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.

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()

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()

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.

New 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.

New 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.

New 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


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.


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


Replaces deprecated unittest method names with the correct ones.



failUnlessEqual(a, b)

assertEqual(a, b)

assertEquals(a, b)

assertEqual(a, b)

failIfEqual(a, b)

assertNotEqual(a, b)

assertNotEquals(a, b)

assertNotEqual(a, b)







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)


Converts basestring to str.


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


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.


Converts except X, T to except X as T.


Converts the exec statement to the exec() function.


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


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


Wraps filter() usage in a list call.


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


Removes from __future__ import new_feature statements.


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


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


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)

is changed to

L = sorted(some_iterable)

Detects sibling imports and converts them to relative imports.


Handles module renames in the standard library.


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


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


Converts intern() to sys.intern().


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)).


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


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


Renames long to int.


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


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


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


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


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


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


Converts octal literals into the new syntax.


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:








isinstance(obj, collections.abc.Sequence)


isinstance(obj, collections.abc.Mapping)


isinstance(obj, numbers.Number)

operator.repeat(obj, n)

operator.mul(obj, n)

operator.irepeat(obj, n)

operator.imul(obj, n)


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)].


Converts the print statement to the print() function.


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.


Converts raw_input() to input().


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


Converts reload() to importlib.reload().


Changes sys.maxint to sys.maxsize.


Replaces backtick repr with the repr() function.


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


Renames StandardError to Exception.


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


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


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


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


Renames unicode to str.


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


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


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


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


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 3.10: Python 3.9 will switch to a PEG parser (see PEP 617), and Python 3.10 may include new language syntax that is not parsable by lib2to3’s LL(1) parser. The lib2to3 module may be removed from the standard library in a future Python version. Consider third-party alternatives such as LibCST or parso.


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