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Compile Python 3.6+ code to Python 2.7+

Project description


Compile Python 3.6+ code to Python 2.7+ compatible code. The idea is quite similar to Babel Develop using the newest interpreter and use (most) new language features and still maintain backward compatibility.


MIT License Supported Python Versions CalVer v202101.1045 PyPI Version PyPI Downloads

Code Quality/CI:

GitHub CI Status GitLab CI Status Type Checked with mypy Code Coverage Code Style: sjfmt

Name role since until
Manuel Barkhau ( author/maintainer 2018-09 -

Ease the Transition from Old Interpreters

If your existing project uses Python2.7, it may not be possible to dedicate a substantial block of time to update all of your code and then flip a switch to start running on Python3. To make matters worse, you may continue to write code only for Python2.7, since that is what your production code will actually run on. With lib3to6 you can start to use Python3 for development and integration (ensuring forward compatibility) and still maintain backward compatibility while you have to deploy for Python2.

An especially attractive feature of Python3.6+ are type annotations and type checking with MyPy. With lib3to6 you can write new code to a higher quality standard without breaking backward compatibility, even if the most recent version you want to support is Python3.5 (which does not support variable annotation for example).

Python Versions and Compatibility

The compiled output is tested using:

  • Python 3.9
  • Python 3.8
  • Python 3.7
  • Python 3.6
  • Python 3.5
  • Python 2.7
  • PyPy 3.6
  • PyPy 3.5

The test-suite for the transpiler is run using:

  • Python 3.9
  • Python 3.8
  • Python 3.7
  • Python 3.6
  • PyPy 3.6

The compiled output may work with other versions of python, such as <=2.6 or >=3.0 <=3.4, but these are not tested.

Usage Caveats

lib3to6 does not add any runtime dependencies of its own, but it does inject code, such as temporary variables and imports from the standard library (itertools and builtins in particular). Any changes will only add a constant O(1) overhead.

lib3to6 does optimistic ast transformations, with the assumption that you're not doing anything too crazy in your code. An example of such a transformation is the support for PEP3102 - Keyword-Only Arguments. lib3to6 will change the function signature to use **kwargs and add locals extracted from kwargs.

$ cat
def compare(a, b, *, key=None):
$ lib3to6
def compare(a, b, **kwargs):
    key = kwargs.get('key', None)

This means that the function signature you can get using the inspect module may not be what you expect for the output of lib3to6.

Per-File Opt-In/Opt-Out

Since lib3to6==v202008.1042 there is support to selectively enable/disable transpilation on a per-file basis.

Any file which starts with a # lib3to6: disabled comment, will not be transpiled. For these, you will have to take care of forward/backward compatibility yourself.

# -*- coding: utf-8 -*-
# lib3to6: disabled
"""A module written to work both with Python2 and 3.

This module doesn't need to be transpiled by lib3to6.

from __future__ import print_function

import sys

PY3 = sys.version_info[0] > 2

if PY3:

Instead of opt-out, you can also take an opt-in approach. You will have to switch the default_mode argument:

package_dir = {"": "src"}

if any(arg.startswith("bdist") for arg in sys.argv):
    import lib3to6
    package_dir = lib3to6.fix(package_dir, default_mode='disabled')

This will leave all files untouched, except for those marked with a # lib3to6: enabled comment.

# lib3to6: enabled
"""A module written to work both with Python2 and 3.

This module doesn't need to be transpiled by lib3to6.

name: str = "Wörld"
print(f"Hello {world}!")

Integration using

The cli command lib3to6 <filename> is nice for demo purposes, but for integration with your project, you may prefer to use it in your file. Contributions for other kinds of integration are most welcome.


import sys
import setuptools

packages = setuptools.find_packages(".")
package_dir = {"": "."}

install_requires = ['typing;python_version<"3.5"']

if any(arg.startswith("bdist") for arg in sys.argv):
    import lib3to6
    package_dir = lib3to6.fix(

        "Programming Language :: Python",
        "Programming Language :: Python :: 2",
        "Programming Language :: Python :: 3",

When you build you package, the contents of the resulting distribution will be the code that was converted by lib3to6.

~/my-module $ python bdist_wheel --python-tag=py2.py3
running bdist_wheel
~/my-module$ ls -1 dist/

~/my-module$ python3 -m pip install dist/my_module-201808.1-py2.py3-none-any.whl
Processing ./dist/my_module-201808.1-py2.py3-none-any.whl
Installing collected packages: my-module
Successfully installed my-module-201808.1

~/my-module$ python2 -m pip install dist/my_module-201808.1-py2.py3-none-any.whl
Processing ./dist/my_module-201808.1-py2.py3-none-any.whl
Installing collected packages: my-module
Successfully installed my-module-201808.1

When testing, make sure you're not importing my_module from your local directory, which is probably the original source code. Instead you can either manipulate your PYTHONPATH, or simply switch directories...

~/$ python3 -c "import my_module"
Hello 世界 from 3.6.5!

~/my-module$ cd ..
~/$ python3 -c "import my_module"
Hello 世界 from 3.6.5!

~$ python2 -c "import my_module"
Hello 世界 from 2.7.15!

Automatic Conversions

Not all new language features have a semantic equivalent in older versions. To the extent these can be detected, an error will be reported when these features are used.

Note that a fix is not applied if the lowest version of python that you are targeting already supports the newer syntax. The conversions are ordered by when the feature was introduced.

PEP 572: Assignment Expressions (aka. the walrus operator)

# Since 3.8
if match1 := pattern1.match(data):
    result =

# From 2.7 to 3.7
match1 = pattern1.match(data)
if match1:
    result =

Some expressions nested expressions in a condition are not so easy, in which case lib3to6 will bend over backwards.

# Since 3.8
while (block := != '':

# From 2.7 to 3.7
__loop_condition = True
while __loop_condition:
    block =
    __loop_condition = block != ''
    if __loop_condition:

PEP 563: Postponed Evaluation of Annotations

# Since 3.7
class SelfRef:
    def method(self) -> SelfRef:

# From 3.0 to 3.6
class SelfRef:
    def method(self) -> 'SelfRef':

Note that this is not a stupid conversion that is applied to all annotations, it is only applied to annotations that are forward references. Backward references are left as is.

# Since 3.7
class BackRef:
    def method(self) -> ForwardRef:

class ForwardRef:
    def method(self) -> BackRef:

# From 3.0 to 3.6
class BackRef:
    def method(self) -> 'ForwardRef':

class ForwardRef:
    def method(self) -> BackRef:

If you're supporting python 2.7, the annotation will of course be elided.

PEP 498: formatted string literals.

# Since 3.6
who = "World"
print(f"Hello {who}!")

# From 2.7 to 3.5
who = "World"
print("Hello {0}!".format(who))

The fixer also converts the newer {var=} syntax, even if you use lib3to6 on a Python version older than 3.8.

# Since 3.6
who = "World"
print(f"Hello {who=}!")

# From 2.7 to 3.5
print("Hello who={0}!".format(who))

Eliding of Annotations

# Since 3.0
def foo(bar: int) -> str:

# In 2.7
def foo(bar):

PEP 515: underscores in numeric literals

# Since 3.6
num = 1_234_567

# From 2.7 to 3.5
num = 1234567

Unpacking generalizations

For literals...

# Since 3.4
x = [*[1, 2], 3]

# From 2.7 to 3.3
x = [1, 2, 3]

For varargs...

# Since 3.4
foo(0, *a, *b)

# From 2.7 to 3.3
foo(*([0] + list(a) + list(b))

For kwargs...

# Since 3.4
foo(**x, y=22, **z)

# From 2.7 to 3.3
import itertools
foo(**dict(itertools.chain(x.items(), {'y': 22}.items(), z.items())))

Note that the import will only be added to your module once.

Keyword only arguments

# Since 3.6
def kwonly_func(*, kwonly_arg=1):

# From 2.7 to 3.5
def kwonly_func(**kwargs):
    kwonly_arg = kwargs.get('kwonly_arg', 1)

Convert class based typing.NamedTuple usage to assignments

import typing

# Since 3.5
class Bar(typing.NamedTuple):
    x: int
    y: str

# From 2.7 to 3.4
Bar = typing.NamedTuple('Bar', [('x', int), ('y', str)])

New Style Classes

# Since 3.0
class Bar:

# Before 3.0
class Bar(object):

Future Imports

All __future__ imports applicable to your target version are prepended to every file.

# -*- coding: utf-8 -*-
# This file is part of the <X> project
# ...
"""A docstring."""

x = True

With target-version=27 (the default).

# -*- coding: utf-8 -*-
# This file is part of the <X> project
# ...
"""A docstring."""
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

x = True

With target-version=3.7

# -*- coding: utf-8 -*-
# This file is part of the <X> project
# ...
"""A docstring."""
from __future__ import annotations

x = True

Note that lib3to6 works mostly at the ast level, but an exception is made for any comments that appear at the top of the file. These are preserved as is, so your shebang, file encoding and licensing headers will be preserved.

Not Supported Features

An (obviously non-exhaustive) list of features which are not supported, either because they involve a semantic change, or because there is no simple ast transformation to make them work across different python versions:

  • PEP 492 - async/await
  • PEP 465 - @/__matmul__ operator
  • PEP 380 - yield from syntax
  • PEP 584 - union operators for dict
  • ordered dictionary (since python 3.6)

Modules with Backports

Some new modules have backports, which lib3to6 will point to:

  • typing
  • pathlib -> pathlib2
  • secrets -> python2-secrets
  • ipaddress -> py2-ipaddress
  • csv -> backports.csv
  • lzma -> backports.lzma
  • enum -> enum34

For a full list of modules for which these warnings and errors apply, please review MAYBE_UNUSABLE_MODULES in src/lib3to6/

For some modules, the backport uses the same module name as the original module in the standard library. By default, lib3to6 will only warn about usage of such modules, since it cannot detect if you're using the module from the backported package (good) or from the standard library (bad if not available in your target version). If you would like to opt-in to hard error messages, you can whitelist modules for which you have the backported package as a dependency.

A good approach to adding such backports as dependencies is to qualify the requirement with a dependency specification, so that users with a newer interpreter use the builtin module and don't install the backport package that they don't need.

These work as arguments for install_requires and also in requirements.txt files.

import setuptools


For testing, you can also pass these as a space separated parameter to the lib3to6 cli command:

$ lib3to6 > /dev/null
WARNING - Use of import 'enum'.
    This module is only available since Python 3.5,
    but you configured target_version=2.7.
WARNING - Use of import 'typing'.
    This module is only available since Python 3.5,
    but you configured target_version=2.7.

import enum
import typing

$ lib3to6 `--install-requires='typing'` > /dev/null
Traceback (most recent call last):
  File "/home/user/.../lib3to6/src/lib3to6/", line 134, in __call__
    raise common.CheckError(errmsg, node)
lib3to6.common.CheckError: - Prohibited import 'enum'.
    This module is available since Python 3.4,
    but you configured target_version='2.7'.
    Use '' instead.

$ lib3to6 `--install-requires='typing enum34'`
import enum
import typing


The main motivation for this project is to be able to use mypy without sacrificing compatibility to older versions of python.

# my_module/
def hello(who: str) -> None:
    import sys
    print(f"Hello {who} from {sys.version.split()[0]}!")

$ pip install lib3to6
$ python -m lib3to6 my_module/
# -*- coding: utf-8 -*-
from __future__ import absolute_import
from __future__ import division
from __future__ import print_function
from __future__ import unicode_literals

def hello(who):
    import sys
    print('Hello {0} from {1}!'.format(who, sys.version.split()[0]))


Fixes are applied to match the semantics of python3 code as close as possible, even when running on a python2.7 interpreter.

Some fixes that have been applied in the above:

- PEP263 magic comment to declare the coding of the python
  source file. This allows the string literal `"世界"` to
  be decoded correctly.
- `__future__` imports have been added. This includes the well
  known print statement -> function change. The unicode_literals
- Type annotations have been removed
- `f""` string -> `"".format()` conversion

Compatibility Matters

I've seen a common gut reaction to lib3to6, which is that we shouldn't care about older versions of Python, Python 2.7 in particular. I would humbly suggest you consider the position of people other than developers who have full control over their development environment and only use CPython. As of this writing (August 2020), the most recent language version supported by alternative interpreters is the following:

Interpreter Version
Stackless 3.7
PyPy 3.6
MicroPython 3.4
IronPython 2.7
Jython 2.7

Notice that even Stackless Python, which has the least effort to keep up with new language features, is nonetheless lagging behind CPython. And even if all you care about is CPython, be aware that the most recent interpreter may not be available on platforms that users care about. For example on, the most recent version of CPython is 3.5 and of PyPy is 2.7.

If you are writing a library and it doesn't need any of the newer runtime features, such as async/await or ordered dictionaries, then I would humbly suggest you do not unnecessarily prevent users of such platforms from using your library.

From a users perspective, only supporting the newest versions of Python might be interpreted as arrogance, but your time as a maintainer isn't free, and you don't owe users of your library anything. Lib3to6 exists to minimize your effort to maintain backward compatibility. If you have difficulties integrating lib3to6 into your packaging process, please report an issue:

On Testing your Project

Projects that use lib3to6 should have a test-suite that is executed with the oldest python version that you want to support, using the converted output generated by lib3to6. While you can develop using a newer version of python, you should not blindly trust lib3to6 as it is very easy to introduce backward incompatible changes if you only test on the most recent interpreter. The most obvious example is that lib3to6 cannot do much to help you if a library produces bytes on Python 2 but str on Python 3.

The easiest way I have found to test a project, is to create a distribution using python bdist_wheel with the above modifications to the, install it and run the test-suite against the installed modules.

How it works

This project works at the level of the python abstract syntax tree (AST). The AST is transformed so that is only uses constructs that are also valid in older versions of python. For example it will translate f-strings to normal strings using the str.format method.

>>> import sys
>>> sys.version_info
>>> import lib3to6
>>> py3_source = 'f"Hello {1 + 1}!"'
>>> cfg = {"fixers": ["f_string_to_str_format"]}
>>> py2_source = lib3to6.transpile_module(cfg, py3_source)

>>> print(py3_source)
f"Hello {1 + 1}!"
>>> print(py2_source)
# -*- coding: utf-8 -*-
"Hello {0}!".format(1 + 1)

At a lower level, this translation is based on detection of the ast.JoinedStr node, which is translated into and AST that can be serialized back into python syntax that will also work on older versions.

>>> print(lib3to6.parsedump_ast(py3_source))
    Str(s='Hello '),
        value=BinOp(left=Num(n=1), op=Add(), right=Num(n=1)),
>>> print(lib3to6.parsedump_ast(py2_source))
        value=Str(s='Hello {0}!'),
    args=[BinOp(left=Num(n=1), op=Add(), right=Num(n=1))],

Checker Errors

Of course this does not cover every aspect of compatibility. Changes in APIs cannot be translated automatically in this way.

An obvious example, is that there is no way to transpile code which uses async and await. In this case, lib3to6 will simply raise a CheckError. This applies only to your source code though, so if import use a library which uses async and await, everything may look fine until you run your tests on python 2.7.

A more subtle example is the change in semantics of the builtin open function.

$ cat
with open("myfile.txt", mode="w", encoding="utf-8") as fobj:
    fobj.write("Hello Wörld!")
$ python2
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: 'encoding' is an invalid keyword argument for this function

Usually there are alternative ways to write equivalent code that works on all versions of python. For these common incompatibilities lib3to6 will raise an error and suggest an alternative, such as in this case using instead.

$ lib3to6
Traceback (Most recent call last):
11  lib3to6      <module>         --> sys.exit(main())
764      __call__         --> return self.main(*args, **kwargs)
717      main             --> rv = self.invoke(ctx)
956      invoke           --> return ctx.invoke(self.callback, **ctx.params)
555      invoke           --> return callback(*args, **kwargs)
55  main             --> fixed_source_text = transpile.transpile_module(cfg, source_text)
260 transpile_module --> checker(cfg, module_tree)
158  __call__         --> raise common.CheckError(msg, node)
CheckError: Prohibited keyword argument 'encoding' to on line 1 of

Here lib3to6 you will give you a CheckError, however it remains your responsibility to write your code so that this syntactic translation is semantically equivalent in both python3 and python2.

lib3to6 uses the python ast module to parse your code. This means that you need a modern python interpreter to transpile from modern python to legacy python interpreter. You cannot transpile features which your interpreter cannot parse. The intended use is for developers of libraries who use the most modern python version, but want their libraries to work on older versions.


The most basic contribution you can make is to provide minimal, reproducible examples of code that should either be converted or which should raise an error.

The project is hosted at, mainly because that's where the CI/CD is configured. GitHub is only used as a copy/backup (and because that seems to be where many people look for things).

You can get started with local development in just a few commands.

user@host:~/ $ git clone
user@host:~/ $ cd lib3to6/
user@host:~/lib3to6/ ⎇master $ make help
user@host:~/lib3to6/ ⎇master $ make conda     # creates conda environments
user@host:~/lib3to6/ ⎇master $ ls ~/miniconda3/envs/
user@host:~/lib3to6_pypy35 lib3to6_py27 lib3to6_py36 lib3to6_py37 lib3to6_py38

The targets in the makefile are set up to use the virtual environments.

user@host:~/lib3to6/ ⎇master $ make fmt
All done! ✨ 🍰 ✨
21 files left unchanged.

user@host:~/lib3to6/ ⎇master $ make lint mypy devtest
isort ... ok
sjfmt ... ok
flake8 .. ok
mypy .... ok

For debugging you may wish to activate a virtual environment anyway.

user@host:~/lib3to6/ ⎇master $ source activate
user@host:~/lib3to6/ ⎇master (lib3to6_py38) $ ipython
Python 3.8.2 | packaged by conda-forge | (default, Apr 24 2020, 08:20:52)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.14.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import lib3to6

In [2]: lib3to6.__file__
Out[2]: '/home/user/lib3to6/src/lib3to6/'

Project Status (as of 2021-01-01): Beta

I have tested with Python 3.9 with only minor updates required since Python 3.8. I've been using this library for over two years on a few projects without much incident. Examples of such projects are:

Future Work

In an ideal world, the project would cover all cases documented on and either:

  1. Transpile to code that will work on any version
  2. Raise an error, ideally pointing to a page and section on or other documentation describing alternative methods of writing backwards compatible code. also contains much info on API changes that might be checked for, but checks and fixers for these will only be written if they are common enough, otherwise it's just too much work (patches are welcome though).


Since starting this project, I've learned of the py-backwards project, which is very, very similar in its approach. I have not evaluated it yet, to determine for what projects it might be a better choice.

Some features that might be implemented

  • PEP 380 - yield from gen syntax might be supported in a basic form by expanding to a for x in gen: yield x. That is not semantically equivalent though and I don't know if it's worth implementing it properly
  • PEP 465 - @ operator could be done by replacing all cases where the operator is used with a __matmul__ method call.


  • Q: Isn't the tagline "Compatibility Matters" ironic, considering that python 3.6+ is required to build a wheel?

  • A: The irony is not lost. The issue is, how to parse source code from a newer version of python than the python interpreter itself supports. You can install lib3to6 on older versions of python, but you'll be limited to the features supported by that version. For example, you won't be able to use f"" strings on python 3.5, but most annotations will work fine.

  • Q: Why keep python2.7 alive? Just let it die already!

  • A: Indeed, and lib3to6 can help with that. Put yourself in the shoes of somebody who is working on an old codebase. It's not realistic hold all other development efforts while the codebase is migrated and tested, while everything else waits.

    Instead an incremental approach is usually the only option. With lib3to6, individual modules of the codebase can be migrated to python3, leaving the rest of the codebase untouched. The project can still run in a python 2.7 environment, while developers increasingly move to using python 3.

    Additionally, lib3to6 is not just for compatibility with python 2.7, it also allows you to use new features like f"" strings and variable annotations, while still maintaining compatibility with older versions of python 3.

  • Q: Why not lib3to2?

  • A: I can't honestly say much about lib3to2. It seems to not be maintained and looking at the source I thought it would be easier to just write something new that worked on the AST level. The scope of lib3to6 is more general than 3to2, as you can use it even if all you care about is converting from python 3.6 to 3.5.

Changelog for


  • Fixes for Python 3.9 and mypy 0.800


  • New #6: per-file opt-in/opt-out using # lib3to6: disabled/# lib3to6: enabled


  • New: Lots more documentation.
  • New #5: Add detection of invalid imports and point to available backports. Use install_requires option to whitelist installed backports.
  • New: Checkers produce better error messages.
  • New: Colouring of diffs when using lib3to6 cli command.
  • New: Checker for yield from syntax on target version doesn't support it
  • New: Checker for @ operator when target version doesn't support it
  • Fix #3: --target-version argument is ignored gitlab../issues/3
  • Fix #4: Remove from __future__ import X when the target version doesn't support it.
  • Fix #4: Convert Forward Reference Annotations to strings gitlab../issues/4 Thank you Faidon Liambotis for your help with testing and helping to debug ❤️.
  • Fix: Don't apply keyword only args fixer for --target-version=3.0 or above.


  • Compatibility fixes for Python 3.8
  • Add support for f-string = specifier
  • Add support for := walrus operator (except inside comprehensions)


  • Fix python 2 builtins were not always overridden correctly.
  • Fix pypy compatibility testing
  • Better mypy coverage


  • Recursively apply some fixers.


  • Move to
  • Use bootstrapit
  • Fix bugs based on use with pycalver


  • CheckErrors include log line numbers

  • Transpile errors now include filenames

  • Added fixers for renamed modules, e.g. .. code-block:: diff

     - import queue
     + try:
     +     import queue
     + except ImportError:
     +     import Queue as queue


  • Better handling of package_dir
  • Change to CalVer Versioning <>_
  • Remove console script in favour of simple python -m lib3to6
  • Rename from three2six -> lib3to6

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