Thinking hard about_python

Daniel Greenfeld
pydanny.com / @pydanny
Daniel Greenfeld
Thinking Hard
About Python

Daniel Greenfeld
@pydanny

Daniel Greenfeld
http://2scoops.org
Danny: 128,546++
Audrey: 121,871++

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What I
want you
to think
of me. One Cartwheel of Many Around the World

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What
I’m
really
like. Myself at 13 in front of the Apple ][

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Overview

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Coding
into
Trouble

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Controversy

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Exceptions

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Avoiding
Technical
Debt

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...first section...

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a.k.a.

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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!

Daniel Greenfeld
Contention
The super() method can create ambiguity.

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Example:
Django

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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.

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However...

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Things I don’t know:
The ancestor chain for
django.views.generic.edit.UpdateView

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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:

Daniel Greenfeld
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?

Daniel Greenfeld
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!

Daniel Greenfeld
from actions.views import ActionUpdateView
for x in ActionUpdateView.mro():
print(x)
Ancestor Chain (MRO)
of ActionUpdateView
MRO = Method Resolution Order
Print the MRO

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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'>

Daniel Greenfeld
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

Daniel Greenfeld
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

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Whew!

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Safe!

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If you’re not careful,
super can cause subtle
inheritance/MRO
problems.

Daniel Greenfeld
• 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)

Daniel Greenfeld
Write a easy-to-use
MRO inspector thingee
that identifies the parent
attributes/methods
specified by the coder.
TODO

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Special cases aren’t special enough to
break the rules.
Although practicality beats purity.*
* Zen of Python, lines 8 and 9

Daniel Greenfeld
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

Daniel Greenfeld
Web2py
Often honors Implicit over Explicit
Follows its own namespace pattern

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# 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

Daniel Greenfeld
# encoding: utf-8
Web2py code sample
I GET IT NOW
Europe taught me
why unicode is
important.

Daniel Greenfeld
# encoding: utf-8
Web2py code sample
OK, Back to the talk...

Daniel Greenfeld
Web2py code sample
# encoding: utf-8
Response object magically exists.
No import necessary
What can I expect in any location?What about namespace pollution?
Written by Massimo himself

Daniel Greenfeld
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

Daniel Greenfeld
Controversy
Special cases aren’t special enough to break the rules.
* Zen of Python, lines 8, 9

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Web2py contends:
* Zen of Python, lines 8, 9

Daniel Greenfeld
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.

Daniel Greenfeld
And that’s okay
Web2py will always
be contentious
Web2py argues practicality
in some very speciﬁc places.
Controversy

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Flask and its global Request object
http://bit.ly/ﬂask-requests
A Little Magic
Goes a Long Way

Silent
Exceptions
are the
Devil

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Exceptions
Unless explicitly silenced.*

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djangopackages.com
• Once a day iterates across all
packages.
• Updates the metadata from:
• Github:
• Bitbucket
• PyPI

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Django Packages
• Sometimes the APIs go down.
• Sometimes the APIs change.
• Sometimes projects get deleted.
• Sometimes the Internets fail
Problems
Catch and report exceptions!

Daniel Greenfeld
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...

Daniel Greenfeld
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)

Daniel Greenfeld
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

Daniel Greenfeld
Exceptions
Unless explicitly silenced.*
My code is
nearly silent
I’ve silenced things
for no good reason

Daniel Greenfeld
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

Daniel Greenfeld
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

Daniel Greenfeld
Exceptions
My code is
nearly silent
I’ve silenced things
for no good reason

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Exceptions

Daniel Greenfeld
Next
up...

Daniel Greenfeld
Decorators
@memoize
def allcaps(string):
return string.upper()
allcaps = memoize(allcaps)
>
Decorators are
easy to explain!
“A decorator is a function that returns a function.”

Daniel Greenfeld
I am Zen
Decorators == Zen of Python

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Until...

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I am not Zen
I need to write a decorator.

Daniel Greenfeld
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

Daniel Greenfeld
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.

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The Dutch Way
Although that way may not be obvious at ﬁrst unless you're Dutch.*

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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
Return function
Return value
set cache
Return value if
args in cache
Datastore

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Whew.

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What about decorators
that accept arguments?

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Oh No.

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Explaining this is
Hard.
That’s because we create a decorator that creates a
parameterized function to wrap the function.

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multiplier decorator
def multiplier(multiple):
def decorator(function):
return function(*args, **kwargs) * multiple
return wrapper
return decorator
@multiplier(5)
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?

Daniel Greenfeld
Whew

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Not Done Yet!

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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)
check_roles(user, roles)
return function(*args, **kwargs)
return wrapper
return decorator
Don’t forget functools!

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Really.

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I’m not doing
class decorators.

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It is not easy
to explain how
to write decorators.

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Contention
While Using
decorators is
Zen...

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Contention
Writing
Decorators
is Not.

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Deep Thought
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.
Decorators are
easy to explain!
Decorators are
hard to explain!

Daniel Greenfeld
Use the
decorator library
https://pypi.python.org/pypi/decorator

Avoiding
Technical
Debt
Part I

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Getting it done
vs.
Technical debt
Now is better than never.
Although never is often better than *right* now.

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• 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)

Daniel Greenfeld
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:

Daniel Greenfeld
Must-have
Documentation
• Installation/Deployment procedures
• Coding standards
• How to run tests
• Version (including __version__)

Daniel Greenfeld
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

Daniel Greenfeld
Getting technical
again...

Avoiding
Technical
Debt
Part II

Daniel Greenfeld
Namespaces
• Extremely powerful
• Useful
• Precise
import re
import os
from twisted.internet import protocol, reactor
from django import forms
from myproject import utils

Daniel Greenfeld
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

Daniel Greenfeld
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)

Thinking hard about_python

Thinking hard about_python

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