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Python Worst Practices

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Python Worst Practices

  1. 1. Python Worst Practices Daniel Greenfeld
  2. 2. 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
  3. 3. 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!
  4. 4. 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
  5. 5. Daniel Greenfeld @pydanny Advice! Do consider using the ‘Fixed Python Practices’ examples in your code You may get complimented
  6. 6. Fundamentals
  7. 7. Daniel Greenfeld @pydanny Python Worst Practice object = MyObject() map = Map() zip = 90213 # common US developer mistake id = 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
  8. 8. Daniel Greenfeld @pydanny Fixed Python Practice obj = MyObject() # necessary abbreviation object_ = MyObject() # Underscore so we don't overwrite map_obj = Map() # combine name w/necessary abbreviation map_ = Map() zip_code = 90213 # Explicit name with US focus postal_code = 90213 # i18n explicit name zip_ = 90213 pk = 34 # pk is often synonymous with id id_ = 34
  9. 9. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice object = MyObject() obj = MyObject() # Use a necessary abbreviation map = Map() object_ = MyObject() # Use underscore so we don't overwrite zip = 90213 # common US developer mistake id = 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
  10. 10. Daniel Greenfeld @pydanny Python Worst Practice Flipping the booleans true = 0 false = 1 True = False Usually done to support an API
  11. 11. Daniel Greenfeld @pydanny 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
  12. 12. Daniel Greenfeld @pydanny Fixed Python Practice 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())
  13. 13. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice true = 0 class CrazyApiConsumer(object): false = 1 True = 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())
  14. 14. Daniel Greenfeld @pydanny Python Worst Practice Identifying variable types with prefixes strColor = "green" boolActive = False intPythonYears = 20 dtPythonFirstUsed = "04/20/2011" Mixing case doesn’t help either
  15. 15. Daniel Greenfeld @pydanny Python Worst Practice Conserving pixels by removing the vowels clr = "green" ctv = False pythnYrs = 20 pthnFrstSd = "04/20/2011"
  16. 16. Daniel Greenfeld @pydanny Python Worst Practice c = "green" a = False p = 20 t = "04/20/2011"
  17. 17. Daniel Greenfeld @pydanny Fixed 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
  18. 18. Daniel Greenfeld @pydanny Fixed Python Practice color = "green" active = False python_years = 20 python_first_used = "04/20/2011" The pixel shortage is over. Use reasonably long variable names.
  19. 19. 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 = False intPythonYears = 20 dtPythonFirstUsed = "04/20/2011"
  20. 20. Daniel Greenfeld @pydanny Python Worst Practice Don’t use enumerate foo = [1, 2, 3] for i, item in zip(range(len(foo)), foo): print i, item
  21. 21. Daniel Greenfeld @pydanny Fixed Python Practice 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
  22. 22. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice foo = [1, 2, 3] foo = [1, 2, 3] zip(range(len(foo)), foo): for i, item in enumerate(foo): print i, item print i, item
  23. 23. Python Worst Practice Present using Different Fonts Dark Text Dire Backgr#nds
  24. 24. Daniel Greenfeld @pydanny Python Worst Practice Present using High Contrast Easy-to-read fonts All devices off All programs off
  25. 25. Daniel Greenfeld @pydanny Side-by-side comparison Worst 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
  26. 26. Classes
  27. 27. Daniel Greenfeld @pydanny Python Worst Practice Implementing 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())
  28. 28. Daniel Greenfeld @pydanny Fixed 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 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)
  29. 29. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice import logging import logging log = 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 we'll 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 = value if __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)
  30. 30. 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
  31. 31. 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()
  32. 32. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice class 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()
  33. 33. Exceptions
  34. 34. Daniel Greenfeld @pydanny Python 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
  35. 35. Daniel Greenfeld @pydanny Fixed Python Practice Use 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)
  36. 36. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice try: 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)
  37. 37. Getting controversial
  38. 38. Daniel Greenfeld @pydanny Python Worst Practice Using exec for dynamic imports imports = "from {0} import {1}".format("random", "randrange") exec(imports) print(randrange(10)) • Hard to debug • Security risk • Sets bad precedents
  39. 39. Daniel Greenfeld @pydanny Fixed 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
  40. 40. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice imports = "from {0} import {1}".format("random", "randrange") import importlib exec(imports) funstuff = importlib.import_module('random') print(randrange(10)) print(funstuff.randrange(10))
  41. 41. Daniel Greenfeld @pydanny Python Worst Practice Generally using lambdas 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]))() • 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!
  42. 42. 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
  43. 43. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice swap = 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]))()
  44. 44. Daniel Greenfeld @pydanny Python 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
  45. 45. Daniel Greenfeld @pydanny Fixed 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”
  46. 46. Daniel Greenfeld @pydanny Python 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
  47. 47. Daniel Greenfeld @pydanny Fixed Python Practice urlpatterns = patterns("", PREREQ_APPS = [ # Django url(r"^$", homepage, name="home"), "django.contrib.admin", url(r"^accounts/", include("accounts.urls")), "django.contrib.auth", url(r"^admin/", include(admin.site.urls)), "django.contrib.contenttypes", url(r"^about/", include("about.urls")), "django.contrib.sessions", url(r"^profiles/", include("profiles.urls")), "django.contrib.sites", url(r"^notices/", include("notification.urls")), "django.contrib.messages", ... "django.contrib.humanize", MIDDLEWARE_CLASSES = [ ) "django.contrib.flatpages", "django.middleware.common.CommonMiddleware", "django.contrib.sessions.middleware.SessionMiddleware", # external "django.middleware.csrf.CsrfViewMiddleware", "notification", # must be first "django.contrib.auth.middleware.AuthenticationMiddleware", "staticfiles", "reversion.middleware.RevisionMiddleware", "uni_form", "django.contrib.messages.middleware.MessageMiddleware", ... ... ] ] Explicit is better This isn’t that much typing, is it? then Implicit
  48. 48. Daniel Greenfeld @pydanny Fixed Python Practice urlpatterns = patterns("", PREREQ_APPS = [ # Django url(r"^$", homepage, name="home"), "django.contrib.admin", url(r"^accounts/", include("accounts.urls")), "django.contrib.auth", url(r"^admin/", include(admin.site.urls)), "django.contrib.contenttypes", url(r"^about/", 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
  49. 49. Daniel Greenfeld @pydanny Fixed Python Practice Use a config file spam = "eggs" [actions] call_view = com.pydanny.nextSlide Read up on config parser http://docs.python.org/library/configparser.html
  50. 50. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice XML simple python files logic heavy python config (.cfg) files
  51. 51. Documentation
  52. 52. Daniel Greenfeld @pydanny Python Worst Practice Bad 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: ... • Do really obvious objects require doc strings? • Complex methods require more than docstrings!
  53. 53. 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 code few 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: ...
  54. 54. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice class 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: ...
  55. 55. 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
  56. 56. Daniel Greenfeld @pydanny Fixed Python Practice • Use Restructured Text • Use Sphinx • Host on http://readthedocs.org
  57. 57. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice
  58. 58. Daniel Greenfeld @pydanny Python 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, 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 *! http://pypi.python.org/pypi/that
  59. 59. Daniel Greenfeld @pydanny Fixed 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 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
  60. 60. Daniel Greenfeld @pydanny Side-by-side comparison Worst Practice Fixed Practice >>> import this >>> import that The Zen of Python, by Tim Peters The 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 aren't 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 you're 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, it's a good sell. If the implementation is hard to explain, it's a bad idea. If the implementation is easy to explain, it won't 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 -- let's do more of those!
  61. 61. Questions?

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