Ansible Runner Python

The ansible-core code runs on both Python 2 and Python 3 because we want Ansible to be able to manage a widevariety of machines. Contributors to ansible-core and to Ansible Collections should be aware of the tips in this document so that they can write code that will run on the same versions of 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 well

Ansible Runner is a tool and python library that helps when interfacing with Ansible directly or as part of another system whether that be through a container image interface, as a standalone tool, or as a Python module that can be. This project wraps the ansiblerunner interface inside a REST API enabling ansible playbooks to be executed and queried from other platforms. The incentive for this is two-fold; provide Ansible integration to non-python projects.

Ansible Runner is a tool and python library that helps when interfacing with Ansible directly or as part of another system whether that be through a container image interface, as a standalone tool, or as a Python module that can be imported. The goal is to provide a stable and consistent interface abstraction to Ansible. Ansible Runner is a tool and python library that helps when interfacing with Ansible directly or as part of another system whether that be through a container image interface, as a standalone tool, or as a Python module that can be.

However, the three types of code do not use the same string strategy. If you’re developing a module or some module_utils code, be sure to read the section on string strategy carefully.

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.

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

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.

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.

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.

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:

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:

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.

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:

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:

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.

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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.

Ansible Runner Python Download

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:

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.

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.common.text.converters.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.common.text.converters.to_native)or back to the string type that their parameters received.

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:

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

The __future__ imports do the following:

absolute_import

Makes imports look in sys.path for the modules beingimported, skipping the directory in which the module doing the importinglives. If the code wants to use the directory in which the module doingthe importing, there’s a new dot notation to do so.

division

Makes division of integers always return a float. If you need tofind the quotient use x//y instead of x/y.

print_function

Changes print from a keyword into a function.

Ansible Run Python

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.

SUMMARY

At this moment, Ansible would fail to run on most remote hosts that do only have python3 installed by default, like Fedora 27 or newer, unless the user exclicitely adds the ansible_python_interpreter=python3 inside the inventory.

This is a real issue because it assumes that the user already knows which is the default ansible interpreter on that host, which may not be something he knows or even controls. Think about dynamic inventories or playbooks that would upgrade a host from a version that had python2 to one that that has python3.

ISSUE TYPE

Feature Idea

COMPONENT NAME

core

ADDITIONAL INFORMATION

This issue was aggravated by the recommandation made https://www.python.org/dev/peps/pep-0394/ which indirectly made newer distributions which ship with python3 by default to not create a symlink from python to python3, even when there was no python2 installed.

As it seems highly unlikely to be able to update the PEP with new recomandations, we should look into findina solution for ansible, one that does not cripple the UX and avoids the need to tune ansible_python_interpreter variable for each host. This should happen under the hood by detecting a missing interpreter and falling back to python3 one (caching may be desired too).