Ansible and Python 3

Ansible maintains a single code base that runs on bothPython 2 and Python 3 because we want Ansible to be able to manage a widevariety of machines. Contributors to Ansible should be aware of the tips inthis document so that they can write code that will run on the same versionsof Python as the rest of Ansible.

To ensure that your code runs on Python 3 as well as on Python 2, learn the tips and tricks and idiomsdescribed here. Most of these considerations apply to all three types of Ansible code:

• controller-side code - code that runs on the machine where you invoke /usr/bin/ansible
• modules - the code which Ansible transmits to and invokes on the managed machine.
• shared module_utils code - the common code that’s used by modules to perform tasks and sometimes used by controller-side code as wellHowever, the three types of code do not use the same string strategy. If you’re developing a module or some module_utils code, be sureto read the section on string strategy carefully.

Minimum version of Python 3.x and Python 2.x

On the controller we support Python 3.5 or greater and Python 2.7 or greater. Module-side, wesupport Python 3.5 or greater and Python 2.6 or greater.

Python 3.5 was chosen as a minimum because it is the earliest Python 3 version adopted as thedefault Python by a Long Term Support (LTS) Linux distribution (in this case, Ubuntu-16.04).Previous LTS Linux distributions shipped with a Python 2 version which users can rely upon insteadof the Python 3 version.

For Python 2, the default is for modules to run on at least Python 2.6. This allowsusers with older distributions that are stuck on Python 2.6 to manage theirmachines. Modules are allowed to drop support for Python 2.6 when one oftheir dependent libraries requires a higher version of Python. This is not aninvitation to add unnecessary dependent libraries in order to force yourmodule to be usable only with a newer version of Python; instead it is anacknowledgment that some libraries (for instance, boto3 and docker-py) willonly function with a newer version of Python.

Note

Python 2.4 Module-side Support:

Support for Python 2.4 and Python 2.5 was dropped in Ansible-2.4. RHEL-5(and its rebuilds like CentOS-5) were supported until April of 2017.Ansible-2.3 was released in April of 2017 and was the last Ansible releaseto support Python 2.4 on the module-side.

Developing Ansible code that supports Python 2 and Python 3

The best place to start learning about writing code that supports both Python 2 and Python 3is Lennart Regebro’s book: Porting to Python 3.The book describes several strategies for porting to Python 3. The one we’reusing is to support Python 2 and Python 3 from a single code base

Understanding strings in Python 2 and Python 3

Python 2 and Python 3 handle strings differently, so when you write code that supports Python 3you must decide what string model to use. Strings can be an array of bytes (like in C) orthey can be an array of text. Text is what we think of as letters, digits,numbers, other printable symbols, and a small number of unprintable “symbols”(control codes).

In Python 2, the two types for these (str for bytes andunicode for text) are often used interchangeably. When dealing onlywith ASCII characters, the strings can be combined, compared, and convertedfrom one type to another automatically. When non-ASCII characters areintroduced, Python 2 starts throwing exceptions due to not knowing what encodingthe non-ASCII characters should be in.

Python 3 changes this behavior by making the separation between bytes (bytes)and text (str) more strict. Python 3 will throw an exception whentrying to combine and compare the two types. The programmer has to explicitlyconvert from one type to the other to mix values from each.

In Python 3 it’s immediately apparent to the programmer when code ismixing the byte and text types inappropriately, whereas in Python 2, code that mixes those typesmay work until a user causes an exception by entering non-ASCII input.Python 3 forces programmers to proactively define a strategy forworking with strings in their program so that they don’t mix text and byte strings unintentionally.

Ansible uses different strategies for working with strings in controller-side code, in:ref: modules <module_string_strategy>, and in module_utils code.

Controller string strategy: the Unicode Sandwich

In controller-side code we use a strategy known as the Unicode Sandwich (namedafter Python 2’s unicode text type). For Unicode Sandwich we know thatat the border of our code and the outside world (for example, file and network IO,environment variables, and some library calls) we are going to receive bytes.We need to transform these bytes into text and use that throughout theinternal portions of our code. When we have to send those strings back out tothe outside world we first convert the text back into bytes.To visualize this, imagine a ‘sandwich’ consisting of a top and bottom layerof bytes, a layer of conversion between, and all text type in the center.

Unicode Sandwich common borders: places to convert bytes to text in controller code

This is a partial list of places where we have to convert to and from byteswhen using the Unicode Sandwich string strategy. It’s not exhaustive butit gives you an idea of where to watch for problems.

Reading and writing to files

In Python 2, reading from files yields bytes. In Python 3, it can yield text.To make code that’s portable to both we don’t make use of Python 3’s abilityto yield text but instead do the conversion explicitly ourselves. For example:

from ansible.module_utils._text import to_text
 
with open('filename-with-utf8-data.txt', 'rb') as my_file:
    b_data = my_file.read()
    try:
        data = to_text(b_data, errors='surrogate_or_strict')
    except UnicodeError:
        # Handle the exception gracefully -- usually by displaying a good
        # user-centric error message that can be traced back to this piece
        # of code.
        pass

Note

Much of Ansible assumes that all encoded text is UTF-8. At somepoint, if there is demand for other encodings we may change that, but fornow it is safe to assume that bytes are UTF-8.

Writing to files is the opposite process:

from ansible.module_utils._text import to_bytes
 
with open('filename.txt', 'wb') as my_file:
    my_file.write(to_bytes(some_text_string))

Note that we don’t have to catch UnicodeError here because we’retransforming to UTF-8 and all text strings in Python can be transformed backto UTF-8.

Filesystem interaction

Dealing with filenames often involves dropping back to bytes because on UNIX-likesystems filenames are bytes. On Python 2, if we pass a text string to thesefunctions, the text string will be converted to a byte string inside of thefunction and a traceback will occur if non-ASCII characters are present. InPython 3, a traceback will only occur if the text string can’t be decoded inthe current locale, but it’s still good to be explicit and have code whichworks on both versions:

import os.path
 
from ansible.module_utils._text import to_bytes
 
filename = u'/var/tmp/くらとみ.txt'
f = open(to_bytes(filename), 'wb')
mtime = os.path.getmtime(to_bytes(filename))
b_filename = os.path.expandvars(to_bytes(filename))
if os.path.exists(to_bytes(filename)):
    pass

When you are only manipulating a filename as a string without talking to thefilesystem (or a C library which talks to the filesystem) you can often getaway without converting to bytes:

import os.path
 
os.path.join(u'/var/tmp/café', u'くらとみ')
os.path.split(u'/var/tmp/café/くらとみ')

On the other hand, if the code needs to manipulate the filename and also talkto the filesystem, it can be more convenient to transform to bytes right awayand manipulate in bytes.

Warning

Make sure all variables passed to a function are the same type.If you’re working with something like os.path.join() which takesmultiple strings and uses them in combination, you need to make sure thatall the types are the same (either all bytes or all text). Mixingbytes and text will cause tracebacks.

Interacting with other programs

Interacting with other programs goes through the operating system andC libraries and operates on things that the UNIX kernel defines. Theseinterfaces are all byte-oriented so the Python interface is byte oriented aswell. On both Python 2 and Python 3, byte strings should be given to Python’ssubprocess library and byte strings should be expected back from it.

One of the main places in Ansible’s controller code that we interact withother programs is the connection plugins’ exec_command methods. Thesemethods transform any text strings they receive in the command (and argumentsto the command) to execute into bytes and return stdout and stderr as byte stringsHigher level functions (like action plugins’ _low_level_execute_command)transform the output into text strings.

Module string strategy: Native String

In modules we use a strategy known as Native Strings. This makes thingseasier on the community members who maintain so many of Ansible’smodules, by not breaking backwards compatibility bymandating that all strings inside of modules are text and converting betweentext and bytes at the borders.

Native strings refer to the type that Python uses when you specify a barestring literal:

"This is a native string"

In Python 2, these are byte strings. In Python 3 these are text strings. Modules should becoded to expect bytes on Python 2 and text on Python 3.

Module_utils string strategy: hybrid

In module_utils code we use a hybrid string strategy. Although Ansible’smodule_utils code is largely like module code, some pieces of it areused by the controller as well. So it needs to be compatible with modulesand with the controller’s assumptions, particularly the string strategy.The module_utils code attempts to accept native strings as inputto its functions and emit native strings as their output.

In module_utils code:

• Functions must accept string parameters as either text strings or byte strings.
• Functions may return either the same type of string as they were given or the native string type for the Python version they are run on.
• Functions that return strings must document whether they return strings of the same type as they were given or native strings.

Module-utils functions are therefore often very defensive in nature.They convert their string parameters into text (using ansible.module_utils._text.to_text)at the beginning of the function, do their work, and then convertthe return values into the native string type (using ansible.module_utils._text.to_native)or back to the string type that their parameters received.

Tips, tricks, and idioms for Python 2/Python 3 compatibility

Use forward-compatibility boilerplate

Use the following boilerplate code at the top of all python filesto make certain constructs act the same way on Python 2 and Python 3:

# Make coding more python3-ish
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type

metaclass = type makes all classes defined in the file into new-styleclasses without explicitly inheriting from object.

The future imports do the following:

See also

• PEP 0328: Absolute Imports
• PEP 0238: Division
• PEP 3105: Print function

Prefix byte strings with b_

Since mixing text and bytes types leads to tracebacks we want to be clearabout what variables hold text and what variables hold bytes. We do this byprefixing any variable holding bytes with b_. For instance:

filename = u'/var/tmp/café.txt'
b_filename = to_bytes(filename)
with open(b_filename) as f:
    data = f.read()

We do not prefix the text strings instead because we only operateon byte strings at the borders, so there are fewer variables that need bytesthan text.

Import Ansible’s bundled python-six library

The third-party python-six library existsto help projects create code that runs on both Python 2 and Python 3. Ansibleincludes a version of the library in module_utils so that other modules can use itwithout requiring that it is installed on the remote system. To make use ofit, import it like this:

from ansible.module_utils import six

Note

Ansible can also use a system copy of six

Ansible will use a system copy of six if the system copy is a laterversion than the one Ansible bundles.

Handle exceptions with as

In order for code to function on Python 2.6+ and Python 3, use thenew exception-catching syntax which uses the as keyword:

try:
    a = 2/0
except ValueError as e:
    module.fail_json(msg="Tried to divide by zero: %s" % e)

Do not use the following syntax as it will fail on every version of Python 3:

try:
    a = 2/0
except ValueError, e:
    module.fail_json(msg="Tried to divide by zero: %s" % e)

Update octal numbers

In Python 2.x, octal literals could be specified as 0755. In Python 3,octals must be specified as 0o755.

String formatting for controller code

Use str.format() for Python 2.6 compatibility

Starting in Python 2.6, strings gained a method called format() to putstrings together. However, one commonly used feature of format() wasn’tadded until Python 2.7, so you need to remember not to use it in Ansible code:

# Does not work in Python 2.6!
new_string = "Dear {}, Welcome to {}".format(username, location)
 
# Use this instead
new_string = "Dear {0}, Welcome to {1}".format(username, location)

Both of the format strings above map positional arguments of the format()method into the string. However, the first version doesn’t work inPython 2.6. Always remember to put numbers into the placeholders so the codeis compatible with Python 2.6.

See also

Python documentation on format strings

Use percent format with byte strings

In Python 3.x, byte strings do not have a format() method. However, itdoes have support for the older, percent-formatting.

b_command_line = b'ansible-playbook --become-user %s -K %s' % (user, playbook_file)

Note

Percent formatting added in Python 3.5

Percent formatting of byte strings was added back into Python 3 in 3.5.This isn’t a problem for us because Python 3.5 is our minimum version.However, if you happen to be testing Ansible code with Python 3.4 orearlier, you will find that the byte string formatting here won’t work.Upgrade to Python 3.5 to test.

See also

Python documentation on percent formatting

Testing modules on Python 3

Ansible modules are slightly harder to code to support Python 3 than normal code from otherprojects. A lot of mocking has to go into unit testing an Ansible module, soit’s harder to test that your changes have fixed everything or to to make surethat later commits haven’t regressed the Python 3 support. Review our testing pagesfor more information.