trick_codes.md

Autovivification 自实现

• dict to tree # or inherit defaultdict

  class Tree(dict):
     def __missing__(self, key):
         value = self[key] = self.__class__()
         return value
     __getattr__ = dict.__getitem__
     __setattr__ = dict.__setitem__
  
  tree = Tree()
  tree.aaaaa.bbbbb.cccccc
  tree.aaaaa.b2222222.ccccw = 9
  print (tree)
  prttree(tree) # see more  
  
  # {'aaaaa': {'b2222222': {'ccccw': 9}, 'bbbbb': {'cccccc': {}}}}
  #   0.> aaaaa
  #       1.> b2222222
  #           2.> ccccw
  #       1.> bbbbb
  #           2.> cccccc

  prttree see more ref1: wikipedia ref2 : gist hrldcpr

  ​

  eg2 : attr to url

  class Url(str):
      __add__ = lambda self,name: Url(str.__add__(self, "/" + str(name)))
      __getattr__ = __add__
  
  url_path = Url('https://github.com').willowj.py_tricks.master + 5 + 8 + 9
  
  print(url_path)
  # https://github.com/willowj/py_tricks/master/5/8/9

  ​

SUM with start param

• lis =  [[1, 2, 3], [4, 5, 6], [7, 8, 9], [1, 5], [5, [9]]]
  
  reduce(lambda x,y:x+y,i)
  sum(lis,[])  # little quicker than reduce and for loop
  
  #[1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 5, 5, [9]]
  #[1, 2, 3, 4, 5, 6, 7, 8, 9, 1, 5, 5, [9]]

  sum(sequence[, start]) -> value

  Return the sum of a sequence of numbers (NOT strings) plus the value of parameter 'start' (which defaults to 0).

  When the sequence is empty, return start. Type: builtin_function_or_method

  ref

IF else <-> and or 一行条件判断

• a = 0 
  b = 7
  
  r1 = "a" if (a > b) else "b"	   # little slower in py3
  
  r2 = a > b and "a" or "b"          # little slower in py2, 

Singleton 单例模式

​ singleton decorator

• import threading
  def singleton(cls):
     instances_dict = dict()
     _lock = threading.Lock()
     def wraper(*args, **kwargs):
         if cls not in instances_dict:
             with _lock :
                 if cls not in instances_dict:
                     instances_dict[cls] = cls(*args, **kwargs)
                 else:
                     instances_dict[cls].__init__(*args, **kwargs)
         return instances_dict[cls]
     return wraper
  
  @singleton
  class Test(object):pass
  
  Test() is Test()

  ​

collection

• namedtuple()	factory function for creating tuple subclasses with named fields
  deque	list-like container with fast appends and pops on either end
  ChainMap	dict-like class for creating a single view of multiple mappings
  Counter	dict subclass for counting hashable objects
  OrderedDict	dict subclass that remembers the order entries were added
  defaultdict	dict subclass that calls a factory function to supply missing values
  UserDict	wrapper around dictionary objects for easier dict subclassing
  UserList	wrapper around list objects for easier list subclassing
  UserString	wrapper around string objects for easier string subclassing

itertools

​

• # Cartesian product
  for p in itertools.product([1, 2, 3], [4, 5]):
      print(p)
      
  (1, 4)
  (1, 5)
  (2, 4)
  (2, 5)
  (3, 4)
  (3, 5)

more :

​ pythontips -intermediatePython 中文版

​ stackoverflow

​ quora

​ jobbole（zh）