映射不同迭代的值

例如，计算多个迭代的每个 i th 元素的平均值：

def average(*args):
    return float(sum(args)) / len(args)  # cast to float - only mandatory for python 2.x

measurement1 = [100, 111, 99, 97]
measurement2 = [102, 117, 91, 102]
measurement3 = [104, 102, 95, 101]

list(map(average, measurement1, measurement2, measurement3))
# Out: [102.0, 110.0, 95.0, 100.0]

如果根据 python 的版本将多个 iterable 传递给 map，则有不同的要求：

• 该函数必须采用与迭代一样多的参数：

  def median_of_three(a, b, c):
      return sorted((a, b, c))[1]
  
  list(map(median_of_three, measurement1, measurement2))
  

  TypeError：median_of_three() 缺少 1 个必需的位置参数：‘c’

  list(map(median_of_three, measurement1, measurement2, measurement3, measurement3))
  

  TypeError：median_of_three() 需要 3 个位置参数，但是给出了 4 个

Python 2.x >= 2.0.1

• map：只要一个 iterable 仍没有被完全消耗，迭代就会迭代，但是从完全消耗的 iterables 中假设 None：

  import operator
  
  measurement1 = [100, 111, 99, 97]
  measurement2 = [102, 117]
  
  # Calculate difference between elements
  list(map(operator.sub, measurement1, measurement2))
  

  TypeError：不支持的操作数类型 - ：‘int’和’NoneType’

• itertools.imap 和 future_builtins.map：只要一个 iterable 停止，映射就会停止：

  import operator
  from itertools import imap
  
  measurement1 = [100, 111, 99, 97]
  measurement2 = [102, 117]
  
  # Calculate difference between elements
  list(imap(operator.sub, measurement1, measurement2))
  # Out: [-2, -6]
  list(imap(operator.sub, measurement2, measurement1))
  # Out: [2, 6]
  

Python 3.x >= 3.0.0

• 一旦迭代停止，映射就会停止：

  import operator
  
  measurement1 = [100, 111, 99, 97]
  measurement2 = [102, 117]
  
  # Calculate difference between elements
  list(map(operator.sub, measurement1, measurement2))
  # Out: [-2, -6]
  list(map(operator.sub, measurement2, measurement1))
  # Out: [2, 6]