Arvados

h1. Hacking Python SDK

{{toc}}

h2. Prerequisites

The FUSE driver requires associated libraries to build:

<pre>

sudo apt-get install libattr1-dev libfuse-dev pkg-config fuse

sudo adduser "$USER" fuse

sudo chmod g+rw /dev/fuse

sudo chown root:fuse /dev/fuse

</pre>

After installing @fuse@ and adding yourself to the @fuse@ group, you need to start a new login session. Make sure the @groups@ command reports that you're in the @fuse@ group.

h2. Get the source code

<pre>

cd

git clone https://github.com/curoverse/arvados.git

</pre>

h2. virtualenv

virtualenv helps you isolate the dependencies for a specific package or environment, much like Bundler does for our Rails applications.  The recommended way to deploy is to build a virtualenv for Arvados development.

To build the virtualenv, run:

<pre>

$ sudo apt-get install virtualenv

$ virtualenv --setuptools VENVDIR

</pre>

(@VENVDIR@ can be a directory anywhere you like, although best practice is to keep it outside your source directory.)

To set up the shell to use the isolated virtualenv environment, run:

<pre>

$ source VENVDIR/bin/activate

</pre>

To learn more about using and configuring virtualenv, read the "virtualenv usage documentation":https://virtualenv.pypa.io/en/latest/virtualenv.html#usage.

h2. Run tests

Strategy:

# Set up the environment to use a dedicated virtualenv

# Run the client library test suite

# Build a client library package and install it to the virtualenv

# Run the FUSE driver test suite

# Build a FUSE driver package and install it to the virtualenv

Note: The test suite brings up a Keep server and an API server to run tests against. For best results:

* Try [[Hacking Keep]] and [[Hacking API Server]] to make sure you have all the right dependencies for running the Keep and API servers.

* Make sure you have a blob_signing_key in services/api/config/application.yml

* Install the keepstore binary.

** Make sure your GOPATH points somewhere, e.g.: @export GOPATH=~/gocode; mkdir -p $GOPATH@

** Install keepstore: @go get git.curoverse.com/arvados.git/services/keepstore@

** (if you don't do anything special, this fetches "master" from git.curoverse.com -- if you want to build a version of keepstore with local modifications, see [[Hacking Keep]])

Script (make sure to edit the first line to refer to your virtualenv):

<pre>

source VENVDIR/bin/activate

cd ~/arvados/sdk/python

python setup.py test

python setup.py install

cd ~/arvados/services/fuse

python setup.py test

python setup.py install

</pre>

h3. Run a single test or test class

<pre>

source VENVDIR/bin/activate

cd ~/arvados/sdk/python

# One test module

python setup.py test --test-suite tests.test_keep_locator

# One test class

python setup.py test --test-suite tests.test_keep_locator.ArvadosKeepLocatorTest

# One test case

python setup.py test --test-suite tests.test_keep_locator.ArvadosKeepLocatorTest.base_locators

</pre>

h2. Builds and versioning

When we build each Python package, the version number is generated from the most recent commit that affected the package.  The format is @0.1.[commit's timestamp formatted as %Y%m%d%H%M%S].[commit's short hash]@.

If you want to make changes in one package and refer to it somewhere else (e.g., have the FUSE package depend on a specific SDK version), you should commit your changes to the original package before anything else.  Then you can consistently refer to the metadata from that dedicated commit in subsequent changes.

h2. Logging

The Python SDK uses Python's built-in logging module to log errors, warnings, and debug messages.  The arvados module sets up logging for messages under "arvados" based on local configuration (e.g., the @ARVADOS_DEBUG@ setting).  Other SDK modules and command-line tools should @import arvados@ and then send messages to a logger under "arvados" to ensure consistent log handling.  Typical setup looks like this:

<pre><code class="python">

import arvados

import logging

logger = logging.getLogger('arvados.YOURTHING')

</code></pre>

Once you've set this up, you can send messages to the logger using methods like @logger.debug()@ and @logger.error()@.  See the "Logger class documentation":https://docs.python.org/2/library/logging.html#logger-objects for full details.

Command-line scripts may reconfigure the @arvados.logger@ object based on additional configuration like command-line switches.  @services/fuse/bin/arv-mount@ demonstrates adjusting the level and setting a custom log handler.

h2. Documenting your code

See "PEP 257":https://www.python.org/dev/peps/pep-0257/ and "PEP 287":https://www.python.org/dev/peps/pep-0287/

h2. Python buffer protocol

Notes on managing buffers efficiently in Python, we don't use this in the python sdk as of this writing (but we might).

http://eli.thegreenplace.net/2011/11/28/less-copies-in-python-with-the-buffer-protocol-and-memoryviews/

Example using bytearray() to allocate a buffer, memoryview() to create a writable slice, and readinto() to write directly to the buffer slice:

<pre>

>>> b = bytearray(20)

>>> c = memoryview(b)

>>> f = open("python.txt", "r")

>>> f.readinto(c[5:10])

5

>>> b

bytearray(b"\x00\x00\x00\x00\x00I\'ve \x00\x00\x00\x00\x00\x00\x00\x00\x00\x00")

</pre>