write changes in place with the -w option, or print the results to stdout; other options adjust which fixers are applied or control the level of reporting.

The conversion works through a set of fixers located in lib2to3.fixes. Each fixer handles a specific class of incompatibilities between Python 2 and Python 3, such as converting print statements to calls to the print() function, updating exception syntax (from "except Exception, e" to "except Exception as e"), and adjusting certain standard library imports to their Python 3 equivalents. The tool also provides fixes for many common Python 2-3 migration scenarios, including division semantics, iterators and items methods, and string/byte handling where relevant.

Limitations: 2to3 cannot automatically fix all Python 2 code paths or all third-party library dependencies; some code relies on behavior that differs between versions or on unported modules with no Python 3 equivalent. In such cases, manual review and additional refactoring are often necessary. While useful for producing a starting point for porting, 2to3 is typically combined with testing and manual changes as part of a broader porting strategy.