29. USING HOOKS

INGRESS (implicit) pyramid.tweens.excview_tween_factory (implicit) myapp.tween_factory1

myapp.tween_factory2 MAIN (implicit)

Specifying neither over nor under is equivalent to specifying under=INGRESS.

If all options for under (or over) cannot be found in the current configuration, it is an error. If some options are specified purely for compatibilty with other tweens, just add a fallback of MAIN or INGRESS. For example, under=(’someothertween’, ’someothertween2’, INGRESS). This constraint will require the tween to be located under both the ‘someothertween’ tween, the ‘someothertween2’ tween, and INGRESS. If any of these is not in the current configuration, this constraint will only organize itself based on the tweens that are present.

29.13.4 Explicit Tween Ordering

Implicit tween ordering is obviously only best-effort. Pyramid will attempt to provide an implicit order of tweens as best it can using hints provided by calls to add_tween(), but because it’s only best-effort, if very precise tween ordering is required, the only surefire way to get it is to use an explicit tween order. The deploying user can override the implicit tween inclusion and ordering implied by calls to add_tween() entirely by using the pyramid.tweens settings value. When used, this settings value must be a list of Python dotted names which will override the ordering (and inclusion) of tween factories in the implicit tween chain. For example:

1	[app:main]
2	use = egg:MyApp
3	pyramid.reload_templates = true
4	pyramid.debug_authorization = false
5	pyramid.debug_notfound = false
6	pyramid.debug_routematch = false
7	pyramid.debug_templates = true
8	pyramid.tweens = myapp.my_cool_tween_factory
9	pyramid.tweens.excview_tween_factory

In the above configuration, calls made during configuration to pyramid.config.Configurator.add_tween() are ignored, and the user is telling the system to use the tween factories he has listed in the pyramid.tweens configuration setting (each is a dotted Python name which points to a tween factory) instead of any tween factories added via pyramid.config.Configurator.add_tween(). The first tween factory in the

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29.13. REGISTERING “TWEENS”

pyramid.tweens list will be used as the producer of the effective Pyramid request handling function; it will wrap the tween factory declared directly “below” it, ad infinitum. The “main” Pyramid request handler is implicit, and always “at the bottom”.

latex-note.png

Pyramid’s own exception view handling logic is implemented as a tween factory function: pyramid.tweens.excview_tween_factory(). If Pyramid exception view handling is desired, and tween factories are specified via the pyramid.tweens configuration setting, the pyramid.tweens.excview_tween_factory() function must be added to the pyramid.tweens configuration setting list explicitly. If it is not present, Pyramid will not perform exception view handling.

29.13.5 Tween Conflicts and Ordering Cycles

Pyramid will prevent the same tween factory from being added to the tween chain more than once using configuration conflict detection. If you wish to add the same tween factory more than once in a configuration, you should either: a) use a tween factory that is a separate globally importable instance object from the factory that it conflicts with b) use a function or class as a tween factory with the same logic as the other tween factory it conflicts with but with a different

__name__ attribute or c) call pyramid.config.Configurator.commit() between calls to pyramid.config.Configurator.add_tween().

If a cycle is detected in implicit tween ordering when over and under are used in any call to “add_tween”, an exception will be raised at startup time.

29.13.6 Displaying Tween Ordering

The ptweens command-line utility can be used to report the current implict and explicit tween chains used by an application. See Displaying “Tweens”.

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29. USING HOOKS

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