# 2to3 — Automated Python 2 to 3 code translation

2to3 is a Python program that reads Python 2.x source code and applies a series of fixers to transform it into valid Python 3.x code. The standard library contains a rich set of fixers that will handle almost all code. 2to3 supporting library `lib2to3` is, however, a flexible and generic library, so it is possible to write your own fixers for 2to3.

Deprecated since version 3.11, will be removed in version 3.13: The `lib2to3` module was marked pending for deprecation in Python 3.9 (raising `PendingDeprecationWarning` on import) and fully deprecated in Python 3.11 (raising `DeprecationWarning`). The `2to3` tool is part of that. It will be removed in Python 3.13.

## Using 2to3

2to3 will usually be installed with the Python interpreter as a script. It is also located in the `Tools/scripts` directory of the Python root.

2to3’s basic arguments are a list of files or directories to transform. The directories are recursively traversed for Python sources.

Here is a sample Python 2.x source file, `example.py`:

    def greet(name):
        print "Hello, {0}!".format(name)
    print "What's your name?"
    name = raw_input()
    greet(name)

It can be converted to Python 3.x code via 2to3 on the command line:

    $ 2to3 example.py

A diff against the original source file is printed. 2to3 can also write the needed modifications right back to the source file. (A backup of the original file is made unless `-n` is also given.) Writing the changes back is enabled with the `-w` flag:

    $ 2to3 -w example.py

After transformation, `example.py` looks like this:

    def greet(name):
        print("Hello, {0}!".format(name))
    print("What's your name?")
    name = input()
    greet(name)

Comments and exact indentation are preserved throughout the translation process.

By default, 2to3 runs a set of predefined fixers. The `-l` flag lists all available fixers. An explicit set of fixers to run can be given with `-f`. Likewise the `-x` explicitly disables a fixer. The following example runs only the `imports` and `has_key` fixers:

    $ 2to3 -f imports -f has_key example.py

This command runs every fixer except the `apply` fixer:

    $ 2to3 -x apply example.py

Some fixers are explicit, meaning they aren’t run by default and must be listed on the command line to be run. Here, in addition to the default fixers, the `idioms` fixer is run:

    $ 2to3 -f all -f idioms example.py

Notice how passing `all` enables all default fixers.

Sometimes 2to3 will find a place in your source code that needs to be changed, but 2to3 cannot fix automatically. In this case, 2to3 will print a warning beneath the diff for a file. You should address the warning in order to have compliant 3.x code.

2to3 can also refactor doctests. To enable this mode, use the `-d` flag. Note that only doctests will be refactored. This also doesn’t require the module to be valid Python. For example, doctest like examples in a reST document could also be refactored with this option.

The `-v` option enables output of more information on the translation process.

Since some print statements can be parsed as function calls or statements, 2to3 cannot always read files containing the print function. When 2to3 detects the presence of the `from __future__ import print_function` compiler directive, it modifies its internal grammar to interpret `print()` as a function. This change can also be enabled manually with the `-p` flag. Use `-p` to run fixers on code that already has had its print statements converted. Also `-e` can be used to make `exec()` a function.

The `-o` or `--output-dir` option allows specification of an alternate directory for processed output files to be written to. The `-n` flag is required when using this as backup files do not make sense when not overwriting the input files.

Added in version 3.2.3: The `-o` option was added.

The `-W` or `--write-unchanged-files` flag tells 2to3 to always write output files even if no changes were required to the file. This is most useful with `-o` so that an entire Python source tree is copied with translation from one directory to another. This option implies the `-w` flag as it would not make sense otherwise.

Added in version 3.2.3: The `-W` flag was added.

The `--add-suffix` option specifies a string to append to all output filenames. The `-n` flag is required when specifying this as backups are not necessary when writing to different filenames. Example:

    $ 2to3 -n -W --add-suffix=3 example.py

Will cause a converted file named `example.py3` to be written.

Added in version 3.2.3: The `--add-suffix` option was added.

To translate an entire project from one directory tree to another use:

    $ 2to3 --output-dir=python3-version/mycode -W -n python2-version/mycode

## Fixers

Each step of transforming code is encapsulated in a fixer. The command `2to3 -l` lists them. As documented above, each can be turned on and off individually. They are described here in more detail.

apply

  Removes usage of `apply()`. For example `apply(function, *args, **kwargs)` is converted to `function(*args, **kwargs)`.

asserts

  Replaces deprecated `unittest` method names with the correct ones.

  From | To
  --- | ---
  `failUnlessEqual(a, b)` | `assertEqual(a, b)`
  `assertEquals(a, b)` | `assertEqual(a, b)`
  `failIfEqual(a, b)` | `assertNotEqual(a, b)`
  `assertNotEquals(a, b)` | `assertNotEqual(a, b)`
  `failUnless(a)` | `assertTrue(a)`
  `assert_(a)` | `assertTrue(a)`
  `failIf(a)` | `assertFalse(a)`
  `failUnlessRaises(exc, cal)` | `assertRaises(exc, cal)`
  `failUnlessAlmostEqual(a, b)` | `assertAlmostEqual(a, b)`
  `assertAlmostEquals(a, b)` | `assertAlmostEqual(a, b)`
  `failIfAlmostEqual(a, b)` | `assertNotAlmostEqual(a, b)`
  `assertNotAlmostEquals(a, b)` | `assertNotAlmostEqual(a, b)`

basestring

  Converts `basestring` to `str`.

buffer

  Converts `buffer` to `memoryview`. This fixer is optional because the `memoryview` API is similar but not exactly the same as that of `buffer`.

dict

  Fixes dictionary iteration methods. `dict.iteritems()` is converted to `dict.items()`, `dict.iterkeys()` to `dict.keys()`, and `dict.itervalues()` to `dict.values()`. Similarly, `dict.viewitems()`, `dict.viewkeys()` and `dict.viewvalues()` are converted respectively to `dict.items()`, `dict.keys()` and `dict.values()`. It also wraps existing usages of `dict.items()`, `dict.keys()`, and `dict.values()` in a call to `list`.

except

  Converts `except X, T` to `except X as T`.

exec

  Converts the `exec` statement to the `exec()` function.

execfile

  Removes usage of `execfile()`. The argument to `execfile()` is wrapped in calls to `open()`, `compile()`, and `exec()`.

exitfunc

  Changes assignment of `sys.exitfunc` to use of the `atexit` module.

filter

  Wraps `filter()` usage in a `list` call.

funcattrs

  Fixes function attributes that have been renamed. For example, `my_function.func_closure` is converted to `my_function.__closure__`.

future

  Removes `from __future__ import new_feature` statements.

getcwdu

  Renames `os.getcwdu()` to `os.getcwd()`.

has_key

  Changes `dict.has_key(key)` to `key in dict`.

idioms

  This optional fixer performs several transformations that make Python code more idiomatic. Type comparisons like `type(x) is SomeClass` and `type(x) == SomeClass` are converted to `isinstance(x, SomeClass)`. `while 1` becomes `while True`. This fixer also tries to make use of `sorted()` in appropriate places. For example, this block

      L = list(some_iterable)
      L.sort()

  is changed to

      L = sorted(some_iterable)

import

  Detects sibling imports and converts them to relative imports.

imports

  Handles module renames in the standard library.

imports2

  Handles other modules renames in the standard library. It is separate from the `imports` fixer only because of technical limitations.

input

  Converts `input(prompt)` to `eval(input(prompt))`.

intern

  Converts `intern()` to `sys.intern()`.

isinstance

  Fixes duplicate types in the second argument of `isinstance()`. For example, `isinstance(x, (int, int))` is converted to `isinstance(x, int)` and `isinstance(x, (int, float, int))` is converted to `isinstance(x, (int, float))`.

itertools_imports

  Removes imports of `itertools.ifilter()`, `itertools.izip()`, and `itertools.imap()`. Imports of `itertools.ifilterfalse()` are also changed to `itertools.filterfalse()`.

itertools

  Changes usage of `itertools.ifilter()`, `itertools.izip()`, and `itertools.imap()` to their built-in equivalents. `itertools.ifilterfalse()` is changed to `itertools.filterfalse()`.

long

  Renames `long` to `int`.

map

  Wraps `map()` in a `list` call. It also changes `map(None, x)` to `list(x)`. Using `from future_builtins import map` disables this fixer.

metaclass

  Converts the old metaclass syntax (`__metaclass__ = Meta` in the class body) to the new (`class X(metaclass=Meta)`).

methodattrs

  Fixes old method attribute names. For example, `meth.im_func` is converted to `meth.__func__`.

ne

  Converts the old not-equal syntax, `<>`, to `!=`.

next

  Converts the use of iterator’s `next()` methods to the `next()` function. It also renames `next()` methods to `__next__()`.

nonzero

  Renames definitions of methods called `__nonzero__()` to `__bool__()`.

numliterals

  Converts octal literals into the new syntax.

operator

  Converts calls to various functions in the `operator` module to other, but equivalent, function calls. When needed, the appropriate `import` statements are added, e.g. `import collections.abc`. The following mapping are made:

  From | To
  --- | ---
  `operator.isCallable(obj)` | `callable(obj)`
  `operator.sequenceIncludes(obj)` | `operator.contains(obj)`
  `operator.isSequenceType(obj)` | `isinstance(obj, collections.abc.Sequence)`
  `operator.isMappingType(obj)` | `isinstance(obj, collections.abc.Mapping)`
  `operator.isNumberType(obj)` | `isinstance(obj, numbers.Number)`
  `operator.repeat(obj, n)` | `operator.mul(obj, n)`
  `operator.irepeat(obj, n)` | `operator.imul(obj, n)`

paren

  Add extra parenthesis where they are required in list comprehensions. For example, `[x for x in 1, 2]` becomes `[x for x in (1, 2)]`.

print

  Converts the `print` statement to the `print()` function.

raise

  Converts `raise E, V` to `raise E(V)`, and `raise E, V, T` to `raise E(V).with_traceback(T)`. If `E` is a tuple, the translation will be incorrect because substituting tuples for exceptions has been removed in 3.0.

raw_input

  Converts `raw_input()` to `input()`.

reduce

  Handles the move of `reduce()` to `functools.reduce()`.

reload

  Converts `reload()` to `importlib.reload()`.

renames

  Changes `sys.maxint` to `sys.maxsize`.

repr

  Replaces backtick repr with the `repr()` function.

set_literal

  Replaces use of the `set` constructor with set literals. This fixer is optional.

standarderror

  Renames `StandardError` to `Exception`.

sys_exc

  Changes the deprecated `sys.exc_value`, `sys.exc_type`, `sys.exc_traceback` to use `sys.exc_info()`.

throw

  Fixes the API change in generator’s `throw()` method.

tuple_params

  Removes implicit tuple parameter unpacking. This fixer inserts temporary variables.

types

  Fixes code broken from the removal of some members in the `types` module.

unicode

  Renames `unicode` to `str`.

urllib

  Handles the rename of `urllib` and `urllib2` to the `urllib` package.

ws_comma

  Removes excess whitespace from comma separated items. This fixer is optional.

xrange

  Renames `xrange()` to `range()` and wraps existing `range()` calls with `list`.

xreadlines

  Changes `for x in file.xreadlines()` to `for x in file`.

zip

  Wraps `zip()` usage in a `list` call. This is disabled when `from future_builtins import zip` appears.

## `lib2to3` — 2to3’s library

Source code: Lib/lib2to3/

Deprecated since version 3.11, will be removed in version 3.13: Python 3.9 switched to a PEG parser (see PEP 617) while lib2to3 is using a less flexible LL(1) parser. Python 3.10 includes new language syntax that is not parsable by lib2to3’s LL(1) parser (see PEP 634). The `lib2to3` module was marked pending for deprecation in Python 3.9 (raising `PendingDeprecationWarning` on import) and fully deprecated in Python 3.11 (raising `DeprecationWarning`). It will be removed from the standard library in Python 3.13. Consider third-party alternatives such as LibCST or parso.

Observera

The `lib2to3` API should be considered unstable and may change drastically in the future.