《Python之禅》中对于Python编程过程中的一些建议

#-----------------------------------------------------------------------
 halve_evens_only = lambda nums: map(lambda i: i/2,\
 filter(lambda i: not i%2, nums))
 #-----------------------------------------------------------------------
def halve_evens_only(nums):
   return [i/2 for i in nums if not i % 2]

a, b = b, a

a = [1,2,3,4,5]
>>> a[::2] # 以步长为2的增量迭代整个list对象
[1,3,5]

>>> a[::-1]
 [5,4,3,2,1]

def function(x, l=[]):     # 不要这么干
def function(x, l=None):    # 更好的一种方式
  if l is None:
    l = []

d = {1: "1", 2: "2", 3: "3"}
 
for key, val in d.items()    # 调用items()后会构建一个完整的list对象
 
for key, val in d.iteritems()  # 只有在迭代时每请求一次才生成一个值

if type(s) == type(""): ...
if type(seq) == list or \
  type(seq) == tuple: ...

if isinstance(s, basestring): ...
if isinstance(seq, (list, tuple)): ...

>>> a=u'aaaa'
>>> print isinstance(a, basestring)
True
>>> print isinstance(a, str)
False

freqs = {}
for c in "abracadabra":
  try:
    freqs[c] += 1
  except:
    freqs[c] = 1

freqs = {}
  for c in "abracadabra":
    freqs[c] = freqs.get(c, 0) + 1

from collections import defaultdict
freqs = defaultdict(int)
  for c in "abracadabra":
    freqs[c] += 1

namedtuple()    # 用指定的域创建元组子类的工厂函数
deque      # 类似list的容器，快速追加以及删除在序列的两端
Counter     # 统计哈希表的dict子类
OrderedDict      # 记录实体添加顺序的dict子类
defaultdict      # 调用工厂方法为key提供缺省值的dict子类

__eq__(self, other)   # 定义相等操作的行为, ==.
__ne__(self, other)   # 定义不相等操作的行为, !=.
__lt__(self, other)   #定义小于操作的行为, <.
__gt__(self, other)   #定义不大于操作的行为, >.
__le__(self, other)   #定义小于等于操作的行为, <=.
__ge__(self, other)   #定义大于等于操作的行为, >=.

x = 3 if (y == 1) else 2

x = 3 if (y == 1) else 2 if (y == -1) else 1

(func1 if y == 1 else func2)(arg1, arg2)

x = (class1 if y == 1 else class2)(arg1, arg2)

class MyClass(object):
  def __init__(self, a, b, c, d):
    self.a, self.b, self.c, self.d = a, b, c, d
 
  def __getitem__(self, item):
    return getattr(self, item)
 
x = MyClass(10, 12, 22, 14)

def __getitem__(self, item):
   if item is Ellipsis:
     return [self.a, self.b, self.c, self.d]
   else:
     return getattr(self, item)

>>> x = MyClass(11, 34, 23, 12)
>>> x[...]
[11, 34, 23, 12]