Python Revisited Day 04 （控制结构与函数）

4.1 控制结构

4.1.1 条件分支

if ....:
	suite1
elif ...:
	suite2
...
elif ...:
	suiteN
else:
	suite_else

条件表达式

expression1 if boolean_expression else expression2

import random
999 if random.random() > 0.5 else 0  #999 or 0

999 + 1 if random.random() > 0.5 else 0  # 1000 or 0
999 + (1 if random.random() > 0.5 else 0) # 1000 or 999

4.1.2 循环

4.1.2.1 while循环

while boolean_expression:
	while_suite
else: #有些时候还蛮有用的 循环正常执行结束后执行
	else_suite

4.1.2.2 for循环

for expression in iterable:
	for_suite
else: #循环正常结束后执行
	else_suite

for i in range(5):
    print(i)
else:
    print('!!!!')

0
1
2
3
4
!!!!

for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print('!!!!')

0
1
2

return 同样会跳过else_suite

4.2 异常处理

4.2.1 捕获与产生异常

try:
	try_suite
except exception_group1 as variable1:
	except_suite1
...
except exception_group1 as variable1:
	except_suiteN
else: #try部分正常执行才会执行这部分
	else_suite
finally: #总会执行  即便异常没有被捕获到 往往用于确保资源被正确释放
	finally_suite

在这里插入图片描述

更加简单的try...finally...

try:
	try_suite
finally:
	finally_suite

产生异常 raise

raise exception(args)
raise exception(args) from original_exception
raise

raise KeyError('dasd')  # KeyError: 'dasd'

4.2.2 自定义异常

class exceptionName(baseException): pass

注意：上面的baseException是指某个已存在的关于异常的类

class WAWAWAError(KeyError):
    pass

tips 用异常跳出深层嵌套循环

flag = False
for i in range(9):
    for j in range(9):
        for k in range(9):
            if i + j +k > 10:
                flag = True
                break
        if flag:
            break
    if flag:
        break
else:
    print(flag)

当i + j + k > 10的时候，我们希望能跳出循环，虽然这个代码块的样子还挺帅的，但是很蠢吧。

class ExitException(Exception): pass
try:
    for i in range(9):
        for j in range(9):
            for k in range(9):
                if i + j +k > 10:
                    raise ExitException()
except ExitException:
    
    print('Come on!')
else:
    print('You will not see me!')

4.3 自定义函数

Tips 参数默认值为可变时危险

给定默认值的时候，参数时在程序执行def时就建立的了，所以，当参数默认值为可变对象的时候，危险。

def append_if_even(x, lst=[]):  #从对象绑定的角度考虑，合情合理
    if x % 2 == 0:
        lst.append(x)
    print(lst)

append_if_even(2) #[2]
append_if_even(2) #[2, 2] 
append_if_even(2) #[2, 2, 2]
append_if_even(2) #[2, 2, 2, 2]

字符串数字元组等都是固定变量

def append_if_even(x, lst=''):
    if x % 2 == 0:
        lst += '?'
    print(lst)
append_if_even(2) # '?'
append_if_even(2) # '?'
append_if_even(2) # '?'
append_if_even(2) # '?'

def append_if_even(x, lst=None):
	lst = [] if lst is None else lst
    if x % 2 == 0:
        lst += '?'
    print(lst)

4.3.1 名称与Docstrings

def simpledoc(real, dream='sky'):
    """ Returns the text I can not control now...
    
    real is any string; dream is the same as well, while it has default value 'sky'.
    Of course, your different people has various dreams, but we all need to confront 
    the real life.
    >>> simpledoc('god')
    "haha happy"
    >>> simpledoc('god', 'earth')
    "don't cry, go forward..."
    """
    if real == 'god' and dream == 'sky':
        return 'haha happy'
    else:
        return "don't cry, go forward..."

4.3.2 参数与参数拆分

def product(*args):
    print(args)
    result = 1
    for arg in args:
        result += arg
    return result
product(2,3,4,5) # (2, 3, 4, 5)  15

*args 后面仍然可以使用关键词参数

def sum_of_powers(*args, power=1): #虽然power不添加默认值不会报错，但是使用的时候必须用关键词参数的形式传入值
    result = 0
    for arg in args:
        result += arg ** power
    return result

* 用于区分位置参数和关键词参数 def f(a, b, *, c = 0): ...

def sum_and_add(a, b, *, c = 0):
    return a + b + c

sum_and_add(1, 2)   # 3
sum_and_add(1, 2, 1) #TypeError
sum_and_add(1, 2, c = 1) # 4

f(**options)

**用于传入参数

options = dict(a = 1, b = 2, c = 1) #如果有多余的参数，会TypeError
sum_and_add(**options) # 4

**用于函数构建

def add(prime = 0, **adds):
    for key, value in adds.items():
        print(key)
        prime += value
    return prime

add(a = 1, b = 2, c = 3) # a b c 6

4.3.3 存取全局范围的变量 global

def remain():
    global REMAIN
    REMAIN = 3
def sum_and_add(a, b):
    remain() #得执行一次
    return a + b + REMAIN
sum_and_add(1, 2) # 6

4.3.4 Lambda 函数

lambda parameters: expression

expression 不能包含分支或循环，也不能包含return或yield。如果expression是一个元组，那么应该用括号包起来。

f = lambda : (1, 2)
f() # (1, 2)

f = lambda x: "" if x == 1 else 's'
f(1) # ''

4.3.5 断言 assert

assert boolean_expression, optional_expression

def product(*args):
    assert all(args), "0 argument"
    result = 1
    for arg in args:
        result *= arg
        
    return result

product(*[1, 2, 3, 4]) # 24