`Lib/lib2to3/main.py`

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py

lib2to3/main.py is the entry point for the 2to3 command-line tool. It parses flags, discovers fixer classes from the lib2to3.fixes package, instantiates a RefactoringTool, and drives it over each file or package directory the user names on the command line. The module is deliberately thin: all the AST walking and source rewriting lives in lib2to3.refactor.

Fixers are plain Python classes, one per transformation, collected under Lib/lib2to3/fixes/. Each class declares a PATTERN (a lib2to3 pattern-language expression that selects AST nodes) and a transform() method that returns a replacement node or None. Common fixers handle print statements, except E, e syntax, .has_key(), dict.iteritems(), and dozens of other Python 2 idioms. The tool applies every enabled fixer in a single pass over the AST.

The main() function wires together the option parser, fixer selection, and RefactoringTool. When -w is given, the tool writes modified files back in place (optionally keeping a .bak backup unless -n suppresses it). --list-fixes exits early after printing the canonical fixer names. The whole lib2to3 package is around 8 000 lines; this entry-point file is about 120 of them.

Map

Lines	Symbol	Role	gopy
1-20	imports	stdlib imports plus `lib2to3.refactor`	n/a
21-40	`warn()`	emit a `DeprecationWarning` for the package itself	n/a
41-70	`main()`	parse argv, select fixers, build `RefactoringTool`	not ported
71-90	fixer discovery	iterate `lib2to3.fixes` to collect fixer names	not ported
91-110	file loop	call `refactor_file()` or `refactor_dir()` per path	not ported
111-120	`__main__` guard	`sys.exit(main())` entry point	not ported

Reading

Package deprecation warning (lines 21 to 40)

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py#L21-40

lib2to3 was deprecated in Python 3.11 and is scheduled for removal. The first thing main.py does on import is call warn() with DeprecationWarning so that any tool embedding the package receives a visible heads-up. Users are directed toward ast plus third-party tools such as libcst.

# warn() is called at module import time
warn("lib2to3 package is deprecated and may be removed in a future version",
     DeprecationWarning, stacklevel=2)

Option parsing (lines 41 to 70)

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py#L41-70

main() builds an optparse.OptionParser with flags for controlling fixer selection (-f, -x), write-back (-w), backup suppression (-n), diff-only mode (--no-diffs), and verbosity (-v). The --list-fixes flag short-circuits the rest of the function, printing every available fixer name and returning 0.

parser.add_option("-w", "--write", action="store_true",
                  help="Write back modified files")
parser.add_option("-n", "--nobackups", action="store_true",
                  help="Don't write backups for modified files")

Fixer discovery (lines 71 to 90)

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py#L71-90

The tool assembles a list of fixer names by inspecting the lib2to3.fixes package directory for modules whose names start with fix_. The -f all shorthand expands to every discovered fixer; individual fixers can be excluded with -x. The resulting list is handed to RefactoringTool as the fixer_names argument.

fixer_names = lib2to3.refactor.get_fixers_from_package("lib2to3.fixes")

File-level refactoring loop (lines 91 to 110)

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py#L91-110

For each path provided on the command line, main() calls either refactor_file() or refactor_dir() on the RefactoringTool instance. The tool walks directories recursively, skipping non-.py files. Diffs are printed to stdout unless --no-diffs is set; files are overwritten only when -w is active.

if os.path.isdir(py_source):
    rt.refactor_dir(py_source, write=options.write, ...)
else:
    rt.refactor_file(py_source, write=options.write, ...)

Entry-point guard (lines 111 to 120)

cpython 3.14 @ ab2d84fe1023/Lib/lib2to3/main.py#L111-120

The standard if __name__ == "__main__" block calls sys.exit(main()) so that the process exit code reflects whether any errors occurred. The 2to3 console script installed by the CPython build system points directly at this guard.

if __name__ == "__main__":
    sys.exit(main())

gopy mirror

Not yet ported.

Map​

Reading​

Package deprecation warning (lines 21 to 40)​

Option parsing (lines 41 to 70)​

Fixer discovery (lines 71 to 90)​

File-level refactoring loop (lines 91 to 110)​

Entry-point guard (lines 111 to 120)​

gopy mirror​

Map