python lambda函數(shù)及三個常用的高階函數(shù)

"""
lambda 參數(shù)列表 : 返回值
lambda 參數(shù)形式：
  1.無參數(shù)：lambda：100
  2.一個參數(shù)：lambda a: a
  3.默認參數(shù)：lambda a, b, c=100: a + b + c
  4.可變參數(shù)：*args,返回元組
  lambda *args: args
  5.可變參數(shù)：*kwargs,返回字典
  lambda **kwargs: kwargs
lambda 應用：簡化代碼
  1.判斷，lambda a, b: a if a > b else b
  2.列表數(shù)據(jù)按字典 key 值排序
"""
# def fn1():
#   return 100
# def de1():
#   r = fn1()
#   print(r)
#   r2 = lambda: 100
#   print(r2)      # 地址：<function main.<locals>.<lambda> at 0x00CCD6E8>
#   print(r2())     # 返回值：100
# def test2():
"""
lambda 測試案例
"""
#   fn1 = lambda a, b: a + b
#   print(fn1(1, 2))
#   fn2 = lambda *args: args
#   print(fn2(1, 2, 3))
#   print(fn2(1, 3))
#   print(fn2(1))    # (1,)
#   fn3 = lambda **kwargs: kwargs
#   print(fn3(name='py', age=30))
#
#   fn4 = lambda a, b: a if a > b else b
#   print(fn4(100, 200))
#
#   student = [{'name': 'a', 'id': '1', 'tel': 'a1'},
#     {'name': 'b', 'id': '2', 'tel': 'b2'},
#     {'name': 'c', 'id': '3', 'tel': 'c3'}]
#   student.sort(key=lambda x: x['name'])
#   print(student)
#   student.sort(key=lambda x: x['name'], reverse=True)
#   print(student)
#   student.sort(key=lambda x: x['id'])
#   print(student)
"""
高階函數(shù)：def add_num(a, b, f)
內置高階函數(shù)：
  1.map(func, list)：
  將func 應用于list中的每一個元素，返回一個迭代器
  2.reduce(func, list)：
  計算結果與下一個數(shù)據(jù)做累積計算，必須有兩個參數(shù)
  from functools import reduce
  3.filter(func, list)：
  過濾掉不符合條件的元素，返回一個filter對象，可用list()轉換
"""
# def add_num(a, b, f):
#   # 傳入函數(shù) f
#   return f(a) + f(b)
# def func(x):
#   return x ** 2
from functools import reduce
def add_2(a, b):
  return a + b
def func3(x):
  return x % 2 == 0
def main():
  """
  體驗高階函數(shù)
  :return:
  """
  # print(abs(-2))     # 求絕對值
  # print(round(0.1))    # 四舍五入
  # s1 = add_num(1, 2, abs)
  # s2 = add_num(1, 2.5, round)
  # print(s1, s2)
  list1 = [1, 2, 3, 4, 5]
  # map(func, list1)
  # print(list(map(func, list1)))
  # s = reduce(add_2, list1)
  # print(s)
  r = filter(func3, list1)
  print(r)    # <filter object at 0x00CAE2E0>
  print(list(r)) # [2, 4]
if __name__ == '__main__':
  main()