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How to Hook to Events

There are three different ways in which byexample can be extended:

  • define zones where to find examples
  • support new languages: how to find them and how to run them
  • perform arbitrary actions during the execution

byexample uses the concept of modules: a python file with some extension classes defined there. Modules can be loaded using --modules <dir> from the command line.

What extension classes will depend of what you want to extend or customize.

In this how-to we will see how to hook to events and perform arbitrary actions during the execution.

Check how to define new zones where to find examples and how to support new finders and languages for a how-to about the first two items.

Let’s show this by example.

How to perform arbitrary actions during the execution: Concern

During the execution of the whole set of examples, byexample will execute some callbacks or hooks at particular moments like before running an example or after it failed.

The set of hooks are collected into the Concern interface (also known as Cross-Cutting Concern).

You can create and add your own to extend the capabilities of byexample:

  • show the progress of the execution
  • log / report generation for export
  • log execution time history for future execution time prediction (estimate)
  • turn on/off debugging, coverage and profile facilities
  • others…

Eg: Dump the Script

Let’s imagine that we want to save in a file all the examples’ code without the expected strings nor anything else.

>>> from byexample.concern import Concern

>>> class DumpScript(Concern):
...     target = 'dump-script'
...     def start_run(self, examples, runners, filepath):
...         self.f = open(filepath + ".script")
...     def end_run(self, failed, user_aborted, crashed):
...         self.f.close()
...     def start_example(self, example, options):
...         self.f.write(example.source)

See the documentation of the class Concern in byexample/ to get a description of all the possible hooks and when they are called.

byexample uses this mechanism to generate a progress bar in byexample/modules/

Concurrency model

Each Concern instance will be created once during the setup of byexample and then it will be created once per job thread.

By default there is only one job thread but more threads can be added with the --jobs option.

If you want to share data among them you will have to use a thread-safe structures created by a sharer and store them in a namespace.

In the concurrency model documentation it is explained and in byexample/modules/ you can see a concrete example.

Changed in byexample 10.0.0. Before 10.0.0 you were forced to use multiprocessing by hand but in 10.0.0 the concurrency model is hidden so you cannot relay on multiprocessing because byexample may not use processes at all! sharer and namespace are objects that hide the details while allowing you to have the same power.

Concern initialization

If you extend Concern and decide to implement your own __init__, you must ensure that you call Concern’s __init__ method passing to it all the keyword-only arguments that you received.

Once done that, you can use the self.cfg property to access any configuration set in byexample including the flags/options set (self.cfg.options).

In the __init__ you can also change the value of target to something different. For a Concern this is typically used to enable/disable the concern object based on the configuration by just setting = "some string" (enable) or = None (disable).

See Extension initialization for more about this and some troubleshooting.

New in byexample 11.0.0: self.cfg was introduced.