python worst practices

7/17/2019 python worst practices

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Python Worst PracticesDaniel Greenfeld



Daniel Greenfeld@pydanny

I do cartwheels

Daniel Greenfeld (@pydanny)

Pythonista at Cartwheel

Djangonaut at Revsys

Co-lead of Django Packages &

Open Comparison

Learned Python at NASA

Fiancé of Audrey Roy




Talk Format

• At least one ‘Python Worst Practice’ slide

• At least one ‘Fixed Python Practice’ slide

•A side-by-side comparison slide

Each section will have three components:

These slides are already online!




Warning!

Don’t use the‘Python Worst Practices’

examples in your code*

You may be hunted down and killed

*Sometimes these are caught byvarious code checking tools




Advice!

Do consider using the‘Fixed Python Practices’

examples in your code

You may get complimented



Fundamentals




Python Worst Practice

object = MyObject()map = Map()

zip = 90213 # common US developer mistakeid = 34 # I still fight this one

I’m guilty

• I still use ‘id’ when I shouldn’t

• I admit I have a problem

•That gives me license to pick on others




Fixed Python Practice

obj = MyObject() # necessary abbreviationobject_ = MyObject() # Underscore so we don't overwrite

map_obj = Map() # combine name w/necessary abbreviationmap_ = Map()

zip_code = 90213 # Explicit name with US focuspostal_code = 90213 # i18n explicit namezip_ = 90213

pk = 34 # pk is often synonymous with idid_ = 34




Side-by-side comparison

object = MyObject()

map = Map()zip = 90213 # common US developer mistake

id = 34 # I still fight this one

obj = MyObject() # Use a necessary abbreviation

object_ = MyObject() # Use underscore so we don't overwrite

map_obj = Map() # combine name with necessary abbreviationmap_ = Map()

zip_code = 90213 # Explicit name with US focus

postal_code = 90213 # International explicit namezip_ = 90213

pk = 34 # pk is often synonymous with id

id_ = 34

Worst Practice Fixed Practice





true = 0

false = 1True = False

Flipping the booleans

Usually done to support an API




This sort of API

def crazy_posting_api(value): """ If a value is supplied

successfully return ‘0’. Otherwise return ‘1’ """

if value:

return 0 return 1





class CrazyApiConsumer(object):

def __init__(self, value): self.value = value

def post(self): # fix booleans in/around the return statement response = crazy_posting_api(self.value) return not bool(response)

cac1 = CrazyApiConsumer("hello")print(cac1.post())cac2 = CrazyApiConsumer("")print(cac2.post())




Side-by-side comparisonWorst Practice Fixed Practice

true = 0

false = 1True = False

class CrazyApiConsumer(object):

def __init__(self, value):

self.value = value

def post(self): # fix booleans in/around the return statement

response = crazy_posting_api(self.value) return not bool(response)

cac1 = CrazyApiConsumer("hello")

print(cac1.post())cac2 = CrazyApiConsumer("")

print(cac2.post())





strColor = "green"boolActive = False

intPythonYears = 20

dtPythonFirstUsed = "04/20/2011"

Identifying variable types with prefixes

Mixing case doesn’t help either





clr = "green"ctv = False

pythnYrs = 20

pthnFrstSd = "04/20/2011"

Conserving pixels by removing the vowels





c = "green"a = False

p = 20

t = "04/20/2011"




Fixed Python Practicecolor = "green"

active = Falsepython_years = 20

python_first_used = "04/20/2011"

Python assumes we are all consenting adults

• Infer from naming schemes the type/purpose

• Don’t be constrained by type





The pixel shortage is over.Use reasonably long variable names.

color = "green"

active = Falsepython_years = 20

python_first_used = "04/20/2011"





strColor = "green"boolActive = FalseintPythonYears = 20

dtPythonFirstUsed = "04/20/2011"

clr = "green"

ctv = FalsepythnYrs = 20

pthnFrstSd = "04/20/2011"

c = "green"

a = Falsep = 20

t = "04/20/2011"

color = "green"active = False

python_years = 20python_first_used = "04/20/2011"





foo = [1, 2, 3]

for i, item in zip(range(len(foo)), foo):

print i, item

Don’t use enumerate





Use enumerate

foo = [1, 2, 3]

for i, item in enumerate(foo):

print i, item

• Memorize the Python built-ins

• Makes your code easier to read

• Proven code





foo = [1, 2, 3]

zip(range(len(foo)), foo):

print i, item

foo = [1, 2, 3]

for i, item in enumerate(foo):

print i, item





Pr ! ent using

Di " erent Fonts

Dark Text Dire Backgr # nds





Present using

High ContrastEasy-to-read fonts

All devices off All programs off





Pr ! ent using Di " erent Fonts

Dark Text Dire Backgr # nds

Present using

High ContrastEasy-to-read fonts

All devices off

All programs off



Classes

D l G f ld




Python Worst PracticeImplementing Java-style getters and setters

import logginglog = logging.getLogger()

class JavaStyle: """ Quiz: what else am I doing wrong here? """

def __init__(self): self.name = ""

def get_name(self): return self.name

def set_name(self, name):

log.debug("Setting the name to %s" % name) if isinstance(name, str): self.name = name else: raise TypeError()

if __name__ == "__main__": j = JavaStyle() j.set_name("pydanny did this back in 2006!")

print(j.get_name())

D i l G f ld




Fixed Python PracticePython properties!


class PythonStyle(object):

def __init__(self): self._name = ""

@property def name(self): return self._name

@name.setter def name(self, value):

""" Because name is probably a string we'll assume that we can infer the type from the variable name""" log.debug("Setting the name to %s" % value) self._name = value

if __name__ == "__main__": p = PythonStyle() p.name = "pydanny doing it the right way" print(p.name)

Accessor

Mutator

D i l G f ld






class JavaStyle: """ Quiz: what else am I doing wrong here? """

def __init__(self): self.name = ""

def get_name(self): return self.name

def set_name(self, name):

log.debug("Setting the name to %s" % name) if isinstance(name, str): self.name = name else: raise TypeError()

if __name__ == "__main__": j = JavaStyle() j.set_name("pydanny did this back in 2006!") print(j.get_name())


class PythonStyle(object):

def __init__(self): self._name = ""

@property def name(self): return self._name

@name.setter

def name(self, value): """ Because name is probably a string we'll assume that we can infer the type from the variable name""" log.debug("Setting the name to %s" % value) self._name = value

if __name__ == "__main__": p = PythonStyle() p.name = "pydanny doing it the right way" print(p.name)

Daniel Greenfeld





class WebService(object):

@property def connect(self): self.proxy = xmlrpc.Server("http://service.xml")

if __name__ == '__main__':

ws = WebService() ws.connect

Using property setters as action methods!

A.K.A. Trying to make your Python code look like Ruby

Daniel Greenfeld






def connect(self): self.proxy = xmlrpc.Server("http://service.xml")

if __name__ == '__main__':

ws = WebService()

ws.connect()

Methods please!

Daniel Greenfeld






@property def connect(self): self.proxy = xmlrpc.Server("http://service.xml")

if __name__ == '__main__':

ws = WebService()

ws.connect


def connect(self):

self.proxy = xmlrpc.Server("http://service.xml")

if __name__ == '__main__':

ws = WebService()

ws.connect()



Exceptions

Daniel Greenfeld





try:

do_akshun(value)except: pass

Passing Generic Exceptions silently

• Ignorance is not bliss

• You have no idea what your system is doing

•Arguably better to not have this in your code

Daniel Greenfeld




Fixed Python PracticeUse specific exceptions and/or logging

class AkshunDoesNotDo(Exception): """ Custom exceptions makes for maintainable code """

pass

try: do_akshun(value)except AttributeError as e:

log.info("Can I get attribution for these slides?") do_bakup_akshun(vlue)except Exception as e: log.debug(str(e)) raise AkshunDoesNotDo(e)

Daniel Greenfeld





try:

do_akshun(value)

except: pass

class AkshunDoesNotDo(Exception):

""" Custom exceptions makes for maintainable code """

pass

try:

do_akshun(value)except AttributeError as e:

log.info("Can I get attribution for these slides?") do_bakup_akshun(vlue)

except Exception as e: log.debug(str(e))

raise AkshunDoesNotDo(e)



Getting controversial

Daniel Greenfeld



@pydanny


• Hard to debug

• Security risk

• Sets bad precedents

imports = "from {0} import {1}".format("random", "randrange")exec(imports)

print(randrange(10))

Using exec for dynamic imports

Daniel Greenfeld



@pydanny


• importlib is in the standard library

• Really explicit

• Direct tie into the Python machinery

import importlibfunstuff = importlib.import_module('random')

print(funstuff.randrange(10))

Using importlib for dynamic imports

Daniel Greenfeld



@pydanny


imports = "from {0} import {1}".format("random", "randrange")

exec(imports)print(randrange(10))

import importlib

funstuff = importlib.import_module('random')print(funstuff.randrange(10))

Daniel Greenfeld



@pydanny


• Too many characters on one line

• Lambdas by design does not have docstrings

• Does not necessarily mean less characters

• I can’t get this sample to work!

swap = lambda a, x, y: lambda f = a.__setitem__: (f(x, (a[x], a[y])), f(y, a[x][0]), f(x, a[x][1]))()

Generally using lambdas

Daniel Greenfeld



@pydanny


• Doc strings that show up nicely in help/Sphinx

• Easier to read

• In Python, functions are first class objects

• Whenever possible avoid using lambdas

def swap(a, x, y): """ Swap two position values in a list """ a[x],a[y] = a[y],a[x]

Daniel Greenfeld@ d



@pydanny

Side-by-side comparisonWorst Practice Fixed Practiceswap = lambda a, x, y:

lambda f = a.__setitem__: (f(x, (a[x], a[y])),

f(y, a[x][0]), f(x, a[x][1]))()

def swap(a, x, y):

""" Swap two position values in a list """ a[x],a[y] = a[y],a[x]

Daniel Greenfeld@ d



@pydanny


• You can’t convince me that XML is the better way

• You are forcing me to learn a new language

<pydanny-ml> <do action="call_view">com.pydanny.nextSlide</do> <global name="spam" value="eggs" /></pydanny-ml>

Configuring your project with XML

Daniel Greenfeld@ d



@pydanny


• Is this the right way?

• This allows conditional logic

• Iterators

• i.e. “Magic Configuration”

spam = "eggs"actions = [ ('call_view', 'com.pydanny.nextSlide')

]

Use Python for configuration!

?

Daniel Greenfeld@ d



@pydanny


INSTALLED_APPS += [p for p in os.listdir(BASE) if os.path.isdir(p)]MIDDLEWARE_CLASSES = [...]def callback(arg, dirname, fnames):

if 'middleware.py' in fnames: m = '%s.middleware' % os.path.split(dirname)[-1] MIDDLEWARE_CLASSES.append(m)

urlpatterns = patterns('', ...)for app in settings.INSTALLED_APPS: if not app.startswith('django'): p = url('^%s/' % app, include('%s.urls') % app) urlpatterns += patterns('', p)

‘Magical configuration code’

Ugh.

http://www.slideshare.net/jacobian/the-best-and-worst-of-django







@pydanny

Fixed Python PracticePREREQ_APPS = [ # Django "django.contrib.admin", "django.contrib.auth", "django.contrib.contenttypes", "django.contrib.sessions", "django.contrib.sites", "django.contrib.messages", "django.contrib.humanize", "django.contrib.flatpages",

# external "notification", # must be first "staticfiles", "uni_form",

... ]

urlpatterns = patterns("",

url(r"^$", homepage, name="home"), url(r"âccounts/", include("accounts.urls")), url(r"âdmin/", include(admin.site.urls)), url(r"âbout/", include("about.urls")), url(r"^profiles/", include("profiles.urls")), url(r"^notices/", include("notification.urls")), ... )

MIDDLEWARE_CLASSES = [ "django.middleware.common.CommonMiddleware", "django.contrib.sessions.middleware.SessionMiddleware" , "django.middleware.csrf.CsrfViewMiddleware", "django.contrib.auth.middleware.AuthenticationMiddleware" , "reversion.middleware.RevisionMiddleware", "django.contrib.messages.middleware.MessageMiddleware" ,

... ]

Explicit is betterthen Implicit

This isn’t that much typing, is it?





@pydanny

Fixed Python PracticePREREQ_APPS = [ # Django "django.contrib.admin", "django.contrib.auth", "django.contrib.contenttypes", "django.contrib.sessions", "django.contrib.sites",

"django.contrib.messages", "django.contrib.humanize", "django.contrib.flatpages",

# external "notification", # must be first "staticfiles", "uni_form",

... ]

urlpatterns = patterns("",

url(r"^$", homepage, name="home"), url(r"âccounts/", include("accounts.urls")), url(r"âdmin/", include(admin.site.urls)), url(r"âbout/", include("about.urls")), url(r"^profiles/", include("profiles.urls")), url(r"^notices/", include("notification.urls")), ... )

MIDDLEWARE_CLASSES = [ "django.middleware.common.CommonMiddleware", "django.contrib.sessions.middleware.SessionMiddleware" , "django.middleware.csrf.CsrfViewMiddleware", "django.contrib.auth.middleware.AuthenticationMiddleware" , "reversion.middleware.RevisionMiddleware", "django.contrib.messages.middleware.MessageMiddleware" , ... ]

Python’s design is predicated on the proposition thatcode is more often read than written.

http://www.slideshare.net/jacobian/the-best-and-worst-of-django/44










@pydanny

Fixed Python PracticeUse a config file

spam = "eggs"

[actions]

call_view = com.pydanny.nextSlide

Read up on config parser

http://docs.python.org/library/configparser.html









@pydanny


XML

logic heavy python

simple python files

config (.cfg) files



Documentation




@pydanny

Python Worst PracticeBad docstrings

• Do really obvious objects require doc strings?

• Complex methods require more than docstrings!

class Pythonista(): # Old style class! """ This class represents a Python programmer """

def code(self): """Write some code """ code, inspiration = Code(), Inspiration() for hour in Effort(): try: code += hour + inspiraion

except CurseWorthyBug: ...




@py a y

Fixed Python Practiceclass Pythonista(object):

def code(self): """ Writes code following these steps 1. Create a space for coding

2. Get some inspiration 3. Loop through some hours of effort 4. Write some code 5. Pull out hair cause of bugs """ code = Code()

inspiration = Inspiration() for hour in Effort(): try: code += hour + inspiraion except CurseWorthyBug: ...

Spend afew minutesdocumenting

the criticalstuff, okay?




@py y


class Pythonista(): # Old style class!

""" This class represents a Python programmer """

def code(self): """Write some code """

code, inspiration = Code(), Inspiration() for hour in Effort():

try: code += hour + inspiraion

except CurseWorthyBug:

...

class Pythonista(object):

def code(self):

""" Writes code following these steps 1. Create a space for coding

2. Get some inspiration 3. Loop through some hours of effort

4. Write some code 5. Pull out hair cause of bugs

"""

code = Code() inspiration = Inspiration() for hour in Effort():

try: code += hour + inspiraion

except CurseWorthyBug: ...





@py y


• Generally not version controlled

•Backups? Mirrors?

• Editing via the web? Ugh.

• No pull requests - smaller group of contributors

Using a wiki for project documentation

“Wikis are where project documentation goes to die” Jacob Kaplan-Moss




py y


• Use Restructured Text

• Use Sphinx

• Host on http://readthedocs.org


http://readthedocs.org/

http://readthedocs.org/




http://pypi.python.org/pypi/that

>>> import thatThe Anti-Zen of Python, by Daniel Greenfeld

Ugly is better than beautiful.Implicit is better than explicit.Complicated is better than complex.Complex is better than simple.Nested is better than flat.

Dense is better than sparse.Line code counts.Special cases are special enough to break the rules.Although purity beats practicality.Errors should always pass silently.Spelchek iz fur loosers.In the face of explicity, succumb to the temptation to guess.There should be many ways to do it.Because only a tiny minority of us are Dutch.Later is the best time to fix something.If the implementation is hard to explain, it's a good sell.If the implementation is easy to explain, it won't take enough time to do.Namespaces are too hard, just use import *!






Fixed Python Practice>>> import thisThe 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!

Core Python





>>> import thatThe Anti-Zen of Python, by Daniel Greenfeld

Ugly is better than beautiful.

Implicit is better than explicit.Complicated is better than complex.Complex is better than simple.Nested is better than flat.Dense is better than sparse.Line code counts.Special cases are special enough to break the rules.Although purity beats practicality.Errors should always pass silently.

Spelchek iz fur loosers.In the face of explicity, succumb to the temptation to guess.There should be many ways to do it.Because only a tiny minority of us are Dutch.Later is the best time to fix something.If the implementation is hard to explain, it's a good sell.If the implementation is easy to explain, it won't take enough time to do.Namespaces are too hard, just use import *!

>>> import thisThe 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!



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