2to3 – Automated Python 2 to 3 code translation

2to3 – Automated Python 2 to 3 code translation

Development Tools for python

The modules described in this chapter help you write software. For example, the pydoc module takes a module and generates documentation based on the module’s contents. The doctest and unittest modules contains frameworks for writing unit tests that automatically exercise code and verify that the expected output is produced. 2to3 can translate Python 2.x source code into valid Python 3.x code.

The list of modules described in this chapter is:

  • typing — Support for type hints
    • Type aliases
    • NewType
    • Callable
    • Generics
    • User-defined generic types
    • The Any type
    • Nominal vs structural subtyping
    • Classes, functions, and decorators
  • pydoc — Documentation generator and online help system
  • doctest — Test interactive Python examples
    • Simple Usage: Checking Examples in Docstrings
    • Simple Usage: Checking Examples in a Text File
    • How It Works
      • Which Docstrings Are Examined?
      • How are Docstring Examples Recognized?
      • What’s the Execution Context?
      • What About Exceptions?
      • Option Flags
      • Directives
      • Warnings
    • Basic API
    • Unittest API
    • Advanced API
      • DocTest Objects
      • Example Objects
      • DocTestFinder objects
      • DocTestParser objects
      • DocTestRunner objects
      • OutputChecker objects
    • Debugging
    • Soapbox
  • unittest — Unit testing framework
    • Basic example
    • Command-Line Interface
      • Command-line options
    • Test Discovery
    • Organizing test code
    • Re-using old test code
    • Skipping tests and expected failures
    • Distinguishing test iterations using subtests
    • Classes and functions
      • Test cases
        • Deprecated aliases
      • Grouping tests
      • Loading and running tests
        • load_tests Protocol
    • Class and Module Fixtures
      • setUpClass and tearDownClass
      • setUpModule and tearDownModule
    • Signal Handling
  • unittest.mock — mock object library
    • Quick Guide
    • The Mock Class
      • Calling
      • Deleting Attributes
      • Mock names and the name attribute
      • Attaching Mocks as Attributes
    • The patchers
      • patch
      • patch.object
      • patch.dict
      • patch.multiple
      • patch methods: start and stop
      • patch builtins
      • Nesting Patch Decorators
      • Where to patch
      • Patching Descriptors and Proxy Objects
    • MagicMock and magic method support
      • Mocking Magic Methods
      • Magic Mock
    • Helpers
      • sentinel
      • DEFAULT
      • call
      • create_autospec
      • ANY
      • FILTER_DIR
      • mock_open
      • Autospeccing
      • Sealing mocks
  • unittest.mock — getting started
    • Using Mock
      • Mock Patching Methods
      • Mock for Method Calls on an Object
      • Mocking Classes
      • Naming your mocks
      • Tracking all Calls
      • Setting Return Values and Attributes
      • Raising exceptions with mocks
      • Side effect functions and iterables
      • Mocking asynchronous iterators
      • Mocking asynchronous context manager
      • Creating a Mock from an Existing Object
    • Patch Decorators
    • Further Examples
      • Mocking chained calls
      • Partial mocking
      • Mocking a Generator Method
      • Applying the same patch to every test method
      • Mocking Unbound Methods
      • Checking multiple calls with mock
      • Coping with mutable arguments
      • Nesting Patches
      • Mocking a dictionary with MagicMock
      • Mock subclasses and their attributes
      • Mocking imports with patch.dict
      • Tracking order of calls and less verbose call assertions
      • More complex argument matching
  • 2to3 – Automated Python 2 to 3 code translation
    • Using 2to3
    • Fixers
    • lib2to3 – 2to3’s library
  • test — Regression tests package for Python
    • Writing Unit Tests for the test package
    • Running tests using the command-line interface
  • test.support — Utilities for the Python test suite
  • test.support.script_helper — Utilities for the Python execution tests

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. lib2to3 could also be adapted to custom applications in which Python code needs to be edited automatically.

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.

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.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.

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.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)

is changed to

L = sorted(some_iterable)


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 __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:

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)


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_valuesys.exc_typesys.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/

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