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Python Best Practices
Lawrence Wachira edited this page Feb 15, 2017
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3 revisions
# UTF-8 Encoding:
# -*- coding: utf-8 -*-
# utf8 is one implement of unicode
# str1.decode('gb2312') str1(gb2312) to unicode(commmon)
# str2.encode('gb2312') str2(unicode(common)) to gb2312
# key word:
# and del from not while
# as elif global or with
# assert else if pass yield
# break except import print
# class exec in raise
# continue finally is return
# def for lambda try
# _* :
# useless for `from module import *`
# __*__ :
# System use
# __* :
# Class private variable
# new-style class A(object): Breadth-First-Search
# old-style class A: Depth-First-Searchlocal -> module -> built-in
local is read-only , when you access it , it will return a copy
global is writeable
easy_install & pip : Python's package managers, use pip
- Very bad
[...]
from modu import *
[...]
x = sqrt(4) # Is sqrt part of modu? A builtin? Defined above?- Better
from modu import sqrt
[...]
x = sqrt(4) # sqrt may be part of modu, if not redefined in between- Best
import modu
[...]
x = modu.sqrt(4) # direct import module , sqrt is visibly part of modu's namespacedef foo():
# do something
def decorator(func):
# manipulate func
return func
foo = decorator(foo) # Manually decorate
@decorator
def bar():
# Do something
# bar() is decorated auto- Bad
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = ""
for n in range(20):
nums += str(n) # slow and inefficient , because a string is immutable
print nums- Good
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = []
for n in range(20):
nums.append(str(n))
print "".join(nums) # much more efficient- Best
# create a concatenated string from 0 to 19 (e.g. "012..1819")
nums = [str(n) for n in range(20)] # more beauty
print "".join(nums)- Note:
foobar = '%s%s' % (foo, bar) # It is OK
foobar = '{0}{1}'.format(foo, bar) # It is better
foobar = '{foo}{bar}'.format(foo=foo, bar=bar) # It is best- Bad
def make_complex(*args):
x, y = args
return dict(**locals()) # local variable- Good
def make_complex(x, y):
return {'x': x, 'y': y}for index, item in enumerate(some_list):
# do something with index and item- In Py3
a, *rest = [1, 2, 3]
# a = 1, rest = [2, 3]
a, *middle, c = [1, 2, 3, 4]
# a = 1, middle = [2, 3], c = 4- Create an ignored variable
filename = 'foobar.txt'
basename, __, ext = filename.rpartition('.')- Bad
if attr == True:
print 'True!'
if attr == None:
print 'attr is None!'- Good
# Just check the value
if attr:
print 'attr is truthy!'
# or check for the opposite
if not attr:
print 'attr is falsey!'
# or, since None is considered false, explicitly check for it
if attr is None:
print 'attr is None!'- Bad
d = {'hello': 'world'}
if d.has_key('hello'):
print d['hello'] # prints 'world'
else:
print 'default_value'- Good
d = {'hello': 'world'}
print d.get('hello', 'default_value') # prints 'world'
print d.get('thingy', 'default_value') # prints 'default_value'
# Or:
if 'hello' in d:
print d['hello']- Bad
# Filter elements greater than 4
a = [3, 4, 5]
b = []
for i in a:
if i > 4:
b.append(i)- Good
a = [3, 4, 5]
b = [i for i in a if i > 4]
# Or:
b = filter(lambda x: x > 4, a)- Bad
# Add three to all list members.
a = [3, 4, 5]
for i in range(len(a)):
a[i] += 3- Good
a = [3, 4, 5]
a = [i + 3 for i in a]
# Or:
a = map(lambda i: i + 3, a)questions = ['name', 'quest', 'favorite color']
answers = ['lancelot', 'the holy grail', 'blue']
for q, a in zip(questions, answers):
print 'What is your {0}? It is {1}.'.format(q, a)
# What is your name? It is lancelot.
# What is your quest? It is the holy grail.
# What is your favorite color? It is blue.- Bad
f = open('file.txt')
a = f.read()
print a
f.close()- Good
with open('file.txt') as f:
for line in f:
print line- Bad
my_very_big_string = """For a long time I used to go to bed early. Sometimes, \
when I had put out my candle, my eyes would close so quickly that I had not even \
time to say “I’m going to sleep.”"""
from some.deep.module.inside.a.module import a_nice_function, another_nice_function, \
yet_another_nice_function- Good
my_very_big_string = (
"For a long time I used to go to bed early. Sometimes, "
"when I had put out my candle, my eyes would close so quickly "
"that I had not even time to say “I’m going to sleep.”"
)
from some.deep.module.inside.a.module import (
a_nice_function, another_nice_function, yet_another_nice_function)What You Wrote
def append_to(element, to=[]):
to.append(element)
return toWhat You Might Have Expected to Happen
my_list = append_to(12)
print my_list # [12]
my_other_list = append_to(42)
print my_other_list # [42]What Does Happen
# [12]
# [12,42]Because:Python’s default arguments are evaluated once when the function is defined, not each time the function is called (like it is in say, Ruby). This means that if you use a mutable default argument and mutate it, you will and have mutated that object for all future calls to the function as well.
What You Should Do Instead
def append_to(element, to=None):
if to is None:
to = []
to.append(element)
return toWhat You Wrote
def create_multipliers():
return [lambda x : i * x for i in range(5)]What You Might Have Expected to Happen
for multiplier in create_multipliers():
print multiplier(2)
# 0 2 4 6 8What Does Happen
# 8 8 8 8 8Because Python’s closures are late binding. This means that the values of variables used in closures are looked up at the time the inner function is called.Here, whenever any of the returned functions are called, the value of i is looked up in the surrounding scope at call time
What You Should Do Instead
def create_multipliers():
return [lambda x, i=i : i * x for i in range(5)]
# Or
from functools import partial
from operator import mul
def create_multipliers():
return [partial(mul, i) for i in range(5)]pip install pep8
pip install requests
pip install Pillow
http://docs.python-guide.org/en/latest/
http://harveyqing.gitbooks.io/python-read-and-write/content/1_python/12.html