Python系列6-Python分支结构if语句

比眉伴天荒 2022-11-12 09:33 260阅读 0赞

### 文章目录 ###

*  一.if 语句介绍
 *   *  1.1 简单的if语句
     *  1.2 if-else语句
     *  1.3 if-elif-else 结构
     *  1.4 使用多个elif 代码块
     *  1.5 省略else模块
     *  1.6 测试多个条件
 *  二.使用if 语句处理列表
 *   *  2.1 检查特殊元素
     *  2.2 确定列表不是空的
     *  2.3 使用多个列表
 *  参考:

# 一.if 语句介绍 #

编程时经常需要检查一系列条件，并据此决定采取什么措施。在Python中，if 语句让你能够检查程序的当前状态，并据此采取相应的措施。

**if 语句语法:**

if <条件判断1>:
        <执行1>
    elif <条件判断2>:
        <执行2>
    elif <条件判断3>:
        <执行3>
    else:
        <执行4>

## 1.1 简单的if语句 ##

最简单的if，一个测试和一个操作。如果测试结果为true，则执行操作

语法:

if conditional_test:
        do something

此时有一个场景，判断登陆用户是否是普通用户。

代码:

user_status = 'Normal';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")

测试记录:

>>> user_status = 'Normal';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ...
    Welcom Normal users;
    >>>

## 1.2 if-else语句 ##

经常需要在条件测试通过了时执行一个操作，并在没有通过时执行另一个操作；在这种情况下，可使用Python提供的if-else 语句。if-else 语句块类似于简单的if 语句，但其中的else 语句让你能够指定条件测试未通过时要执行的操作。

语法:

if conditional_test:
        do something
    else:
        do another thing

此时有一个场景，判断登陆用户是否是普通用户,如果不是，给一个输出  
代码:

user_status = 'Normal';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    else:
        print("You are not a Normal users;")
    
    user_status = 'N/A';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    else:
        print("You are not a Normal users;")

测试记录:

>>> user_status = 'Normal';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... else:
    ...     print("You are not a Normal users;")
    ...
    Welcom Normal users;
    >>>
    >>> user_status = 'N/A';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... else:
    ...     print("You are not a Normal users;")
    ...
    You are not a Normal users;
    >>>

## 1.3 if-elif-else 结构 ##

经常需要检查超过两个的情形，为此可使用Python提供的if-elif-else 结构。Python只执行if-elif-else 结构中的一个代码块，它依次检查每个条件测试，直到遇到通过  
了的条件测试。测试通过后，Python将执行紧跟在它后面的代码，并跳过余下的测试。

语法:

if conditional_test1:
        do something1
    elif conditional_test2:
        do something2
    else:
        do another thing

此时有一个场景，判断登陆用户是否是普通用户或者VIP，如果都不是给一个其它输出  
代码:

user_status = 'Normal';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    else:
        print("You are not my users;")
    
    user_status = 'N/A';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    else:
        print("You are not my users;")
    
    user_status = 'VIP';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    else:
        print("You are not my users;")

测试记录:

>>> user_status = 'Normal';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... else:
    ...     print("You are not my users;")
    ...
    Welcom Normal users;
    >>> user_status = 'N/A';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... else:
    ...     print("You are not my users;")
    ...
    You are not my users;
    >>>
    >>>
    >>> user_status = 'VIP';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... else:
    ...     print("You are not my users;")
    ...
    Welcom respect VIP users;
    >>>

## 1.4 使用多个elif 代码块 ##

可根据需要使用任意数量的elif 代码块

语法:

if conditional_test1:
        do something1
    elif conditional_test2:
        do something2
    elif conditional_test3:
        do something3
    else:
        do another thing

此时有一个场景，判断登陆用户是否是普通用户、VIP或者SVIP，如果都不是给一个其它输出  
代码:

user_status = 'Normal';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    elif user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")
    else:
        print("You are not my users;")
    
    user_status = 'N/A';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    elif user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")
    else:
        print("You are not my users;")
    
    user_status = 'VIP';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    elif user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")
    else:
        print("You are not my users;")
    
    user_status = 'SVIP';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    elif user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")
    else:
        print("You are not my users;")

测试记录:

>>> user_status = 'Normal';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... elif user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ... else:
    ...     print("You are not my users;")
    ...
    Welcom Normal users;
    >>> user_status = 'N/A';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... elif user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ... else:
    ...     print("You are not my users;")
    ...
    You are not my users;
    >>> user_status = 'VIP';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... elif user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ... else:
    ...     print("You are not my users;")
    ...
    Welcom respect VIP users;
    >>> user_status = 'SVIP';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... elif user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ... else:
    ...     print("You are not my users;")
    ...
    Welcom respect SVIP users,you are my father;
    >>>
    >>>

## 1.5 省略else模块 ##

Python并不要求if-elif 结构后面必须有else 代码块。  
语法:

if conditional_test1:
        do something1
    elif conditional_test2:
        do something2
    elif conditional_test3:
        do something3

此时有一个场景，判断登陆用户是否是普通用户、VIP或者SVIP，如果都不是不给出输出

代码:

user_status = 'N/A';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    elif user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    elif user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")

测试记录:

>>> user_status = 'N/A';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ... elif user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ... elif user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ...
    >>>
    >>>

## 1.6 测试多个条件 ##

if-elif-else 结构功能强大，但仅适合用于只有一个条件满足的情况：遇到通过了的测试后，Python就跳过余下的测试。这种行为很好，效率很高，让你能够测试一个特定的条件。

然而，有时候必须检查你关心的所有条件。在这种情况下，应使用一系列不包含elif 和else 代码块的简单if 语句。在可能有多个条件为True ，且你需要在每个条件为True时都采取相应措施时，适合使用这种方法。

语法:

if conditional_test1:
        do something1
    
    
    if conditional_test2:
        do something2
    
    
    if conditional_test3:
        do something3

此时有一个场景，判断登陆用户是否是普通用户、VIP或者SVIP，并给出相应输出

代码:

user_status = 'SVIP';
    if  user_status == 'Normal':
        print("Welcom " + user_status + " users;")
    
    
    if user_status == 'VIP':
        print("Welcom respect " + user_status + " users;")
    
        
    if user_status == 'SVIP':
        print("Welcom respect " + user_status + " users,you are my father;")

测试记录:

>>> user_status = 'SVIP';
    >>> if  user_status == 'Normal':
    ...     print("Welcom " + user_status + " users;")
    ...
    >>>
    >>> if user_status == 'VIP':
    ...     print("Welcom respect " + user_status + " users;")
    ...
    >>>
    ... if user_status == 'SVIP':
    ...     print("Welcom respect " + user_status + " users,you are my father;")
    ...
    Welcom respect SVIP users,you are my father;
    >>>
    >>>
    >>>

# 二.使用if 语句处理列表 #

通过结合使用if 语句和列表，可完成一些有趣的任务：对列表中特定的值做特殊处理；高效地管理不断变化的情形。

## 2.1 检查特殊元素 ##

现在有一个场景，有一个列表是当前市面上主流的数据库，公司对数据库进行技术选型，如果员工选择MySQL，需要输出 “MySQL is the most popular Databases”

代码:

dbs = ['Oracle','MySQL','SQL Server','Postgresql','Redis']
    
    for db in dbs:
        if db == 'MySQL':
            print("MySQL is the most popular Databases")

测试记录:

>>> dbs = ['Oracle','MySQL','SQL Server','Postgresql','Redis']
    >>> for db in dbs:
    ...     if db == 'MySQL':
    ...         print("MySQL is the most popular Databases")
    ...
    MySQL is the most popular Databases
    >>>

## 2.2 确定列表不是空的 ##

到目前为止，对于处理的每个列表都做了一个简单的假设，即假设它们都至少包含一个元素。我们马上就要让用户来提供存储在列表中的信息，因此不能再假设循环运行时列表不是空的。有鉴于此，在运行for 循环前确定列表是否为空很重要。

代码:

dbs = []
    
    if dbs:
        for db in dbs:
            if db == 'MySQL':
                print("MySQL is the most popular Databases")
    else:
        print("The dbs is empty!")

测试记录:

>>> dbs = []
    >>>
    >>> if dbs:
    ...     for db in dbs:
    ...         if db == 'MySQL':
    ...             print("MySQL is the most popular Databases")
    ... else:
    ...     print("The dbs is empty!")
    ...
    The dbs is empty!
    >>>

## 2.3 使用多个列表 ##

现在有一个场景，有一个列表是当前市面上主流的数据库，公司目前已在使用几种数据库，需要区分开公司使用的数据库和未使用的数据库。

代码:

dbs = ['Oracle','MySQL','SQL Server','Postgresql','Redis']
    use_dbs = ['Oracle','MySQL','Redis']
    
    if dbs:
        for db in dbs:
            if db in use_dbs:
                print("We have used "+ db + ";")
            else:
                print("We have not use "+ db + ";")
    else:
        print("The dbs is empty!")

测试记录:

>>> dbs = ['Oracle','MySQL','SQL Server','Postgresql','Redis']
    >>> use_dbs = ['Oracle','MySQL','Redis']
    >>>
    >>> if dbs:
    ...     for db in dbs:
    ...         if db in use_dbs:
    ...             print("We have used "+ db + ";")
    ...         else:
    ...             print("We have not use "+ db + ";")
    ... else:
    ...     print("The dbs is empty!")
    ...
    We have used Oracle;
    We have used MySQL;
    We have not use SQL Server;
    We have not use Postgresql;
    We have used Redis;
    >>>

# 参考: #

1.Python编程:从入门到实践