Conditional statements decide the flow of program execution. In programming whenever we need to make execute any special blocks based on the decision then we use the conditional statements.
条件语句决定程序执行的流程。 在编程中，只要我们需要根据决策执行任何特殊的块，就使用条件语句 。
No need to be confused about it because we are using it in our daily life when we need to make some decisions and based on these decisions, we decide what should we have to do next. Similar situations can occur while programming also where we need to make some decisions based on conditions given and based on these decisions we will execute the block of code.
无需对此感到困惑，因为当我们需要做出一些决定时，我们就在日常生活中使用它，并根据这些决定来决定下一步该做什么。 在编程时也会发生类似情况，在这种情况下，我们需要根据给定的条件做出一些决策，并根据这些决策执行代码块。
Conditional statements available in the Python are,
Python中可用的条件语句是
if statements
如果陈述
if...else statements
如果...其他陈述
if...elif...else statements
if ... elif ... else语句
Nested if statements
嵌套if语句
1)Python if语句 (1) Python if statement)
It is one of the most common conditional statements in which some conditions are provided and if the condition is true then block under the if the condition will be executed.
它是最常见的条件语句之一，其中提供了一些条件，如果条件为true，则在条件将被执行时阻塞。
Syntax:
句法：
if condition:
# what we want to execute here.
Example:
例：
# input age
age=int(input('what is your age: '))
# checking the condition
if age>=18:
print('You are Grown-up now !')
Output
输出量
RUN 1:
what is your age: 21
You are Grown-up now !
RUN 2:
what is your age: 15
2)Python if ... else语句 (2) Python if...else statement )
In the above, we have seen that if the condition is true then block under if will execute then one thing is, comes to our mind that what happens when the condition will be false. So, to overcome this problem we are using if...else statements.
在上面的内容中，我们已经看到，如果条件为true，则在条件执行时阻塞，然后发生一件事，我们想到的是，条件为false时会发生什么。 因此，为了克服这个问题，我们使用了if ... else语句 。
Syntax:
句法：
if condition:
# what we want to execute here.
else:
# what we want to execute here.
If the condition is true, then it will execute the block of if statements otherwise else statement.
如果条件为true ，则它将执行if语句的块，否则执行else语句。
Example:
例：
# input age
age=int(input('what is your age: '))
# checking the condition
if age>=18:
print('You are Grown-up now !')
else:
print('You are Young!')
Output
输出量
RUN 1:
what is your age: 21
You are Grown-up now !
RUN 2:
what is your age: 15
You are Young!
3)Python if ... elif ... else语句 (3) Python if...elif...else statement)
These conditional statements use where we have to check multiple conditions in the program. If these will not true that is false then the else blocks only execute.
这些条件语句用于我们必须检查程序中的多个条件的地方。 如果这些都不是 ，则返回true ，否则else块仅执行。
Syntax:
句法：
if condition:
# what we want to execute here.
elif conditions:
# what we want to execute here.
else:
# what we want to execute here.
Example:
例：
# input the age
n=int(input('Enter marks: '))
# checking the conditions
if n>=90:
print('Excellent')
elif n<90 and n>=75:
print('Passed')
else:
print('Fail')
Output
输出量
RUN 1:
Enter marks: 95
Excellent
RUN 2:
Enter marks: 80
Passed
RUN 3:
Enter marks: 63
Fail
4)Python嵌套的if语句 (4) Python Nested if statement)
As we all have familiar with the word nested which means one statement inside another statement same in the programming nested if statements mean an if statement inside another if statement that is we can place one if statements inside another if statements.
众所周知，嵌套一词意味着在编程中相同的另一个语句中的一个语句，嵌套的if语句意味着另一个if语句内部的if语句，我们可以将一个if语句放在另一个if语句中。
Syntax:
句法：
if condition:
# what we want to execute here.
if condition:
# what we want to execute here.
else:
# what we want to execute here.
Note: In Python, true and false are written as True and False. Since Python follow the indentation rule so the statements must write under the indentations of if statements or other.
注意：在Python中， true和false分别写为True和False 。 由于Python遵循缩进规则，因此语句必须在if语句或其他语句的缩进下编写。
Now, we will see an example based on the above conditional statements which will show us the grade of students.
现在，我们将基于上述条件陈述看到一个示例，该示例将向我们显示学生的成绩。
Example:
例：
# input the age
n=int(input('Enter marks: '))
# checking the conditions
if n>=75:
if n >=95:
print('Excellent')
else:
print('Pass')
else:
print('Fail')
Output
输出量
RUN 1:
Enter marks: 96
Excellent
RUN 2:
Enter marks: 89
Pass
RUN 3:
Enter marks: 69
Fail
翻译自: https://www.includehelp.com/python/conditional-statements-if-if-else-if-elif-else-and-nested-if.aspx
