0
Daniel Greenfeld
pydanny.com / @pydanny
Daniel Greenfeld
Thinking Hard
About Python
Daniel Greenfeld
pydanny.com / @pydanny
@pydanny
Daniel Greenfeld
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http://2scoops.org
Danny: 128,546++
Audrey: 121,871++
Daniel Greenfeld
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What I
want you
to think
of me. One Cartwheel of Many Around the World
Daniel Greenfeld
pydanny.com / @pydanny
What
I’m
really
like. Myself at 13 in front of the Apple ][
Daniel Greenfeld
pydanny.com / @pydanny
Overview
Daniel Greenfeld
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Coding
into
Trouble
Daniel Greenfeld
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Controversy
Daniel Greenfeld
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Exceptions
Daniel Greenfeld
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Avoiding
Technical
Debt
Daniel Greenfeld
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...first section...
Coding
into
Trouble
Daniel Greenfeld
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a.k.a.
Super()
Troubles
Daniel Greenfeld
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Circle
The super method calls the
parent class, which is Circle
import math
class ...
Daniel Greenfeld
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Contention
The super() method can create ambiguity.
Daniel Greenfeld
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Example:
Django
Daniel Greenfeld
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Class Based Generic Views
• Composition
• Inheritance
• Subclassing
• Polymorphism...
Daniel Greenfeld
pydanny.com / @pydanny
However...
Daniel Greenfeld
pydanny.com / @pydanny
Things I don’t know:
The ancestor chain for
django.views.generic.edit.UpdateView
Daniel Greenfeld
pydanny.com / @pydanny
django.views.generic.edit.UpdateView
django.views.generic.detail.SingleObjectTempl...
Daniel Greenfeld
pydanny.com / @pydanny
def form_valid(self, form):
verb_form = verb_form_base(self.request.POST)
if verb_...
Daniel Greenfeld
pydanny.com / @pydanny
class ActionUpdateView(
LoginRequiredMixin, # django-braces
ActionBaseView, # inhe...
Daniel Greenfeld
pydanny.com / @pydanny
from actions.views import ActionUpdateView
for x in ActionUpdateView.mro():
print(...
Daniel Greenfeld
pydanny.com / @pydanny
Ancestor Chain
(MRO)
<class 'actions.views.ActionUpdateView'>
<class 'braces.views...
Daniel Greenfeld
pydanny.com / @pydanny
from actions.views import ActionUpdateView
for x in [x for x in ActionUpdateView.m...
Daniel Greenfeld
pydanny.com / @pydanny
Ancestor Chain
(MRO) of
<class 'actions.views.ActionUpdateView'>
<class 'django.vi...
Daniel Greenfeld
pydanny.com / @pydanny
Whew!
Daniel Greenfeld
pydanny.com / @pydanny
Safe!
Daniel Greenfeld
pydanny.com / @pydanny
If you’re not careful,
super can cause subtle
inheritance/MRO
problems.
Daniel Greenfeld
pydanny.com / @pydanny
• Hope that anyone else maintaining this
project isn’t going to kill me.
• Convert...
Daniel Greenfeld
pydanny.com / @pydanny
Write a easy-to-use
MRO inspector thingee
that identifies the parent
attributes/me...
Controversy
Daniel Greenfeld
pydanny.com / @pydanny
Special cases aren’t special enough to
break the rules.
Although practicality beat...
Daniel Greenfeld
pydanny.com / @pydanny
Zen of Python
$ python -c “import this”
The Zen of Python, by Tim Peters
Beautiful...
Daniel Greenfeld
pydanny.com / @pydanny
Special cases aren’t special enough to
break the rules.
Although practicality beat...
Daniel Greenfeld
pydanny.com / @pydanny
Web2py
Often honors Implicit over Explicit
Follows its own namespace pattern
Daniel Greenfeld
pydanny.com / @pydanny
# encoding: utf-8
# https://github.com/mdipierro/evote/blob/master/models/menu.py
...
Daniel Greenfeld
pydanny.com / @pydanny
# encoding: utf-8
# https://github.com/mdipierro/evote/blob/master/models/menu.py
...
Daniel Greenfeld
pydanny.com / @pydanny
# encoding: utf-8
# https://github.com/mdipierro/evote/blob/master/models/menu.py
...
Daniel Greenfeld
pydanny.com / @pydanny
Web2py code sample
# encoding: utf-8
# https://github.com/mdipierro/evote/blob/mas...
Daniel Greenfeld
pydanny.com / @pydanny
Contention
• Explicit is better than implicit
• In the name of ambiguity, refuse t...
Daniel Greenfeld
pydanny.com / @pydanny
Controversy
Special cases aren’t special enough to break the rules.
Although pract...
Daniel Greenfeld
pydanny.com / @pydanny
Special cases aren’t special enough to break the rules.
Although practicality beat...
Daniel Greenfeld
pydanny.com / @pydanny
Web2py contends:
• Implicit behaviors means Web2py is easier for
beginners to lear...
Daniel Greenfeld
pydanny.com / @pydanny
And that’s okay
Web2py will always
be contentious
Web2py argues practicality
in so...
Daniel Greenfeld
pydanny.com / @pydanny
Flask and its global Request object
http://bit.ly/flask-requests
A Little Magic
Goe...
Daniel Greenfeld
pydanny.com / @pydanny
Flask and its global Request object
http://bit.ly/flask-requests
A Little Magic
Goe...
Exceptions
Silent
Exceptions
are the
Devil
Daniel Greenfeld
pydanny.com / @pydanny
Exceptions
Errors should never pass silently.
Unless explicitly silenced.*
* Zen o...
Daniel Greenfeld
pydanny.com / @pydanny
djangopackages.com
• Once a day iterates across all
packages.
• Updates the metada...
Daniel Greenfeld
pydanny.com / @pydanny
Django Packages
• Sometimes the APIs go down.
• Sometimes the APIs change.
• Somet...
Daniel Greenfeld
pydanny.com / @pydanny
Old package_updater.py
...
for package in Package.objects.all():
try:
package.fetc...
Daniel Greenfeld
pydanny.com / @pydanny
What I was doing
>>> try:
... a = b
... except Exception as e:
... print(e)
...
na...
Daniel Greenfeld
pydanny.com / @pydanny
What I wanted
>>> a = b
Traceback (most recent call last):
File "<stdin>", line 1,...
Daniel Greenfeld
pydanny.com / @pydanny
Exceptions
Errors should never pass silently.
Unless explicitly silenced.*
My code...
Daniel Greenfeld
pydanny.com / @pydanny
Getting what I want
>>> class CustomErrorHandler(Exception):
... def __init__(self...
Daniel Greenfeld
pydanny.com / @pydanny
PackageUpdaterException
Nice message
Full traceback
All errors
caught
class Packag...
Daniel Greenfeld
pydanny.com / @pydanny
Exceptions
Errors should never pass silently.
Unless explicitly silenced.
My code ...
Daniel Greenfeld
pydanny.com / @pydanny
Exceptions
Errors should never pass silently.
Unless explicitly silenced.
Daniel Greenfeld
pydanny.com / @pydanny
Next
up...
The Dutch Way
Daniel Greenfeld
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Decorators
@memoize
def allcaps(string):
return string.upper()
def allcaps(string)...
Daniel Greenfeld
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I am Zen
Decorators == Zen of Python
Daniel Greenfeld
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Until...
Daniel Greenfeld
pydanny.com / @pydanny
I am not Zen
I need to write a decorator.
Daniel Greenfeld
pydanny.com / @pydanny
You try to shoot yourself in the foot, only
to realize there’s no need, since Guid...
Daniel Greenfeld
pydanny.com / @pydanny
Decorators
@memoize
def allcaps(string):
return string.upper()
def allcaps(string)...
Daniel Greenfeld
pydanny.com / @pydanny
Decorator Template
http://pydanny-event-notes.readthedocs.org/en/latest/SCALE10x/p...
Daniel Greenfeld
pydanny.com / @pydanny
The Dutch Way
There should be one-- and preferably only one --obvious way to do it...
Daniel Greenfeld
pydanny.com / @pydanny
Decorator
implementation
def memoize(func):
cache = {}
def memoized(*args):
if arg...
Daniel Greenfeld
pydanny.com / @pydanny
Whew.
Daniel Greenfeld
pydanny.com / @pydanny
What about decorators
that accept arguments?
Daniel Greenfeld
pydanny.com / @pydanny
Oh No.
Daniel Greenfeld
pydanny.com / @pydanny
Explaining this is
Hard.
That’s because we create a decorator that creates a
param...
Daniel Greenfeld
pydanny.com / @pydanny
multiplier decorator
def multiplier(multiple):
def decorator(function):
def wrappe...
Daniel Greenfeld
pydanny.com / @pydanny
Whew
Daniel Greenfeld
pydanny.com / @pydanny
Oh No.
Daniel Greenfeld
pydanny.com / @pydanny
Not Done Yet!
Daniel Greenfeld
pydanny.com / @pydanny
authentication
decorator
@authorization('admin')
def do_admin_thing(user):
# do so...
Daniel Greenfeld
pydanny.com / @pydanny
Whew
Daniel Greenfeld
pydanny.com / @pydanny
Really.
Daniel Greenfeld
pydanny.com / @pydanny
I’m not doing
class decorators.
Daniel Greenfeld
pydanny.com / @pydanny
It is not easy
to explain how
to write decorators.
Daniel Greenfeld
pydanny.com / @pydanny
Contention
While Using
decorators is
Zen...
Daniel Greenfeld
pydanny.com / @pydanny
Contention
Writing
Decorators
is Not.
Daniel Greenfeld
pydanny.com / @pydanny
Deep Thought
There should be one-- and preferably only one --obvious way to do it....
Daniel Greenfeld
pydanny.com / @pydanny
Use the
decorator library
https://pypi.python.org/pypi/decorator
Avoiding
Technical
Debt
Part I
Daniel Greenfeld
pydanny.com / @pydanny
Getting it done
vs.
Technical debt
Now is better than never.
Although never is oft...
Daniel Greenfeld
pydanny.com / @pydanny
• Tests
• Documentation
Some things take time
Risk: Deploying broken code
Risk: pr...
Daniel Greenfeld
pydanny.com / @pydanny
Easy Test Patterns
• Always make sure your test harness can
run
• Try using tests ...
Daniel Greenfeld
pydanny.com / @pydanny
Must-have
Documentation
• Installation/Deployment procedures
• Coding standards
• ...
Daniel Greenfeld
pydanny.com / @pydanny
Easy Test Patterns
• Always make sure your test harness can run
• Try using tests ...
Daniel Greenfeld
pydanny.com / @pydanny
Getting technical
again...
Avoiding
Technical
Debt
Part II
Daniel Greenfeld
pydanny.com / @pydanny
Namespaces
• Extremely powerful
• Useful
• Precise
import re
import os
from twiste...
Daniel Greenfeld
pydanny.com / @pydanny
import * makes
development faster[1]
• Extremely powerful
• Useful
• Imports every...
Daniel Greenfeld
pydanny.com / @pydanny
Comparing two modules
def compare(mod1, mod2):
title = 'nComparing {0}, {1}:'.form...
Daniel Greenfeld
pydanny.com / @pydanny
>>> re.sys == os.sys
True
>>> re.error == os.error
False
Comparing two modules
>>>...
Daniel Greenfeld
pydanny.com / @pydanny
Breaking built-ins
def compare_builtins(mod1):
print("nComparing {0} to builtins:"...
Daniel Greenfeld
pydanny.com / @pydanny
Breaking built-ins
>>> compare_builtins(re)
Comparing re to builtins:
* GLOBAL: co...
Daniel Greenfeld
pydanny.com / @pydanny
The open() story
from os import *after
before
Breaks
all
the
things!
Help on built...
Daniel Greenfeld
pydanny.com / @pydanny
Beginner pro-tip
Be careful of tutorials that use import *.
Daniel Greenfeld
pydanny.com / @pydanny
Contention
import * is not for beginners.
import * is people who really know Pytho...
Summary
Daniel Greenfeld
pydanny.com / @pydanny
Stay
this
person
Myself at 13 in front of the Apple ][
Daniel Greenfeld
pydanny.com / @pydanny
Admit What
You Don't
Know
Daniel Greenfeld
pydanny.com / @pydanny
Stay out of
your comfort
Zone
Daniel Greenfeld
pydanny.com / @pydanny
Grow
Daniel Greenfeld
pydanny.com / @pydanny
What I Want To Know
• Twisted
• Numpy
• SciPy
• Tulip
• C
• Etc.
Daniel Greenfeld
pydanny.com / @pydanny
If I continue
to Learn
Daniel Greenfeld
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I Get
To Be
This
Person
Daniel Greenfeld
pydanny.com / @pydanny
Think
Hard
Daniel Greenfeld
pydanny.com / @pydanny
Thank you
• Armin Ronacher
• nephila.it
• Richard Jones
• Raymond Hettiger
• EuroP...
Daniel Greenfeld
pydanny.com / @pydanny
Thank you
• Matt Harrison
• Ola Sendecka
• Kenneth Love
• Lennart Regebro
• Paul H...
Daniel Greenfeld
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One More
Thing...
Daniel Greenfeld
pydanny.com / @pydanny
Finis
Daniel Greenfeld
pydanny.com / @pydanny
Q & A
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Thinking hard about_python

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My keynote speech from EuroPython, this talk explores what it is like being a developer in a community filled with experts from around the world. The goal of the talk is to provide useful content for beginners and topics of discussion for more advanced developers, while also focusing on Python’s strengths. Video of this talk is at http://www.youtube.com/watch?v=7TImWbnUDeI

Published in: Technology

Transcript of "Thinking hard about_python"

  1. 1. Daniel Greenfeld pydanny.com / @pydanny Daniel Greenfeld Thinking Hard About Python
  2. 2. Daniel Greenfeld pydanny.com / @pydanny @pydanny
  3. 3. Daniel Greenfeld pydanny.com / @pydanny http://2scoops.org Danny: 128,546++ Audrey: 121,871++
  4. 4. Daniel Greenfeld pydanny.com / @pydanny What I want you to think of me. One Cartwheel of Many Around the World
  5. 5. Daniel Greenfeld pydanny.com / @pydanny What I’m really like. Myself at 13 in front of the Apple ][
  6. 6. Daniel Greenfeld pydanny.com / @pydanny Overview
  7. 7. Daniel Greenfeld pydanny.com / @pydanny Coding into Trouble
  8. 8. Daniel Greenfeld pydanny.com / @pydanny Controversy
  9. 9. Daniel Greenfeld pydanny.com / @pydanny Exceptions
  10. 10. Daniel Greenfeld pydanny.com / @pydanny Avoiding Technical Debt
  11. 11. Daniel Greenfeld pydanny.com / @pydanny ...first section...
  12. 12. Coding into Trouble
  13. 13. Daniel Greenfeld pydanny.com / @pydanny a.k.a.
  14. 14. Super() Troubles
  15. 15. Daniel Greenfeld pydanny.com / @pydanny Circle The super method calls the parent class, which is Circle import math class Circle(object): def __init__(self, radius): self.radius = radius def area(self): return self.radius ** 2 *math.pi def __repr__(self): return '{0} as area {1}'.format( self.__class__.__name__, self.area() ) class Donut(Circle): def __init__(self, outer, inner): super().__init__(outer) self.inner = inner def area(self): outer, inner = self.radius, self.inner return Circle(outer).area() - Circle(inner).area() What if our inheritance isn’t simple? >>> Circle(10) Circle as area 314.159265359 >>> Donut(10, 5) 235.619449019 Superclassing is so easy!
  16. 16. Daniel Greenfeld pydanny.com / @pydanny Contention The super() method can create ambiguity.
  17. 17. Daniel Greenfeld pydanny.com / @pydanny Example: Django
  18. 18. Daniel Greenfeld pydanny.com / @pydanny Class Based Generic Views • Composition • Inheritance • Subclassing • Polymorphism • Lots of other big words used to impress other developers, students, your boss, your doctor, Capoiera mestre, dog, cat, friends, family, and other people who generally don’t care about such things.
  19. 19. Daniel Greenfeld pydanny.com / @pydanny However...
  20. 20. Daniel Greenfeld pydanny.com / @pydanny Things I don’t know: The ancestor chain for django.views.generic.edit.UpdateView
  21. 21. Daniel Greenfeld pydanny.com / @pydanny django.views.generic.edit.UpdateView django.views.generic.detail.SingleObjectTemplateResponseMixin django.views.generic.base.TemplateResponseMixin django.views.generic.edit.BaseUpdateView django.views.generic.edit.ModelFormMixin django.views.generic.edit.FormMixin django.views.generic.detail.SingleObjectMixin django.views.generic.edit.ProcessFormView django.views.generic.base.View The ancestor chain for django.views.generic.edit.UpdateView:
  22. 22. Daniel Greenfeld pydanny.com / @pydanny def form_valid(self, form): verb_form = verb_form_base(self.request.POST) if verb_form.is_valid(): form.instance.verb_attributes = verb_form.cleaned_data return super().form_valid(form) A form_valid() implementation OMG Which form_valid() am I calling?
  23. 23. Daniel Greenfeld pydanny.com / @pydanny class ActionUpdateView( LoginRequiredMixin, # django-braces ActionBaseView, # inherits from AuthorizedForProtocolMixin AuthorizedforProtocolEditMixin, # Checks rights on edit views VerbBaseView, # Gets one of 200+ verb forms UpdateView): # django.views.generic.BaseView def form_valid(self, form): verb_form = verb_form_base(self.request.POST) if verb_form.is_valid(): form.instance.verb_attributes = verb_form.cleaned_data return super().form_valid(form) A form_valid() implementationOMG! OMG! OMG!
  24. 24. Daniel Greenfeld pydanny.com / @pydanny from actions.views import ActionUpdateView for x in ActionUpdateView.mro(): print(x) Ancestor Chain (MRO) of ActionUpdateView MRO = Method Resolution Order Print the MRO
  25. 25. Daniel Greenfeld pydanny.com / @pydanny Ancestor Chain (MRO) <class 'actions.views.ActionUpdateView'> <class 'braces.views.LoginRequiredMixin'> <class 'actions.views.ActionBaseView'> <class 'core.views.AuthorizedForProtocolMixin'> <class 'core.views.AuthorizedforProtocolEditMixin'> <class 'verbs.views.VerbBaseView'> <class 'django.views.generic.edit.UpdateView'> <class 'django.views.generic.detail.SingleObjectTemplateResponseMixin'> <class 'django.views.generic.base.TemplateResponseMixin'> <class 'django.views.generic.edit.BaseUpdateView'> <class 'django.views.generic.edit.ModelFormMixin'> <class 'django.views.generic.edit.FormMixin'> <class 'django.views.generic.detail.SingleObjectMixin'> <class 'django.views.generic.edit.ProcessFormView'> <class 'django.views.generic.base.View'> <type 'object'>
  26. 26. Daniel Greenfeld pydanny.com / @pydanny from actions.views import ActionUpdateView for x in [x for x in ActionUpdateView.mro() if hasattr(x, "form_valid")]: print(x) Ancestor Chain (MRO) of ActionUpdateView Filter the MRO list to only include classes with a form_valid() nethod
  27. 27. Daniel Greenfeld pydanny.com / @pydanny Ancestor Chain (MRO) of <class 'actions.views.ActionUpdateView'> <class 'django.views.generic.edit.UpdateView'> <class 'django.views.generic.edit.BaseUpdateView'> <class 'django.views.generic.edit.ModelFormMixin'> <class 'django.views.generic.edit.FormMixin'> super’s chosen form_valid() ancestor Current class
  28. 28. Daniel Greenfeld pydanny.com / @pydanny Whew!
  29. 29. Daniel Greenfeld pydanny.com / @pydanny Safe!
  30. 30. Daniel Greenfeld pydanny.com / @pydanny If you’re not careful, super can cause subtle inheritance/MRO problems.
  31. 31. Daniel Greenfeld pydanny.com / @pydanny • Hope that anyone else maintaining this project isn’t going to kill me. • Convert to a functional view. • Explore better patterns. Possible mitigations for this view. • return UpdateView.form_valid(self, form)
  32. 32. Daniel Greenfeld pydanny.com / @pydanny Write a easy-to-use MRO inspector thingee that identifies the parent attributes/methods specified by the coder. TODO
  33. 33. Controversy
  34. 34. Daniel Greenfeld pydanny.com / @pydanny Special cases aren’t special enough to break the rules. Although practicality beats purity.* * Zen of Python, lines 8 and 9
  35. 35. Daniel Greenfeld pydanny.com / @pydanny Zen of Python $ python -c “import this” The Zen of Python, by Tim Peters Beautiful is better than ugly. Explicit is better than implicit. Simple is better than complex. Complex is better than complicated. Flat is better than nested. Sparse is better than dense. Readability counts. Special cases aren't special enough to break the rules. Although practicality beats purity. Errors should never pass silently. Unless explicitly silenced. In the face of ambiguity, refuse the temptation to guess. There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you're Dutch. Now is better than never. Although never is often better than *right* now. If the implementation is hard to explain, it's a bad idea. If the implementation is easy to explain, it may be a good idea. Namespaces are one honking great idea -- let's do more of those! PEP-0020
  36. 36. Daniel Greenfeld pydanny.com / @pydanny Special cases aren’t special enough to break the rules. Although practicality beats purity.* * Zen of Python, lines 8 and 9
  37. 37. Daniel Greenfeld pydanny.com / @pydanny Web2py Often honors Implicit over Explicit Follows its own namespace pattern
  38. 38. Daniel Greenfeld pydanny.com / @pydanny # encoding: utf-8 # https://github.com/mdipierro/evote/blob/master/models/menu.py # this file is released under public domain and # you can use without limitations response.title = 'Voting Service' response.subtitle = None ## read more at http://dev.w3.org/html5/markup/meta.name.html response.meta.author = 'Your Name <you@example.com>' response.meta.description = 'a cool new app' response.meta.keywords = 'web2py, python, framework' response.meta.generator = 'Web2py Web Framework' # snip more content that I cut in the name of brevity Web2py code sample
  39. 39. Daniel Greenfeld pydanny.com / @pydanny # encoding: utf-8 # https://github.com/mdipierro/evote/blob/master/models/menu.py # this file is released under public domain and # you can use without limitations response.title = 'Voting Service' response.subtitle = None ## read more at http://dev.w3.org/html5/markup/meta.name.html response.meta.author = 'Your Name <you@example.com>' response.meta.description = 'a cool new app' response.meta.keywords = 'web2py, python, framework' response.meta.generator = 'Web2py Web Framework' # snip more content that I cut in the name of brevity Web2py code sample I GET IT NOW Europe taught me why unicode is important.
  40. 40. Daniel Greenfeld pydanny.com / @pydanny # encoding: utf-8 # https://github.com/mdipierro/evote/blob/master/models/menu.py # this file is released under public domain and # you can use without limitations response.title = 'Voting Service' response.subtitle = None ## read more at http://dev.w3.org/html5/markup/meta.name.html response.meta.author = 'Your Name <you@example.com>' response.meta.description = 'a cool new app' response.meta.keywords = 'web2py, python, framework' response.meta.generator = 'Web2py Web Framework' # snip more content that I cut in the name of brevity Web2py code sample OK, Back to the talk...
  41. 41. Daniel Greenfeld pydanny.com / @pydanny Web2py code sample # encoding: utf-8 # https://github.com/mdipierro/evote/blob/master/models/menu.py # this file is released under public domain and # you can use without limitations response.title = 'Voting Service' response.subtitle = None ## read more at http://dev.w3.org/html5/markup/meta.name.html response.meta.author = 'Your Name <you@example.com>' response.meta.description = 'a cool new app' response.meta.keywords = 'web2py, python, framework' response.meta.generator = 'Web2py Web Framework' # snip more content that I cut in the name of brevity Response object magically exists. No import necessary What can I expect in any location?What about namespace pollution? Written by Massimo himself
  42. 42. Daniel Greenfeld pydanny.com / @pydanny Contention • Explicit is better than implicit • In the name of ambiguity, refuse the temptation to guess • Namespaces are one honking great idea -- let's do more of those! Web2py violates these 3 koans: * Zen of Python, lines 2, 12, 19
  43. 43. Daniel Greenfeld pydanny.com / @pydanny Controversy Special cases aren’t special enough to break the rules. Although practicality beats purity.* * Zen of Python, lines 8, 9
  44. 44. Daniel Greenfeld pydanny.com / @pydanny Special cases aren’t special enough to break the rules. Although practicality beats purity.* Web2py contends: * Zen of Python, lines 8, 9
  45. 45. Daniel Greenfeld pydanny.com / @pydanny Web2py contends: • Implicit behaviors means Web2py is easier for beginners to learn. • The Web2py namespace pattern is easy to learn. • For experienced developers, commonly repeated imports are boilerplate. Note:This is my interpretation of Web2py design considerations. Personal side note:Web2py is very easy to install.
  46. 46. Daniel Greenfeld pydanny.com / @pydanny And that’s okay Web2py will always be contentious Web2py argues practicality in some very specific places. Controversy Special cases aren’t special enough to break the rules. Although practicality beats purity.
  47. 47. Daniel Greenfeld pydanny.com / @pydanny Flask and its global Request object http://bit.ly/flask-requests A Little Magic Goes a Long Way
  48. 48. Daniel Greenfeld pydanny.com / @pydanny Flask and its global Request object http://bit.ly/flask-requests A Little Magic Goes a Long Way
  49. 49. Exceptions
  50. 50. Silent Exceptions are the Devil
  51. 51. Daniel Greenfeld pydanny.com / @pydanny Exceptions Errors should never pass silently. Unless explicitly silenced.* * Zen of Python, lines 10 and 11
  52. 52. Daniel Greenfeld pydanny.com / @pydanny djangopackages.com • Once a day iterates across all packages. • Updates the metadata from: • Github: • Bitbucket • PyPI
  53. 53. Daniel Greenfeld pydanny.com / @pydanny Django Packages • Sometimes the APIs go down. • Sometimes the APIs change. • Sometimes projects get deleted. • Sometimes the Internets fail Problems Catch and report exceptions!
  54. 54. Daniel Greenfeld pydanny.com / @pydanny Old package_updater.py ... for package in Package.objects.all(): try: package.fetch_metadata() package.fetch_commits() except socket_error, e: text += "nFor '%s', threw a socket_error: %s" % (package.title, e) continue # snip lots of other exceptions except Exception as e: text += "nFor '%s', General Exception: %s" % (package.title, e) continue # email later https://github.com/opencomparison/opencomparison/blob/master/package/management/commands/package_updater.py http://bit.ly/Q8v9xk Um...
  55. 55. Daniel Greenfeld pydanny.com / @pydanny What I was doing >>> try: ... a = b ... except Exception as e: ... print(e) ... name 'b' is not defined What’s the error type?!? Where is my stack trace?!? (and it’s wrong)
  56. 56. Daniel Greenfeld pydanny.com / @pydanny What I wanted >>> a = b Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'b' is not defined Traceback Error type Error message
  57. 57. Daniel Greenfeld pydanny.com / @pydanny Exceptions Errors should never pass silently. Unless explicitly silenced.* My code is nearly silent I’ve silenced things for no good reason * Zen of Python, lines 10 and 11
  58. 58. Daniel Greenfeld pydanny.com / @pydanny Getting what I want >>> class CustomErrorHandler(Exception): ... def __init__(self, error): ... print(error) ... print(type(error)) ... >>> try: ... a=b ... except Exception as e: ... raise CustomErrorHandler(e) ... name 'b' is not defined Traceback (most recent call last): File "<stdin>", line 4, in <module> __main__.CustomErrorHandler NameError Traceback Error message For this example print == log No color because it’s a print statement Error Type
  59. 59. Daniel Greenfeld pydanny.com / @pydanny PackageUpdaterException Nice message Full traceback All errors caught class PackageUpdaterException(Exception): def __init__(self, error, title): log_message = "For {title}, {error_type}: {error}".format( title=title, error_type=type(error), error=error ) logging.error(log_message) logging.exception(error) for package in Package.objects.all(): try: try: package.fetch_metadata() package.fetch_commits() except Exception as e: raise PackageUpdaterException(e, package.title) except PackageUpdaterException: continue Loop forward
  60. 60. Daniel Greenfeld pydanny.com / @pydanny Exceptions Errors should never pass silently. Unless explicitly silenced. My code is nearly silent I’ve silenced things for no good reason
  61. 61. Daniel Greenfeld pydanny.com / @pydanny Exceptions Errors should never pass silently. Unless explicitly silenced.
  62. 62. Daniel Greenfeld pydanny.com / @pydanny Next up...
  63. 63. The Dutch Way
  64. 64. Daniel Greenfeld pydanny.com / @pydanny Decorators @memoize def allcaps(string): return string.upper() def allcaps(string): return string.upper() allcaps = memoize(allcaps) > Decorators are easy to explain! “A decorator is a function that returns a function.”
  65. 65. Daniel Greenfeld pydanny.com / @pydanny I am Zen Decorators == Zen of Python
  66. 66. Daniel Greenfeld pydanny.com / @pydanny Until...
  67. 67. Daniel Greenfeld pydanny.com / @pydanny I am not Zen I need to write a decorator.
  68. 68. Daniel Greenfeld pydanny.com / @pydanny You try to shoot yourself in the foot, only to realize there’s no need, since Guido thoughtfully shot you in the foot years ago. -- Nick Mathewson, comp.lang.python http://starship.python.net/~mwh/quotes.html Ouch
  69. 69. Daniel Greenfeld pydanny.com / @pydanny Decorators @memoize def allcaps(string): return string.upper() def allcaps(string): return string.upper() allcaps = memoize(allcaps) > Decorators are easy to explain! “A decorator is a function that returns a function.”
  70. 70. Daniel Greenfeld pydanny.com / @pydanny Decorator Template http://pydanny-event-notes.readthedocs.org/en/latest/SCALE10x/python-decorators.html#decorator-template def decorator(function_to_decorate): def wrapper(*args, **kwargs): # do something before invoation result = func_to_decorate(*args, **kwargs) # do something after return result # update wrapper.__doc__ and .func_name # or functools.wraps return wrapper Result is returned when the wrapper is done When decorated function is called decorator returns wrapper Wrapper function does things before and after the function is called here. Wrapper function does things before and after the function is called here.
  71. 71. Daniel Greenfeld pydanny.com / @pydanny The Dutch Way There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you're Dutch.* * Zen of Python, lines 13 and 14
  72. 72. Daniel Greenfeld pydanny.com / @pydanny Decorator implementation def memoize(func): cache = {} def memoized(*args): if args in cache: return cache[args] result = cache[args] = func(*args) return result return memoized @memoize def allcaps(string): return string.upper() Return function Return value set cache Return value if args in cache Datastore
  73. 73. Daniel Greenfeld pydanny.com / @pydanny Whew.
  74. 74. Daniel Greenfeld pydanny.com / @pydanny What about decorators that accept arguments?
  75. 75. Daniel Greenfeld pydanny.com / @pydanny Oh No.
  76. 76. Daniel Greenfeld pydanny.com / @pydanny Explaining this is Hard. That’s because we create a decorator that creates a parameterized function to wrap the function.
  77. 77. Daniel Greenfeld pydanny.com / @pydanny multiplier decorator def multiplier(multiple): def decorator(function): def wrapper(*args, **kwargs): return function(*args, **kwargs) * multiple return wrapper return decorator @multiplier(5) def allcaps(string): return string.upper() Multiplier function sets the state for the multiple argument When decorated function is called the decorator function returns the wrapper function Result is returned when the wrapper is done. Wrapper function does: What am I supposed to highlight?
  78. 78. Daniel Greenfeld pydanny.com / @pydanny Whew
  79. 79. Daniel Greenfeld pydanny.com / @pydanny Oh No.
  80. 80. Daniel Greenfeld pydanny.com / @pydanny Not Done Yet!
  81. 81. Daniel Greenfeld pydanny.com / @pydanny authentication decorator @authorization('admin') def do_admin_thing(user): # do something administrative return user import functools def authorization(roles): def decorator(function): @functools.wraps(function) def wrapper(*args, **kwargs): check_roles(user, roles) return function(*args, **kwargs) return wrapper return decorator Don’t forget functools!
  82. 82. Daniel Greenfeld pydanny.com / @pydanny Whew
  83. 83. Daniel Greenfeld pydanny.com / @pydanny Really.
  84. 84. Daniel Greenfeld pydanny.com / @pydanny I’m not doing class decorators.
  85. 85. Daniel Greenfeld pydanny.com / @pydanny It is not easy to explain how to write decorators.
  86. 86. Daniel Greenfeld pydanny.com / @pydanny Contention While Using decorators is Zen...
  87. 87. Daniel Greenfeld pydanny.com / @pydanny Contention Writing Decorators is Not.
  88. 88. Daniel Greenfeld pydanny.com / @pydanny Deep Thought There should be one-- and preferably only one --obvious way to do it. Although that way may not be obvious at first unless you're Dutch. If the implementation is hard to explain, it's a bad idea. If the implementation is easy to explain, it may be a good idea. Although practicality beats purity. Decorators are easy to explain! Decorators are hard to explain!
  89. 89. Daniel Greenfeld pydanny.com / @pydanny Use the decorator library https://pypi.python.org/pypi/decorator
  90. 90. Avoiding Technical Debt Part I
  91. 91. Daniel Greenfeld pydanny.com / @pydanny Getting it done vs. Technical debt Now is better than never. Although never is often better than *right* now. * Zen of Python, lines 15 and 16
  92. 92. Daniel Greenfeld pydanny.com / @pydanny • Tests • Documentation Some things take time Risk: Deploying broken code Risk: problems upgrading dependencies Risk: Forgetting install/deploy Risk: Multiple coding standards (Risks of skipping them)
  93. 93. Daniel Greenfeld pydanny.com / @pydanny Easy Test Patterns • Always make sure your test harness can run • Try using tests instead of the shell/repl. • After the first deadline, reject any incoming code that drops coverage. • Use coverage.py For developers racing to meet deadlines:
  94. 94. Daniel Greenfeld pydanny.com / @pydanny Must-have Documentation • Installation/Deployment procedures • Coding standards • How to run tests • Version (including __version__)
  95. 95. Daniel Greenfeld pydanny.com / @pydanny Easy Test Patterns • Always make sure your test harness can run • Try using tests instead of the shell/repl. • Reject any incoming code that drops coverage. • Use coverage.py
  96. 96. Daniel Greenfeld pydanny.com / @pydanny Getting technical again...
  97. 97. Avoiding Technical Debt Part II
  98. 98. Daniel Greenfeld pydanny.com / @pydanny Namespaces • Extremely powerful • Useful • Precise import re import os from twisted.internet import protocol, reactor from django import forms from myproject import utils
  99. 99. Daniel Greenfeld pydanny.com / @pydanny import * makes development faster[1] • Extremely powerful • Useful • Imports everything at once! [2] from re import * from os import * from twisted import * from django.forms import * from myproject.utils import * [1]Warning: import * can be dangerous [2]Warning: import * can be dangerous
  100. 100. Daniel Greenfeld pydanny.com / @pydanny Comparing two modules def compare(mod1, mod2): title = 'nComparing {0}, {1}:'.format( mod1.__name__, mod2.__name__ ) print(title) for x in dir(mod1): for y in dir(mod2): if x == y and not x.startswith('_'): print("* " + x)
  101. 101. Daniel Greenfeld pydanny.com / @pydanny >>> re.sys == os.sys True >>> re.error == os.error False Comparing two modules >>> import re >>> import os >>> compare(os, re) Comparing os, re: * sys * error import * can get you into trouble from re import * from os import *
  102. 102. Daniel Greenfeld pydanny.com / @pydanny Breaking built-ins def compare_builtins(mod1): print("nComparing {0} to builtins:".format(mod1.__name__)) for x in dir(mod1): for y in dir(globals()['__builtins__']): if x == y and not x.startswith('_'): print("* GLOBAL: {0}".format(x)) Checks to see if a module has items that match any Python built-in.
  103. 103. Daniel Greenfeld pydanny.com / @pydanny Breaking built-ins >>> compare_builtins(re) Comparing re to builtins: * GLOBAL: compile >>> compare_builtins(os) Comparing os to builtins: * GLOBAL: open from re import * from os import * Breaks compile() built-in. Annoying but infrequent problem. Breaks open() built-in. This can drive you crazy. Compare ‘re’ Compare ‘os’
  104. 104. Daniel Greenfeld pydanny.com / @pydanny The open() story from os import *after before Breaks all the things! Help on built-in function open in module __builtin__: open(...) open(name[, mode[, buffering]]) -> file object Open a file using the file() type, returns a file object. This is the preferred way to open a file. See file.__doc__ for further information. Help on built-in function open in module posix: open(...) open(filename, flag [, mode=0777]) -> fd Open a file (for low level IO).
  105. 105. Daniel Greenfeld pydanny.com / @pydanny Beginner pro-tip Be careful of tutorials that use import *.
  106. 106. Daniel Greenfeld pydanny.com / @pydanny Contention import * is not for beginners. import * is people who really know Python. __all__ = ["echo", "surround", "reverse"]
  107. 107. Summary
  108. 108. Daniel Greenfeld pydanny.com / @pydanny Stay this person Myself at 13 in front of the Apple ][
  109. 109. Daniel Greenfeld pydanny.com / @pydanny Admit What You Don't Know
  110. 110. Daniel Greenfeld pydanny.com / @pydanny Stay out of your comfort Zone
  111. 111. Daniel Greenfeld pydanny.com / @pydanny Grow
  112. 112. Daniel Greenfeld pydanny.com / @pydanny What I Want To Know • Twisted • Numpy • SciPy • Tulip • C • Etc.
  113. 113. Daniel Greenfeld pydanny.com / @pydanny If I continue to Learn
  114. 114. Daniel Greenfeld pydanny.com / @pydanny I Get To Be This Person
  115. 115. Daniel Greenfeld pydanny.com / @pydanny Think Hard
  116. 116. Daniel Greenfeld pydanny.com / @pydanny Thank you • Armin Ronacher • nephila.it • Richard Jones • Raymond Hettiger • EuroPython • PyKonik • Łukasz Langa • Tomasz Paczkowski
  117. 117. Daniel Greenfeld pydanny.com / @pydanny Thank you • Matt Harrison • Ola Sendecka • Kenneth Love • Lennart Regebro • Paul Hildebrandt • Audrey Roy
  118. 118. Daniel Greenfeld pydanny.com / @pydanny One More Thing...
  119. 119. Daniel Greenfeld pydanny.com / @pydanny Finis
  120. 120. Daniel Greenfeld pydanny.com / @pydanny Q & A
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