Python系列6-Python分支结构if语句

作者: 只是甲 | 来源:发表于2021-03-23 13:56 被阅读0次

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

Python系列6-Python分支结构if语句

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:

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