understanding Pyramid Tweens

1. Tweens
Alex Conrad
@alexconrad
Code Monkey
at SurveyMonkey
PloneConf 2011
Nov 5, 2011

2. What are Pyramid Tweens?

3. omg tweens r soooo, like, totally awesome!!!!!!1!
lol
(totally friend me on twtr)
^^

4. They are NOT
pre-teenager girls
(too old for toys, too young for boys)

5. Pyramid Tweens were named
after the word “between”

6. Pyramid Tweens
are middlewares

7. Pyramid Tweens live between
Pyramid code and your application views

8. Pyramid Tweens are NOT
WSGI middlewares

10. REQUEST

11. REQUEST

12. So why use Tweens?

13. To apply a common behavior
on all requests

14. Think of them as decorators
applied to each of your Pyramid views.

15. @foo
@bar
def useless_view(request):
return {}

16. These are not
Tweens, but actual
@foo
decorators, ok?
@bar
def useless_view(request):
return {}

17. Tweens can be stacked together

18. Pyramid Tweens
vs.
WSGI Middlewares

19. WSGI
Tweens
middleware
Pipelining YES YES
Error catching YES YES
WSGI compliant YES NO
Access to
application NO YES
(registry, template
engine, ...)

20. How does it work?

21. Create a tween factory: a function

22. A tween factory takes 2 arguments:
- handler
- registry

23. A tween factory returns a function:
a tween
(just like a decorator does)

24. The tween function takes 1 argument:
- request
(just like a view would)

25. The tween function returns a response object
(just like a view would)

26. A tween factory is called
at application startup

27. A tween is called
per request

28. The tween factory decides
if the tween should be part of the tween stack

29. def useless_tween_factory(handler, registry):
if registry.settings['useless'] == '1':
def useless_tween(request):
response = handler(request)
return response
return useless_tween
return handler
handler: the next tween in the stack
registry: the pyramid registry

30. def useless_tween_factory(handler, registry):
if registry.settings['useless'] == '1':
def useless_tween(request):
response = handler(request)
return response
return useless_tween
return handler
handler: the next tween in the stack
registry: the pyramid registry
this is a tween

31. Tween positioning

32. REQUEST

33. REQUEST

34. REQUEST

35. REQUEST
---------------------- INGRESS ----------------------
------------------------ MAIN ------------------------

36. 3 ways of positioning tweens
- implicit
- implicit (hint)
- explicit

37. 3 ways of positioning tweens
- implicit
- implicit (hint)
- explicit

38. Implicit positioning
from pyramid.config import Configurator
# ... more imports
def main(global_config, **settings):
config = Configurator(...)
config.add_tween('useless.useless_tween_factory')
config.add_tween('useless.null_tween_factory')
# ... more stuff
return config.make_wsgi_app()

39. Implicit positioning
from pyramid.config import Configurator
# ... more imports
def main(global_config, **settings):
config = Configurator(...)
config.add_tween('useless.useless_tween_factory')
config.add_tween('useless.null_tween_factory')
# ... more stuff REQUEST
return config.make_wsgi_app()

40. INGRESS
useless.null_tween_factory
useless.useless_tween_factory
pyramid.tweens.excview_tween_factory
MAIN

41. REQUEST
INGRESS
useless.null_tween_factory
useless.useless_tween_factory
pyramid.tweens.excview_tween_factory
MAIN

42. 3 ways of positioning tweens
- implicit
- implicit (hint)
- explicit

43. Suggested positioning (hint)
config.add_tween('useless.useless_tween_factory')
config.add_tween('useless.null_tween_factory',
over=pyramid.tweens.MAIN,
under='useless.useless_tween_factory')

44. Tween constants
pyramid.tweens.INGRESS
pyramid.tweens.EXCVIEW
pyramid.tweens.MAIN

45. You cannot position a tween
before INGRESS (over)
nor after MAIN (under)

46. 3 ways of positioning tweens
- implicit
- implicit (hint)
- explicit

47. Explicit positioning
# development.ini
[app:main]
...
pyramid.tweens = useless.useless_tween_factory
useless.null_tween_factory
pyramid.tweens.excview_tween_factory

48. WARNING:
Explicit tween ordering
(via the .ini file)
will ignore any calls to
config.add_tween( … )

49. WARNING:
[app:main]
...
pyramid.includes = pyramid_debugtoolbar
pyramid.tweens = useless.useless_tween_factory
useless.null_tween_factory
pyramid.tweens.excview_tween_factory

50. WARNING:
[app:main]
...
pyramid.includes = pyramid_debugtoolbar
pyramid.tweens = useless.useless_tween_factory
useless.null_tween_factory
pyramid.tweens.excview_tween_factory

51. Explicitly tween positioning requires
that you declare ALL tweens.

52. Listing tweens

53. $ paster ptweens development.ini#main
Position Name
­­­­­­­­ ­­­­
­ INGRESS
0 useless.useless_tween_factory
1 useless.null_tween_factory
2 pyramid.tweens.excview_tween_factory
­ MAIN

54. Tweens you might be using:
- pyramid_debugtoolbar
- pyramid_tm

55. pyramid_debugtoolbar

56. Debug Toolbar renders and injects HTML
into your response

57. Although it can decide whether or not it should
render HTML depending on your host address

58. Transaction Manager
(pyramid_tm)

59. For every request,
it creates a new transaction
and closes it on response.

60. Transaction Manager
is NOT about database transactions

61. pyramid_tm relies on the transaction package

62. 1
Register your commit / rollback action with
the transaction package (join() the transaction)

63. 2
pyramid_tm calls:
transaction.begin() on request
then
transaction.commit() or transaction.rollback()

64. 3
the transaction package will then
call subscribed callbacks

65. Why is this useful?

66. One single point of commit.

67. Avoid sending an email
“Your account has been created! o/”
when your DB write actually failed

68. Things you could do:
- commit your DB transaction
- invalidate cache
- send an email
...

69. I already have try/except blocks in my Pyramid
views, I don't need pyramid_tm.
Until you get a stupid Exception outside your
block (e.g., in the renderer) for whatever reason.
Also know as: a bug.

70. Pyramid registers SQLAlchemy automatically
extension=ZopeTransactionExtension()

71. Did you know?
pyramid_tm is responsible
for the existence of Pyramid tweens.
It was the first use case for tweens.

72. Now you are tween experts!
Use wisely.

73. Thank you !!!
let's totally follow each other like,
on twitter!!!1! LOOOL :D
@alexconrad