Python Worst Practices

Python Worst Practices Daniel Greenfeld

Daniel Greenfeld @pydannyI 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

Daniel Greenfeld @pydanny Talk Format Each section will have three components:• At least one ‘Python Worst Practice’ slide• At least one ‘Fixed Python Practice’ slide• A side-by-side comparison slide These slides are already online!

Daniel Greenfeld @pydanny Warning! Don’t use the‘Python Worst Practices’ examples in your code*You may be hunted down and killed *Sometimes these are caught by various code checking tools

Daniel Greenfeld @pydanny Advice! Do consider using the‘Fixed Python Practices’ examples in your code You may get complimented

Fundamentals

Daniel Greenfeld @pydanny Python Worst Practiceobject = MyObject()map = Map()zip = 90213 # common US developer mistakeid = 34 # I still fight this oneI’m guilty • I still use ‘id’ when I shouldn’t • I admit I have a problem • That gives me license to pick on others

Daniel Greenfeld @pydanny Fixed Python Practiceobj = MyObject() # necessary abbreviationobject_ = MyObject() # Underscore so we dont overwritemap_obj = Map() # combine name w/necessary abbreviationmap_ = Map()zip_code = 90213 # Explicit name with US focuspostal_code = 90213 # i18n explicit namezip_ = 90213pk = 34 # pk is often synonymous with idid_ = 34

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practiceobject = MyObject() obj = MyObject() # Use a necessary abbreviationmap = Map() object_ = MyObject() # Use underscore so we dont overwritezip = 90213 # common US developer mistakeid = 34 # I still fight this one map_obj = Map() # combine name with necessary abbreviation map_ = Map() zip_code = 90213 # Explicit name with US focus postal_code = 90213 # International explicit name zip_ = 90213 pk = 34 # pk is often synonymous with id id_ = 34

Daniel Greenfeld @pydannyPython Worst Practice Flipping the booleans true = 0 false = 1 True = False Usually done to support an API

Daniel Greenfeld @pydanny This sort of APIdef crazy_posting_api(value): """ If a value is supplied successfully return ‘0’. Otherwise return ‘1’ """ if value: return 0 return 1

Daniel Greenfeld @pydannyFixed Python Practiceclass 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())

Daniel Greenfeld @pydannySide-by-side comparisonWorst Practice Fixed Practicetrue = 0 class CrazyApiConsumer(object):false = 1True = False 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())

Daniel Greenfeld @pydannyPython Worst Practice Identifying variable types with preﬁxes strColor = "green" boolActive = False intPythonYears = 20 dtPythonFirstUsed = "04/20/2011" Mixing case doesn’t help either

Daniel Greenfeld @pydannyPython Worst PracticeConserving pixels by removing the vowelsclr = "green"ctv = FalsepythnYrs = 20pthnFrstSd = "04/20/2011"

Daniel Greenfeld @pydannyPython Worst Practice c = "green" a = False p = 20 t = "04/20/2011"

Daniel Greenfeld @pydannyFixed Python Practice color = "green" active = False python_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

Daniel Greenfeld @pydannyFixed Python Practicecolor = "green"active = Falsepython_years = 20python_first_used = "04/20/2011" The pixel shortage is over. Use reasonably long variable names.

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice c = "green" a = False color = "green" p = 20 t = "04/20/2011" active = False python_years = 20 python_first_used = "04/20/2011" clr = "green" ctv = False pythnYrs = 20 pthnFrstSd = "04/20/2011"strColor = "green"boolActive = FalseintPythonYears = 20dtPythonFirstUsed = "04/20/2011"

Daniel Greenfeld @pydanny Python Worst Practice Don’t use enumeratefoo = [1, 2, 3]for i, item in zip(range(len(foo)), foo): print i, item

Daniel Greenfeld @pydannyFixed Python Practice Use enumeratefoo = [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

Daniel Greenfeld @pydannySide-by-side comparisonWorst Practice Fixed Practicefoo = [1, 2, 3] foo = [1, 2, 3]zip(range(len(foo)), foo): for i, item in enumerate(foo): print i, item print i, item

Python Worst Practice Present using Diﬀerent Fonts Dark Text Dire Backgr#nds

Daniel Greenfeld @pydannyPython Worst Practice Present using High Contrast Easy-to-read fonts All devices off All programs off

Daniel Greenfeld @pydannySide-by-side comparisonWorst Practice Fixed Practice Present using Present using Diﬀerent Fonts High Contrast Dark Text Easy-to-read fonts Dire Backgr#nds All devices off All programs off

Classes

Daniel Greenfeld @pydannyPython Worst PracticeImplementing Java-style getters and setters import logging log = 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())

Daniel Greenfeld @pydannyFixed Python Practice Python properties! import logging log = logging.getLogger() class PythonStyle(object): def __init__(self): self._name = "" Accessor @property def name(self): return self._name Mutator @name.setter def name(self, value): """ Because name is probably a string well 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 @pydanny Side-by-side comparison Worst Practice Fixed Practiceimport logging import logginglog = logging.getLogger() log = logging.getLogger()class JavaStyle: class PythonStyle(object): """ Quiz: what else am I doing wrong here? """ def __init__(self): def __init__(self): self._name = "" self.name = "" @property def get_name(self): def name(self): return self.name return self._name def set_name(self, name): @name.setter log.debug("Setting the name to %s" % name) def name(self, value): if isinstance(name, str): """ Because name is probably a string well assume that we can self.name = name infer the type from the variable name""" else: log.debug("Setting the name to %s" % value) raise TypeError() self._name = valueif __name__ == "__main__": if __name__ == "__main__": j = JavaStyle() p = PythonStyle() j.set_name("pydanny did thisp.namein 2006!") doing it the right way" back = "pydanny print(j.get_name()) print(p.name)

Daniel Greenfeld @pydanny Python Worst Practice Using property setters as action methods!class WebService(object): @property def connect(self): self.proxy = xmlrpc.Server("http://service.xml")if __name__ == __main__: ws = WebService() ws.connect A.K.A.Trying to make your Python code look like Ruby

Daniel Greenfeld @pydanny Fixed Python Practice Methods please!class WebService(object): def connect(self): self.proxy = xmlrpc.Server("http://service.xml")if __name__ == __main__: ws = WebService() ws.connect()

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practiceclass WebService(object): class WebService(object): @property def connect(self): def connect(self): self.proxy = xmlrpc.Server("http://service.xml") self.proxy = xmlrpc.Server("http://service.xml")if __name__ == __main__: if __name__ == __main__: ws = WebService() ws.connect ws = WebService() ws.connect()

Exceptions

Daniel Greenfeld @pydannyPython Worst Practice Passing Generic Exceptions silently try: do_akshun(value) except: pass• Ignorance is not bliss• You have no idea what your system is doing• Arguably better to not have this in your code

Daniel Greenfeld @pydanny Fixed Python Practice Use speciﬁc exceptions and/or loggingclass AkshunDoesNotDo(Exception): """ Custom exceptions makes for maintainable code """ passtry: 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 @pydannySide-by-side comparisonWorst Practice Fixed Practicetry: class AkshunDoesNotDo(Exception): do_akshun(value) """ Custom exceptions makes for maintainable code """except: pass 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 Python Worst Practice Using exec for dynamic importsimports = "from {0} import {1}".format("random", "randrange")exec(imports)print(randrange(10)) • Hard to debug • Security risk • Sets bad precedents

Daniel Greenfeld @pydannyFixed Python Practice Using importlib for dynamic imports import importlib funstuff = importlib.import_module(random) print(funstuff.randrange(10))• importlib is in the standard library• Really explicit• Direct tie into the Python machinery

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practiceimports = "from {0} import {1}".format("random", "randrange") import importlibexec(imports) funstuff = importlib.import_module(random)print(randrange(10)) print(funstuff.randrange(10))

Daniel Greenfeld @pydanny Python Worst Practice Generally using lambdasswap = lambda a, x, y: lambda f = a.__setitem__: (f(x, (a[x], a[y])), f(y, a[x][0]), f(x, a[x][1]))() • 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!

Daniel Greenfeld @pydanny Fixed Python Practice def swap(a, x, y): """ Swap two position values in a list """ a[x],a[y] = a[y],a[x]• Doc strings that show up nicely in help/Sphinx• Easier to read• In Python, functions are ﬁrst class objects• Whenever possible avoid using lambdas

Daniel Greenfeld @pydanny Side-by-side comparisonWorst Practice Fixed Practiceswap = lambda a, x, y: def swap(a, x, y): lambda f = a.__setitem__: """ Swap two position values in a list """ (f(x, (a[x], a[y])), a[x],a[y] = a[y],a[x] f(y, a[x][0]), f(x, a[x][1]))()

Daniel Greenfeld @pydannyPython Worst Practice Conﬁguring your project with XML<pydanny-ml> <do action="call_view">com.pydanny.nextSlide</do> <global name="spam" value="eggs" /></pydanny-ml>• You can’t convince me that XML is the better way• You are forcing me to learn a new language

Daniel Greenfeld @pydannyFixed Python Practice ? Use Python for conﬁguration! spam = "eggs" actions = [ (call_view, com.pydanny.nextSlide) ] • Is this the right way? • This allows conditional logic • Iterators • i.e. “Magic Conﬁguration”

Daniel Greenfeld @pydannyPython Worst Practice ‘Magical conﬁguration code’ 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) Ugh. http://www.slideshare.net/jacobian/the-best-and-worst-of-django

Daniel Greenfeld @pydanny Fixed Python Practice urlpatterns = patterns("",PREREQ_APPS = [ # Django url(r"^$", homepage, name="home"), "django.contrib.admin", url(r"âccounts/", include("accounts.urls")), "django.contrib.auth", url(r"âdmin/", include(admin.site.urls)), "django.contrib.contenttypes", url(r"âbout/", include("about.urls")), "django.contrib.sessions", url(r"^profiles/", include("profiles.urls")), "django.contrib.sites", url(r"^notices/", include("notification.urls")), "django.contrib.messages", ... "django.contrib.humanize", MIDDLEWARE_CLASSES = [ ) "django.contrib.flatpages", "django.middleware.common.CommonMiddleware", "django.contrib.sessions.middleware.SessionMiddleware", # external "django.middleware.csrf.CsrfViewMiddleware", "notification", # must be first "django.contrib.auth.middleware.AuthenticationMiddleware", "staticfiles", "reversion.middleware.RevisionMiddleware", "uni_form", "django.contrib.messages.middleware.MessageMiddleware", ... ... ] ]Explicit is better This isn’t that much typing, is it? then Implicit

Daniel Greenfeld @pydanny Fixed Python Practice urlpatterns = patterns("",PREREQ_APPS = [ # Django url(r"^$", homepage, name="home"), "django.contrib.admin", url(r"âccounts/", include("accounts.urls")), "django.contrib.auth", url(r"âdmin/", include(admin.site.urls)), "django.contrib.contenttypes", url(r"âbout/", include("about.urls")), "django.contrib.sessions", url(r"^profiles/", include("profiles.urls")), "django.contrib.sites", url(r"^notices/", include("notification.urls")), "django.contrib.messages", ... "django.contrib.humanize", MIDDLEWARE_CLASSES = [ ) "django.contrib.flatpages", "django.middleware.common.CommonMiddleware", "django.contrib.sessions.middleware.SessionMiddleware", # external "django.middleware.csrf.CsrfViewMiddleware", "notification", # must be first "django.contrib.auth.middleware.AuthenticationMiddleware", "staticfiles", "reversion.middleware.RevisionMiddleware", "uni_form", "django.contrib.messages.middleware.MessageMiddleware", ... ... ] ] Python’s design is predicated on the proposition that code is more often read than written. http://www.slideshare.net/jacobian/the-best-and-worst-of-django/44

Daniel Greenfeld @pydanny Fixed Python Practice Use a config filespam = "eggs"[actions]call_view = com.pydanny.nextSlide Read up on config parserhttp://docs.python.org/library/configparser.html

Daniel Greenfeld @pydannySide-by-side comparisonWorst Practice Fixed PracticeXML simple python fileslogic heavy python config (.cfg) files

Documentation

Daniel Greenfeld @pydanny Python Worst Practice Bad docstringsclass 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: ... • Do really obvious objects require doc strings? • Complex methods require more than docstrings!

Daniel Greenfeld @pydanny Fixed Python Practice 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 Spend a 4. Write some codefew minutes 5. Pull out hair cause of bugs """documenting code = Code() the critical inspiration = Inspiration() for hour in Effort(): stuff, okay? try: code += hour + inspiraion except CurseWorthyBug: ...

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practiceclass Pythonista(): # Old style class! class Pythonista(object): """ This class represents a Python programmer """ def code(self): def code(self): """ Writes code following these steps """Write some code """ 1. Create a space for coding code, inspiration = Code(), Inspiration() 2. Get some inspiration for hour in Effort(): 3. Loop through some hours of effort try: 4. Write some code code += hour + inspiraion 5. Pull out hair cause of bugs except CurseWorthyBug: """ ... code = Code() inspiration = Inspiration() for hour in Effort(): try: code += hour + inspiraion except CurseWorthyBug: ...

Daniel Greenfeld @pydanny Python Worst Practice Using a wiki for project documentation “Wikis are where project documentation goes to die” Jacob Kaplan-Moss• Generally not version controlled• Backups? Mirrors?• Editing via the web? Ugh.• No pull requests - smaller group of contributors

Daniel Greenfeld @pydannyFixed Python Practice• Use Restructured Text• Use Sphinx• Host on http://readthedocs.org

Daniel Greenfeld @pydannySide-by-side comparisonWorst Practice Fixed Practice

Daniel Greenfeld @pydannyPython Worst Practice >>> import that The 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, its a good sell. If the implementation is easy to explain, it wont take enough time to do. Namespaces are too hard, just use import *! http://pypi.python.org/pypi/that

Daniel Greenfeld @pydannyFixed Python Practice >>> 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 arent 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 youre Dutch. Now is better than never. Although never is often better than *right* now. If the implementation is hard to explain, its a bad idea. If the implementation is easy to explain, it may be a good idea. Namespaces are one honking great idea -- lets do more of those! Core Python

Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice >>> import this>>> import that The Zen of Python, by Tim PetersThe Anti-Zen of Python, by Daniel Greenfeld Beautiful is better than ugly.Ugly is better than beautiful. Explicit is better than implicit.Implicit is better than explicit. Simple is better than complex.Complicated is better than complex. Complex is better than complicated.Complex is better than simple. Flat is better than nested.Nested is better than flat. Sparse is better than dense.Dense is better than sparse. Readability counts.Line code counts. Special cases arent special enough to break the rules.Special cases are special enough to break the rules. Although practicality beats purity.Although purity beats practicality. Errors should never pass silently.Errors should always pass silently. Unless explicitly silenced.Spelchek iz fur loosers. In the face of ambiguity, refuse the temptation to guess.In the face of explicity, succumb to the temptation to guess. There should be one-- and preferably only one --obvious way to do it.There should be many ways to do it. Although that way may not be obvious at first unless youre Dutch.Because only a tiny minority of us are Dutch. Now is better than never.Later is the best time to fix something. Although never is often better than *right* now.If the implementation is hard to explain, its a good sell. If the implementation is hard to explain, its a bad idea.If the implementation is easy to explain, it wont take enough time to do. If the implementation is easy to explain, it may be a good idea.Namespaces are too hard, just use import *! Namespaces are one honking great idea -- lets do more of those!

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Python Worst Practices

by Daniel Greenfeld on Apr 28, 2011

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