This document discusses various Python anti-patterns and better practices for writing clean Python code. It provides examples of bad practices like catching all exceptions, not using enumerate() or zip() when iterating over multiple lists, and string concatenation for formatting. Better alternatives shown include specifying exception types, using enumerate() and zip(), and string formatting methods. The document also recommends using context managers, generators, unpacking values, and the any() and all() built-in functions.

1. PYTHON ANTI-PATTERNS
(BUT MOSTLY TIPS 'N TRICKS)
RIK VAN ACHTERBERG

2. CATCHING EXCEPTIONS
Don't:
try:
<somecode>
except:
<handleALLexceptions>
You never know whatyou're gonna catch.
try:
<somecode>
exceptIndexError:
<handlespecificexception>
except(IOError,OSError):
<handlespecificexceptions>
exceptStandardErrorase:
printe.message
<handlebaseexception>
exceptExceptionase:
<wheneverythingelsefails>
raisee

3. Instead, do:
NOT USING ENUMERATE()
Don't:
>>>my_list=['a','b','c']
foridxinrange(0,len(my_list)):
printidx,my_list[idx]
0a
1b
2c
>>>counter=0
foreachinmy_list:
printcounter,my_list[counter]
counter+=1
>>>foridx,iteminenumerate(my_list):
printidx,item
0a
1b
2c

4. Good:
ITERATING OVER SEVERAL LISTS
Bad:
>>>a=[1,2,3]
b=[4,5,6]
foreachina:
idx=a.index(each)
printeach,b[idx]
14
25
36
>>>fora_item,b_iteminzip(a,b):
printa_item,b_item
14
25
36
>>>zip(a,[])
[]
>>>zip([1,2],['a'])
[(1,'a')]

5. Better:
USE DEFAULT VALUES ON DICTS
Possible:
try:
a=some_dict[variable]
exceptIndexError:
a=None
a=None
ifvariableinsome_dict:
a=some_dict[variable]
a=some_dict.get(variable,None)

6. EAFP
Asking permission:
ifos.exists(some_file):
open(some_file)
else:
...
Asking forgiveness:
try:
open(some_file)
exceptIOError:
...
Prevents race conditions
Exceptions are cheap in Python

7. Butaccording to Python Style Guide, parentheses are preferred:
BREAKING LONG LINES
This is possible:
ifais"somevalue"
andbis"someothervalue"
andcisnot"thisothervalue":
pass
if(ais"somevalue"
andbis"someothervalue"
andcisnot"thisothervalue"):
pass

8. Better:
USE CONTEXT MANAGERS
Mostnotably, for open()
f=open('file.txt','r')
printf.read()
f.close()
withopen('file.txt','r')asf:
printf.read()
The file willalways be properlyclosed
"contextmanagers provide __enter__() and __exit__()
methods thatare invoked on entryto and exitfrom the
bodyof the with statement."

9. CLASS METHOD CASCADING
classSomeClass(object):
deflight_on(self):
[...]
returnself
defsound_on(self):
[...]
returnself
>>>sc=SomeClass()
>>>sc.light_on().sound_on()
Onlyworks when classes inheritfrom object

10. Shorter (and cleaner)
USING LIST COMPREHENSION
colors=['blue','green','yellow','red','brown','black']
x=[]
foritemincolors:
if'e'initem:
x.append(item)
>>>x
['blue','green','yellow','red']
>>>[cforcincolors]
['blue','green','yellow','red','brown','black']
>>>[cforcincolorsif'e'incolor]
['blue','green','yellow','red']
>>>[c.upper()forcincolorsif'e'incolor]
['BLUE','GREEN','YELLOW','RED']
no nested loop necessary
more concise

11. OR DICT COMPREHENSION, FOR THAT
MATTER
colors=['blue','green','yellow','red','brown','black']
>>>dict([(c,len(c))forcincolors])
{'blue':4,'brown':5,'yellow':6,'green':5,'black':5,'red':3}
#isthesameas
>>>{c:len(c)forcincolors}
{'blue':4,'brown':5,'yellow':6,'green':5,'black':5,'red':3}

12. Instead, write
USING EXPLICIT FALSE
Values thatare considered false are:
None
False
Zero of anynumeric type
Anyemptycontainer, e.g. '',(),[],{}
So, this is unnecessary:
iflen(some_list)>0:
...
ifinteger_value<1:
...
ifsome_list:
...
ifnotinteger_value:
...
Does notapplyto sets!

13. CHECKING FOR NONE TYPES
BAD:
ifobj==None:
...
ifobj!=None:
...
ifnotobj==None:
...
"Comparisons to singletons like None should always
be done with is or is not, never the equalityoperators." (PEP8)
Classes are free to implementcomparison.
So __eq__() or __cmp__() can be defined in anyway
So playsafe - compare objectidentities using is
also, is is faster!
ifobjisNone:
...
ifobjisnotNone:
...

14. Better:
STRING FORMATTING
Don'tuse concatenation.
>>>"Thevalueis"+str(12)+"!"
'Thevalueis12!'
>>>"Thevalueis%s!"%12
'Thevalueis12!'
>>>"Thevaluesare%sand%s"%("One","Two")
'ThevaluesareOneandTwo'
>>>"Thevalueis%d!"%12
'Thevalueis12!'
>>>"Good%(daypart)sthere%(name)s!"%{'daypart':'morning','name':'Bob'}
'GoodmorningthereBob!'
>>>"Thevalueis{0},Irepeat,it's{0}!".format(12)
"Thevalueis12,Irepeat,it's12!"
>>>"Hellothere,mynameis{0[name]}andI'm{0[age]}yearsold".format(
{'name':"Hank",'age':68})
"Hellothere,mynameisHankandI'm68yearsold"

15. Applies to listcomprehension:Generator expression
USING GENERATOR FUNCTIONS
Possible
defget_numbers():
data=[]
foreachinget_data():
ifeach>10:
data.append(each)
returndata
Possiblybetter
defget_numbers():
foreachinget_data():
ifeach>10:
yieldeach
>>>data_generator=(dfordinget_data()ifd>10)
printdata_generator
<generatorobject<genexpr>at0x3027640>
>>>foriindata_generator:
>>>...printi
[11,12,13]

16. When you can do this:
UNPACKING VALUES
Whydo this:
>>>do_something()
('Operationsucceeded',True)
>>>values=do_something()
message=values[0]
success=values[1]
>>>message,success=do_something()
printmessage,success
OperationsucceededTrue

17. All():
USING ANY() AND ALL() BUILTINS
Any():
defany(iterable):
forelementiniterable:
ifelement:
returnTrue
returnFalse
>>>any([True,True,False])
True
defall(iterable):
forelementiniterable:
ifnotelement:
returnFalse
returnTrue
>>>all([True,True,False])
False
>>>all([True,True,True])
True

18. for &else
except&else
"ELSE" IN UNEXPECTED PLACES
(think of itas "nobreak")
while &else
whilei<10:
...
else:
... #Willbeexecutedwheni>=10-notonabreak!
foriteminmy_list:
...
else:
... #Willbeexecutedwhenforclauseendsnormally(withoutbreak)
try:
something()
exceptRuntimeError():
...
else:
... #Willbeexecutedwhennoexceptionwascaught
finally:
... #Willbeexecutedinanycase

19. THANKS. FURTHER READING:
http://lignos.org/py_antipatterns/
https://gist.github.com/sloria/7001839
http://google-styleguide.googlecode.com/svn/trunk/pyguide.html
http://docs.python-guide.org/en/latest/writing/style/
http://stevenloria.com/python-best-practice-patterns-by-vladimir-keleshev-not
es/
https://docs.python.org/2/howto/doanddont.html
http://blog.grahampoulter.com/2009/01/bad-python.html
http://stackoverflow.com/questions/576988/python-specific-antipatterns-and-ba
d-practices
http://www.slideshare.net/pydanny/python-worst-practices
http://www.scribd.com/doc/39946630/Python-Idioms
http://www.jeffknupp.com/blog/2012/10/04/writing-idiomatic-python/
https://stackoverflow.com/questions/101268/hidden-features-of-python?rq=1
http://forums.udacity.com/questions/2005120/implicit-boolean-evaluation-aka-t
ruthinessfalsiness
https://mail.python.org/pipermail/python-ideas/2009-October/006155.html(disc
ussionabout"else")