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diff --git a/content/development/exposing_function_wsme.rst b/content/development/exposing_function_wsme.rst new file mode 100644 index 0000000..da97997 --- /dev/null +++ b/content/development/exposing_function_wsme.rst @@ -0,0 +1,200 @@ +Exposing function in python module using entry_points, WSME in a Flask webapp +############################################################################# + +:date: 2014-06-07 18:10 +:slug: entry_point_wsme.rst +:tags: python, programming, wsme, rest, setuptools +:author: copyninja +:summary: In this post I try to explain how to expose a function in a + python module as REST service using WSME from an Flask + application using setuptools entry_points feature. + +The heading might be ambiguous, but I couldn't figure out better +heading so let me start by explaining what I'm trying to solve here. + +Problem +------- + +I have a python module which contains a function which I want to +expose as a REST web service in a Flask application. I use WSME for +Flask application, which actually needs signature of function in +question and problem comes to picture because function to be exposed +is foreign to Flask application, it resides in separate python module. + + +Solution +-------- + +While reading `Julien Danjou's <http://julien.danjou.info/>`_ *Hackers +Guide To Python* book I came across the setuptools `entry_points` +concept which can be used to extend existing feature of a tool like +plug-ins. So here I'm going to use this `entry_points` feature from +setuptools to provide function in the module which can expose the +signature of function[s] to be exposed through REST. Of course this +means I need to modify module in question to write entry_points and +function for giving out signature of function to be exposed. + +I will explain this with small example. I have a dummy module which +provides a add function and a function which exposes the add functions +signature. + +.. code-block:: python + + + def add(a, b): + return a + b + + def expose_rest_func(): + return [add, int, int, int] + +This is stored in `dummy/__init__.py` file. I use `pbr` tool to +package my python module. Below is content of `setup.cfg` file. + +.. code-block:: python + + [metadata] + name = dummy + author = Vasudev Kamath + author-email = kamathvasudev@gmail.com + summary = Dummy module for testing purpose + version = 0.1 + license = MIT + description-file = + README.rst + requires-python = >= 2.7 + + [files] + packages = + dummy + + [entry_points] = + myapp.api.rest = + rest = dummy:expose_rest_func + + +The special thing in above file is `entry_points` section, which +defines function to be hooked into entry_point. In our case +entry_point `myapp.api.rest` is used by our Flask application to +interact with modules which expose them. The function which will be +got accessing the entry_point is `expose_rest_func` which gives the +function to be exposed its arg types and return types as a list. + +If we are looking at only supporting python3 it was sufficient to know +function name only and use `function annotations +<http://legacy.python.org/dev/peps/pep-3107>`_ in function +definition. Since I want to support both python2 and python3 this is +out of question. + +Now, just run the following command in virtualenv to get the module +installed. + +.. code-block:: shell-session + + PBR_VERSION=0.1 python setup.py sdist + pip install dist/dummy_module-0.1.tar.gz + +Now if you want to see if the module is exposing entry_point or not +just use `entry_point_inspector` tool after installing you will get a +command called `epi` if you run it as follows you should note the +dummy_module in its output + +.. code-block:: shell-session + + epi group list + +-----------------------------+ + | Name | + +-----------------------------+ + | cliff.formatter.completion | + | cliff.formatter.list | + | cliff.formatter.show | + | console_scripts | + | distutils.commands | + | distutils.setup_keywords | + | egg_info.writers | + | epi.commands | + | flake8.extension | + | setuptools.file_finders | + | setuptools.installation | + | myapp.api.rest | + | stevedore.example.formatter | + | stevedore.test.extension | + | wsme.protocols | + +-----------------------------+ + +So our entry_point is exposed now, we need to access it in our Flask +application and expose the function using WSME. It is done by below +code. + +.. code-block:: python + + from wsmeext.flask import signature + + import flask + import pkg_resources + + + def main(): + app = flask.Flask(__name__) + app.config['DEBUG'] = True + for entrypoint in pkg_resources.iter_entry_points('myapp.api.rest'): + # Ugly but fix is only supporting python3 + func_signature = entrypoint.load()() + app.route('/' + func_signature[0].__name__, methods=['POST'])( + signature(func_signature[-1], + *func_signature[1:-1])(func_signature[0])) + app.run() + + if __name__ == '__main__': + main() + +entry_point `myapp.api.rest` are iterated using the pkg_resources +package provided by setuptools, when I load the entry_point I get back +the function to be used which is called in same place to get function +signature. Then I'm calling Flask and WSME decorator functions (yeah +instead of decorating I'm using them directly over function to be +exposed). + +The code looks bit ugly at the place where I'm accessing list using +slices but I can't help it due to limitation of python2 with python3 +there is new packing and unpacking stuff which makes code look bit +more cooler see below. + +.. code-block:: python + + from wsmeext.flask import signature + + import flask + import pkg_resources + + + def main(): + app = flask.Flask(__name__) + app.config['DEBUG'] = True + for entrypoint in pkg_resources.iter_entry_points('silpa.api.rest'): + func, *args, rettype = entrypoint.load()() + app.route('/' + func.__name__, methods=['POST'])( + signature(rettype, *args)(func_signature[0])) + app.run() + + if __name__ == '__main__': + main() + +You can access the service at `http://localhost:5000/add` depending on +`Accept` header of HTTP you will get either XML or JSON response. If +you access it from browser you will get XML response. + +Usecase +------- + +Now if you are wondering what is the reason behind this experience, +this is for `SILPA Project <http://silpa.org.in>`_. I'm trying to +implement REST service for all Indic language computing module. Since +all these module are independent of SILPA which is a Flask web app I +had to find a way to achieve this, and this is what I came up with. + +Conclusion +---------- + +I'm not sure if there is any other approaches to achieve this, if +there I would love to hear about them. You can write your comments and +suggestion over `email <http://scr.im/vasudev>`_ |