python各种推导式分析

variable = [out_exp_res for out_exp in input_list if out_exp == 2]
  out_exp_res:　　列表生成元素表达式，可以是有返回值的函数。
  for out_exp in input_list：　　迭代input_list将out_exp传入out_exp_res表达式中。
  if out_exp == 2：　　根据条件过滤哪些值可以。

multiples = [i for i in range(30) if i % 3 is 0]
print(multiples)
# Output: [0, 3, 6, 9, 12, 15, 18, 21, 24, 27]

def squared(x):
    return x*x
multiples = [squared(i) for i in range(30) if i % 3 is 0]
print multiples
#  Output: [0, 9, 36, 81, 144, 225, 324, 441, 576, 729]

multiples = (i for i in range(30) if i % 3 is 0)
print(type(multiples))
#  Output: <type 'generator'>

mcase = {'a': 10, 'b': 34, 'A': 7, 'Z': 3}
mcase_frequency = {
    k.lower(): mcase.get(k.lower(), 0) + mcase.get(k.upper(), 0)
    for k in mcase.keys()
    if k.lower() in ['a','b']
}
print mcase_frequency
#  Output: {'a': 17, 'b': 34}

mcase = {'a': 10, 'b': 34}
mcase_frequency = {v: k for k, v in mcase.items()}
print mcase_frequency
#  Output: {10: 'a', 34: 'b'}

squared = {x**2 for x in [1, 1, 2]}
print(squared)
# Output: set([1, 4])