1. 循环与流程控制
(1) for
myList1 = ["A", "B", "C", "D"]# 正序遍历for item in myList1: print(item)# 倒序遍历for item in reversed(myList1): print(item)myList2 = list(range(1,5))# [1, 2, 3, 4]# 等效于其它语言的for(i=0; i<4; i++)for index in range(4): myList2.append(myList1[index])print(myList2)# [1, 2, 3, 4, 'A', 'B', 'C', 'D']# 按索引遍历for index in range(len(myList2)): print("索引", index, "对应的元素是:", myList2[index])
for循环本质上是通过不断调用迭代器(Iterator)的next()
函数来实现的:
from collections import IterablemyList = [3, 6, 9]for i in myList: print(i)# 下面的代码与上面的for循环等效if isinstance(myList, Iterable): myIter = iter(myList)while True: try: x = next(myIter) print(x) except StopIteration: break
(2) while
myList = ["A", "B", "C", "D"]index = 0while index < len(myList): print(myList[index]) index += 1
(3) break和continue
myList = list(range(10))# 循环到5后跳出index = 0while True: print(myList[index]) if index==5: break index += 1# 循环遇到偶数略过,只打印奇数for item in myList: if int(item)%2==0: continue print(item)
2. 函数
# 定义一个空函数def voidFun(): pass# 定义一个求和函数def getSum(x, y): if not isinstance(x, (int, float)) and isinstance(y, (int, float)): raise TypeError("参数'x'和'y'必须为数字") return x + y# 定义一个同时返回最大和最小值的函数def getMinAndMax(x, y, z): return min(x, y, z), max(x, y, z)print(voidFun())# Noneprint(getSum(3, 5))# 8multiValue = getMinAndMax(3, 9, 6)print(multiValue)# 多参数返回其实是一个Tuple# (3, 9)minValue, maxValue = getMinAndMax(3, 9, 6)print(minValue, maxValue)# 3 9
默认参数示例:
def enroll(name, age, sex="male", gold=0, country="China"): print("name:",name,"\n", "age:",age,"\n", "sex:",sex,"\n", "gold:",gold,"\n", "country:",country)enroll("wayne", 23)# wayne 23 male 0 Chinaenroll("tina", 19, "female")# tina 19 female 0 Chinaenroll("andy", 28, gold=15)# andy 28 male 15 China
默认参数需要注意的一点是:默认值最好设置为不可变值(例如字符串或数字),否则会带来意料之外的状况:
def addItemToList(myList=[]): myList.append("End") print(myList)# 提供参数连续调用2次是正常的,因为每次是以新的参数值传入addItemToList([1, 2])addItemToList([1, 2])# [1, 2, 'End']# [1, 2, 'End']# 当使用默认参数连续调用2次时,第1次已经改变了默认参数指向的ListaddItemToList()addItemToList()# ['End']# ['End', 'End']
针对上面的例子,可以修改默认值为None
:
def addItemToList(myList=None): if myList is None: myList = [] myList.append("End") print(myList)
可变参数示例:
def getSum(*numbers): sum = 0 for i in numbers: sum += i print(sum) return sumgetSum()# 0getSum(1, 2, 3)# 6myTuple = (1, 3, 5)getSum(*myTuple)# 9
关键字参数示例:
def enroll(name, age, **params): if "gold" in params: print('传了"gold"这个参数') print("params:",params)enroll("wayne", 23, gold=15, sex="male")# 传了"gold"这个参数# params: {'gold': 15, 'sex': 'male'}
限制关键字参数必须要以key=value
的形式提供参数,它的定义是以*
分隔:
def enroll(name, age, *, gold, country): pass# enroll("wayne", 23, 15, country="China") 未提供关键字将报错enroll("wayne", 23, gold=15, country="China")
如果函数中间存在一个可变参数,在这个可变参数后面的参数自动会被认定为限制关键字参数:
def enroll(name, age, *skills, gold, country): pass# 未提供关键字"gold"将报错# enroll("wayne", 23, "java", "python", 15, country="China") enroll("wayne", 23, "java", "python", gold=15, country="China")
3. 生成器(generator)
当创建一个List或Array时,当元素个数越多,则占用的内存就越高;generator能很好的解决内存占用问题,它的特点是边循环边计算,在处理类似斐波拉契数列的场合非常有用。
定义generator和定义函数比较类似,python通过判断是否含有yield
关键字来加以区分: def fib(max): n, a, b = 0, 0, 1 while n < max: yield b a, b = b, a+b n = n + 1for i in fib(10): print(i)
4. 函数参数
python支持函数参数,示例如下:
def add(x, y): return x+ydef multiply(x, y): return x*ydef compute(x, y, fun): return fun(x, y)print(compute(2, 5, add))print(compute(3, 6, multiply))
map
函数可以让一个Iterable里面的每一个元素执行同一个函数:
def square(x): return x * xmyTuple = tuple(map(square, (2, 4, 6)))# (4, 16, 36)myList = list(map(str,[1, 3, 5]))# ['1', '3', '5']
reduce
函数把一个函数作用在一个序列上,这个函数接收两个参数,reduce
把结果继续和序列的下一个元素做累计计算:
from functools import reducedef multiply(x, y): return x*yresult = reduce(multiply, [1, 2, 3, 4])print(result)# 24
lambda
表达式提供了一个更加简练的函数式语法来写匿名方法,它的调用形式是lambda argument_list: expression
,argument_list是参数列表,expression是表达式,表达式中出现的参数需要在参数列表中有定义。
用法一:将lambda函数赋值给一个变量,通过这个变量间接调用该lambda函数:
add = lambda x,y:x+yadd(3,5)
用法二:将lambda函数赋值给其他函数,从而将其他函数用该lambda函数替换:
import time# 屏蔽sleep函数time.sleep = lambda x:None time.sleep(10)print("test")
用法三:将lambda函数作为参数传递给其他函数。
myList = list(map(lambda x: x*x, [2, 3, 4]))print(myList)# [4, 9, 16]
filter
函数用于过滤列表,需要实现一个过滤函数:
# 过滤出奇数def is_odd(n): return n%2 == 1myList = list(filter(is_odd, [1, 2, 3, 4, 5, 6, 7, 8, 9]))print(myList)# [1, 3, 5, 7, 9]# 过滤出非空字符串def not_empty(s): return s and s.strip()myList = list(filter(not_empty, ['A', '', 'B', None, 'C', ' ']))print(myList)# ['A', 'B', 'C']
sorted
函数用于排序,它接收3个参数:列表,key函数,reverse(正向/反向)。调用sorted函数后并不更改原列表,而是返回一个排序后的可循环列表:
# 默认按升序排列,注意字符串是以ASCII码大小来排序的myList = ["a", "B", "c", "D"]print(sorted(myList))# ['B', 'D', 'a', 'c']# 如果字符串要以"A-Z"的顺序排序,可以先将字母全部转换成大写或小写myTuple = ("BOY", "apple", "Pig", "monkey")print(sorted(myTuple, key=str.lower))# ['apple', 'BOY', 'monkey', 'Pig']# 按绝对值的倒序排序myList = [15, 23, -9, 100, -36]print(sorted(myList, key=abs, reverse=True))# [100, -36, 23, 15, -9]