Python 简明教程

Python if-else Statement

Python if else Statement

Python 中的 if-else 语句用于在 if statement 中的条件为真时执行一段代码，在条件为假时执行另一段代码。

The if-else statement in Python is used to execute a block of code when the condition in the if statement is true, and another block of code when the condition is false.

Syntax of if-else Statement

Python 中 if-else 语句的语法如下 −

The syntax of an if-else statement in Python is as follows −

if boolean_expression:
  # code block to be executed
  # when boolean_expression is true
else:
  # code block to be executed
  # when boolean_expression is false

如果布尔表达式求值为 TRUE，则 if 块中的语句将被执行，否则 else 块的语句将被执行。

If the boolean expression evaluates to TRUE, then the statement(s) inside the if block will be executed otherwise statements of the else block will be executed.

Flowchart of if-else Statement

此流程图显示了如何使用 if-else 语句 −

This flowchart shows how if-else statement is used −

如果 expr 为真，则执行 stmt1、2、3 块，然后默认流继续 stmt7。但是，如果 expr 为假，则 stmt4、5、6 块运行，然后默认流继续。

If the expr is True, block of stmt1, 2, 3 is executed then the default flow continues with stmt7. However, if the expr is False, block stmt4, 5, 6 runs then the default flow continues.

以下流程图的 Python 实现如下：

Python implementation of the above flowchart is as follows −

if expr==True:
   stmt1
   stmt2
   stmt3
else:
   stmt4
   stmt5
   stmt6
Stmt7

Python if-else Statement Example

让我们通过以下示例了解 if-else 语句的用法。这里，变量年龄可以取不同的值。如果表达式 age > 18 为真，则会显示有资格投票的消息，否则将显示没有资格投票的消息。以下流程图说明了此逻辑 −

Let us understand the use of if-else statements with the following example. Here, variable age can take different values. If the expression age > 18 is true, then eligible to vote message will be displayed otherwise not eligible to vote message will be displayed. Following flowchart illustrates this logic −

现在，让我们看看 Python 中上述流程图的实现。

Now, let’s see the Python implementation the above flowchart.

age=25
print ("age: ", age)
if age >=18:
   print ("eligible to vote")
else:
   print ("not eligible to vote")

执行此代码后，您将获得以下 output −

On executing this code, you will get the following output −

age: 25
eligible to vote

若要测试 else 代码块，请将年龄更改为 12，然后再次运行代码。

To test the else block, change the age to 12, and run the code again.

age: 12
not eligible to vote

Python if elif else Statement

if elif else 语句允许你检查多个表达式是否为 TRUE，并且一旦某个条件判断为 TRUE，则执行一段代码块。

The if elif else statement allows you to check multiple expressions for TRUE and execute a block of code as soon as one of the conditions evaluates to TRUE.

类似于 else 代码块，elif 代码块也是可选的。但是，一个程序只能包含一个 else 代码块，而一个 if 代码块之后可以有多个 elif 代码块。

Similar to the else block, the elif block is also optional. However, a program can contains only one else block whereas there can be an arbitrary number of elif blocks following an if block.

Syntax of Python if elif else Statement

if expression1:
   statement(s)
elif expression2:
   statement(s)
elif expression3:
   statement(s)
else:
   statement(s)

How if elif else Works?

关键字elif是else if的缩写形式。它允许在第一个 if 语句之后将逻辑排列在 elif 语句级联中。如果第一个 if 语句判断为 false，后续的 elif 语句将逐一判断，并且只要满足任意一个 elif 语句，代码都将跳出级联。

The keyword elif is a short form of else if. It allows the logic to be arranged in a cascade of elif statements after the first if statement. If the first if statement evaluates to false, subsequent elif statements are evaluated one by one and comes out of the cascade if any one is satisfied.

级联中的最后一个语句是 else 代码块，当所有前面 if/elif 条件都失败时，else 代码块将参与其中。

Last in the cascade is the else block which will come in picture when all preceding if/elif conditions fails.

Example

假设购买时有不同的折扣分层−

Suppose there are different slabs of discount on a purchase −

20% on amount exceeding 10000,
10% for amount between 5-10000,
5% if it is between 1 to 5000.
no discount if amount<1000

以下流程图说明了这些条件：

The following flowchart illustrates these conditions −

我们可以使用 if-else 语句为上述逻辑编写 Python 代码−

We can write a Python code for the above logic with if-else statements −

amount = 2500
print('Amount = ',amount)
if amount > 10000:
   discount = amount * 20 / 100
else:
   if amount > 5000:
      discount = amount * 10 / 100
   else:
      if amount > 1000:
         discount = amount * 5 / 100
      else:
         discount = 0

print('Payable amount = ',amount - discount)

设置金额以测试所有可能条件：800、2500、7500 和 15000。输出将相应地变化−

Set amount to test all possible conditions: 800, 2500, 7500 and 15000. The outputs will vary accordingly −

Amount: 800
Payable amount = 800
Amount: 2500
Payable amount = 2375.0
Amount: 7500
Payable amount = 6750.0
Amount: 15000
Payable amount = 12000.0

虽然该代码可以正常运行，但如果你查看每个 if 和 else 语句的缩进层次不断增加，你会发现，如果有更多条件，代码将很难管理。

While the code will work perfectly fine, if you look at the increasing level of indentation at each if and else statement, it will become difficult to manage if there are still more conditions.

Python if elif else Statement Example

elif 语句使代码易于阅读和理解。以下是使用 if elif else 语句实现相同逻辑的 Python 代码−

The elif statement makes the code easy to read and comprehend. Following is the Python code for the same logic with if elif else statements −

amount = 2500
print('Amount = ',amount)
if amount > 10000:
   discount = amount * 20 / 100
elif amount > 5000:
   discount = amount * 10 / 100
elif amount > 1000:
   discount = amount * 5 / 100
else:
   discount=0

print('Payable amount = ',amount - discount)

以上代码的输出如下所示 -

The output of the above code is as follows −

Amount: 2500
Payable amount = 2375.0