文章目录

4.1 控制结构

4.1.1 条件分支

if ....:	suite1elif ...:	suite2...elif ...:	suiteNelse:	suite_else

条件表达式

expression1 if boolean_expression else expression2

import random999 if random.random() > 0.5 else 0  #999 or 0

999 + 1 if random.random() > 0.5 else 0  # 1000 or 0999 + (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_suiteelse: #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_suiteelse: #循环正常结束后执行	else_suite

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

01234!!!!

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

012

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_suiteexcept exception_group1 as variable1:	except_suite1...except exception_group1 as variable1:	except_suiteNelse: #try部分正常执行才会执行这部分	else_suitefinally: #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

更加简单的try…finally…

try:	try_suitefinally:	finally_suite

产生异常 raise

raise exception(args)raise exception(args) from original_exceptionraise

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 = Falsefor 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:        breakelse:    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): passtry:    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 resultproduct(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 + csum_and_add(1, 2)   # 3sum_and_add(1, 2, 1) #TypeErrorsum_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TypeErrorsum_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 primeadd(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 = 3def sum_and_add(a, b):    remain() #得执行一次    return a + b + REMAINsum_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 resultproduct(*[1, 2, 3, 4]) # 24

练习

import osimport sysWORD_FORWARD = "Choose filename: "WORD_CONTINUE = "Press Enter to continue..."WORD_OPTION1 = "[A/a]dd [D/d]elete [S/s]ave    [Q/q]uit [a]: "WORD_OPTION2 = "[A/a]dd [Q/q]uit [a]: "WORD_ERROR_FILENAME = "Sorry, {0} is not found..."WORD_ERROR_OPTION1 = "ERROR: invalid choice--enter one of 'AaDdSsQq'"WORD_ERROR_OPTION2 = "ERROR: invalid choice--enter one of 'AaQq'"WORD_FILES_ZERO = "-- no items are in list --"WORD_ADD_ITEM = "Add item: "WORD_DELETE_ITEM = "Delete item number (or 0 to cancel): "WORD_ERROR_DELETE = "The number exceeds the limits..."WORD_SAVE_ITEM = "Saved {0} item{1} to {2}"WORD_SAVE_UNSAVED = "Save unsaved changes (y/n) [y]: "def filename_and_set():    f = None    files = []    global filename    try:        filename = input(WORD_FORWARD)        if filename[-4:] != '.txt':  #.txt 代替.lst            filename += '.txt'        f = open(filename)        for item in f:            files.append(item.rstrip())    except FileNotFoundError:        pass    finally:        if f is not None:            f.close()    return filesdef delete_item(files):    flag = input(WORD_DELETE_ITEM)    try:        flag = int(flag)        if flag is 0:            pass        else:            files.pop(flag - 1)    except ValueError:        print("Integer is need...")    except IndexError:        print(WORD_ERROR_DELETE)def save_item(files):    f = None    n = len(files)    try:        f = open(filename, 'w', encoding='utf8')        for item in files:            f.write(item + '\n')        print(WORD_SAVE_ITEM.format(n,                                    's' if n > 1 else '',                                    filename))    except:        print('ERROR: SAVE...')    finally:        if f is not None:            f.close()def quit_item(files):    n = len(files)    flag = input(WORD_SAVE_UNSAVED)    if flag is 'y' or not flag:        save_item(files)    sys.exit()def option1(files, label):    if label == 'A' or label == 'a' or not label:        files.append(input(WORD_ADD_ITEM))    elif label == 'D' or label == 'd':        delete_item(files)    elif label == 'S' or label =='s':        save_item(files)    elif label == 'Q' or label == 'q':        quit_item(files)    else:        print(WORD_ERROR_OPTION1)def option2(files, label):    if label == 'A' or label == 'a' or not label:        files.append(input(WORD_ADD_ITEM))    elif label == 'Q' or label == 'q':        quit_item(files)    else:        print(WORD_ERROR_OPTION2)def screen_show(files):    n = len(files)    if n != 0:        files.sort()    if not n:        print(WORD_FILES_ZERO)        label = input(WORD_OPTION2)        option2(files, label)    else:        for i in range(n):            print("{0}: {1}".format(i+1, files[i]))        label = input(WORD_OPTION1)        option1(files, label)def main():    files = filename_and_set()    count = 1    while True:        count += 1        if count > 10:            break        screen_show(files)        print('\n\n')main()