timid 0.1.2

Templating and Expressions

Many actions and modifiers allow Jinja2-style templates to be specified for values, which enhances reusability of test description components. Jinja2-style expressions can also be used; an example would be the “when” modifier, which provides simple conditional control of an action. Template variables can be set on the command line, read from a YAML file, or set up directly in the test description.

Security

Timid provides a way to mark both template variables and environment variables as being “sensitive”. This is to allow security-sensitive data, such as usernames and passwords, to be used, while ensuring that that sensitive data is scrubbed from any verbose or debugging output from Timid itself. For template variables, this can only be done from the test description file, but environment variables can also be marked sensitive by listing them in the TIMID_SENSITIVE environment variable, separated by your system’s path separator character. (On UNIX and Linux systems, this character would be the “:” character.) The TIMID_SENSITIVE environment variable will also be present in the environment of any subordinate processes, updated with any additional environment variables marked as sensitive by the test description; this can be used by test scripts to omit sensitive information from the environment in debugging output.

Creating a New Action

Actions perform the actual test step. Creating a new action is a matter of extending timid.Action. In the new action class, the schema class attribute must be set to a JSONSchema description of the expected configuration; and the __call__() method, taking as its sole argument a context object, must be defined to implement the actual action; it should return an instance of timid.StepResult. The timid.Action class declares a __init__() method taking four arguments (a context object, the name of the action (the key read from the test description), the configuration for the action (the value for that key), and a step address object); it validates the configuration, then stores the last three arguments in the name, config, and step_addr attributes, respectively. (The context object should not be stored; it will be passed in to the __call__() method.)

There are two types of actions. By default, all timid.Action subclasses are instantiated while reading the test description, then their __call__() methods are invoked in order during the actual test run–or not invoked at all, if a syntax check is being performed. However, it is possible to create a “step action”, an action invoked immediately after it is read from the test description; this is used, for instance, to implement the “include” step, which reads steps from another file and inserts them in place of the “include” step. These are implemented by setting the step_action class attribute to True and having __call__() return a list of timid.Step objects, instead of a timid.StepResult object.

Creating a New Modifier

Modifiers modify a step in some fashion, such as by running the step in a loop or applying a conditional prior to invoking the step. Creating a new action is a matter of extending timid.Modifier. Like actions, the new subclass must have a schema class attribute set to a JSONSchema description of the expected configuration, and a __init__() method identical to that for timid.Action is also implemented; however, modifiers do not have a __call__() method, and the class attribute priority must be set to an integer value. The priority attribute controls the order in which modifiers are applied while running a step, with lower values invoked before higher values.

A modifier actually consists of a set of hook functions. The timid.Modifier superclass contains default implementations of these hook functions, so a developer need only override the ones needed to implement the modifier.

The first hook is the action_conf() hook, which takes 5 arguments: a context object, the class implementing the modified action, the name of the action (key read from the test description), the configuration for the action (the __init__() method receives the configuration for the modifier), and a step address object. The hook function must return the configuration that should be passed to the action class, giving the modifier the opportunity to alter the configuration.

The remaining two hooks are the pre_call() and post_call() methods, which are invoked prior to and after calling the action’s __call__() method, respectively. The pre_call() method can return a timid.StepResult object, which aborts further processing (including the call to the action) and proceeds with invoking any post_call() methods. The post_call() method receives the timid.StepResult object and can modify it or even replace it entirely, by returning a different object. The pre_call() method takes 4 arguments: a context object, a “pre_mod” list, a “post_mod” list, and the instance of the timid.Action subclass. The post_call() method gets 5 arguments: a context object, the result of the call (an instance of timid.StepResult), the instance of the timid.Action subclass, a “post_mod” list, and a “pre_mod” list. The “pre_mod” and “post_mod” lists are lists of timid.Modifier instances that have lower priority and higher priority, respectively. It should also be noted that post_call() is called in the inverse order of pre_call().

Context Objects

The context object passed to the actions and modifier methods provides several services throughout Timid. The verbose attribute contains an integer value controlling the verbosity of Timid’s output (0 means no output at all), and debug is a boolean indicating whether debugging is enabled. The variables attribute contains a dictionary of template variables, and environment contains the environment variables. (The environment dictionary-like object also allows control of the current working directory, by setting its cwd attribute, and its call() method should be used to invoke external programs.)

Timid provides an interface to Jinja2, and two utility methods on the context object facilitate this: the template() method takes a string and returns a callable of one argument that will render the template, and expression() works similarly for Jinja2 expressions. (It is safe to pass objects other than strings to these two methods as well; the result will still be a callable of one argument, but no template expansion will be performed.) The context object should be passed to the callable returned by template() and expression().

The usual way to use the template() and expression() methods is to override the __init__() method of the timid.Action or timid.Modifier subclass; the method should invoke the superclass’s version of __init__() (using a super() expression), and would then process the configuration, saving the callables produced by calling template() and expression(). Then, where the values are used in the action’s __call__() or the modifier’s hook methods, simply pass the context to the callable and use the result as the actual value to use.

Step Addresses

To aid debugging, each action or modifier has a step address object associated with it. The address has three attributes: the filename from which the step was read (fname); the 0-based index of the step within the file (idx); and the key for the list containing the steps (key). (This latter attribute will be None if the file was a simple list of steps.) The object also has a straightforward string representation which includes the filename, key, and step index (1-based; that is, if idx is 3, the string will identify the step as step 4).

Creating an Extension

Extensions are the most powerful extensibility mechanism in Timid. Creating one is a matter of extending timid.Extension and implementing the desired hook methods, similar to creating a new modifier, except that a Timid extension must implement an activate() class method if it actually intends to do anything. Additionally, a timid.Extension subclass must set the priority class attribute to a numerical value, just like a timid.Modifier subclass; extension hook functions will be called in the order dictated by the priorities.

The first hook method that an extension may implement is the prepare() method. This must be a class method, and will receive as its sole argument an argparse.ArgumentParser instance, which the extension may use to declare new command line options. All extensions will have their prepare() method called during Timid initialization.

Once the command line has been processed by argparse.ArgumentParser, each extension’s activate() method will be called with a context object and an argparse.Namespace containing the results of the command line processing. This method must also be a class method, and must return either None or an instance of the timid.Extension subclass; if it returns None, the extension is treated as inactive and no other hook methods will be called.

The remaining hook methods are all instance methods, called on the object returned by the activate() method. The read_steps() method is called with a context object and a list of timid.Step instances; the extension may perform any in-place modifications to the list of steps that are appropriate. The pre_step() and post_step() methods are called before and after executing a step, respectively; pre_step() is called with a context object, the timid.Step instance, and the index of the step, and may return a True value to cause the step to be skipped. The post_step() method is called with the same arguments, and a fourth argument, which will be a timid.StepResult object, which it may alter in place; the return value of post_step() is ignored. Note that post_step() is called in extension order, in contrast to the post_call() method of timid.Modifier instances.

The final hook function is the finalize() method, which is called just before the command line tool exits. It is called with a context object and the result, which will typically be None for success, or a text string indicating an error. (It could also be called with an Exception instance if an error occurred.) This method’s return value will replace the result.

Debugging Extensions

The implementation of extensions explicitly ignores exceptions raised by a given extension. This would make it difficult to debug a newly developed extension, so Timid provides a debugging mechanism: the TIMID_EXTENSION_DEBUG environment variable may be set to an integer value, with larger values resulting in more verbose debugging. If TIMID_EXTENSION_DEBUG is present in the environment with no value, or with a non-integer value, the debugging level will be set to 1; a debugging level of 0 (or any negative value) is exactly the same as if TIMID_EXTENSION_DEBUG was not present in the environment at all.

Note that this environment variable is checked directly from the environment, unlike the TIMID_SENSITIVE environment variable. This means that the value used by extension debugging cannot be altered by any instructions in the test description; only child processes can be affected by such instructions. Even command line environment variable manipulations are ignored for the purposes of extension debugging. This design decision was made so that debugging could be enabled before even calling the extension prepare() method, which is called before any argument processing is done, and thus prior to reading any test description files.