Python3 Conditional Control

Python conditional statements determine which code block to execute based on the execution result (True or False) of one or more statements.

The execution process of conditional statements can be briefly understood through the following diagram:

Code execution process:

Conditional Judgment Keywords

Keyword / Function Description Example
if Conditional judgment statement, executes the code block when the condition is True if x > 0:
elif Multi-condition judgment branch (else if) elif x == 0:
else Executed when all conditions are not satisfied else:
pass Empty statement, used as a placeholder to ensure complete syntax if x > 0: pass
match Structured pattern matching (Python 3.10+, similar to switch) match x: case 1: ...

if Statement

The general form of an if statement in Python is as follows:

if condition_1: statement_block_1 elif condition_2: statement_block_2 else: statement_block_3
  • If "condition_1" is True, the "statement_block_1" block statement will be executed
  • If "condition_1" is False, "condition_2" will be evaluated
  • If "condition_2" is True, the "statement_block_2" block statement will be executed
  • If "condition_2" is False, the "statement_block_3" block statement will be executed

In Python,elifis used instead ofelse if, so the keywords of the if statement are:if – elif – else。

Note:

  • 1. A colon must be used after each condition:, indicating that the following is the statement block to be executed when the condition is met.
  • 2. Use indentation to divide statement blocks. Statements with the same number of indents together form a statement block.
  • 3. There is noswitch...casestatement in Python, but in version 3.10, thematch...casestatement was added, with similar functionality. See below for details.

Gif demo:

Example

The following is a simple if example:

Example

#!/usr/bin/python3 var1 = 100 if var1: print ("1 - if expression condition is true") print (var1) var2 = 0 if var2: print ("2 - if expression condition is true") print (var2) print ("Good bye!")

After executing the above code, the output is:

1 - if 表达式条件为 true
100
Good bye!

From the result, it can be seen that because variable var2 is 0, the statement inside the corresponding if was not executed.

The following example demonstrates the calculation and judgment of a dog's age:

Example

#!/usr/bin/python3 age = int(input("Please enter your dog's age:")) print("") if age <= 0: print("You're kidding me!") elif age == 1: print("Equivalent to a 14-year-old human.") elif age == 2: print("Equivalent to a 22-year-old human.") elif age > 2: human = 22 + (age -2)*5 print("Corresponding human age:", human) ### Exit prompt input("Press enter key to exit")

Save the above script in the dog.py file and execute the script:

$ python3 dog.py 
请输入你家狗狗的年龄: 1

相当于 14 岁的人。
点击 enter 键退出

The following are commonly used operators in if:

Operator Description
< Less than
<= Less than or equal to
> Greater than
>= Greater than or equal to
== Equal to, compares whether two values are equal
!= Not equal to

Example

#!/usr/bin/python3 # The program demonstrates the == operator # Using numbers print(5 == 6) # Using variables x = 5 y = 8 print(x == y)

The output of the above example:

False
False

The high_low.py file demonstrates numeric comparison operations:

Example

#!/usr/bin/python3 # This example demonstrates a number guessing game number = 7 guess = -1 print("Number guessing game!") while guess != number: guess = int(input("Please enter the number you guessed:")) if guess == number: print("Congratulations, you guessed it right!") elif guess < number: print("The guessed number is too small...") elif guess > number: print("The guessed number is too big...")

After executing the above script, the example output is as follows:

$ python3 high_low.py 
数字猜谜游戏!
请输入你猜的数字：1
猜的数字小了...
请输入你猜的数字：9
猜的数字大了...
请输入你猜的数字：7
恭喜，你猜对了！

if Nesting

In nested if statements, you can place an if...elif...else structure inside another if...elif...else structure.

if 表达式1:
    语句
    if 表达式2:
        语句
    elif 表达式3:
        语句
    else:
        语句
elif 表达式4:
    语句
else:
    语句

Example

# !/usr/bin/python3 num=int(input("Enter a number:")) if num%2==0: if num%3==0: print ("The number you entered is divisible by both 2 and 3") else: print ("The number you entered is divisible by 2, but not by 3") else: if num%3==0: print ("The number you entered is divisible by 3, but not by 2") else: print ("The number you entered is divisible by neither 2 nor 3")

Save the above program to the test_if.py file. After execution, the output is:

$ python3 test.py 
输入一个数字：6
你输入的数字可以整除 2 和 3

match...case

Python 3.10 added match...caseconditional judgment, so there is no need to use a series ofif-elseto judge anymore.

The object after match will be matched in turn with the content after case. If the match is successful, the matched expression is executed; otherwise, it is skipped directly._It can match anything.

SVG: match...case execution flowchart subject box match subject Down arrow case <pattern_1> ? Match action_1 No match case <pattern_2> ? Match action_2 No match case <pattern_3> ? Match action_3 No match case _ (wildcard) Must match wildcard

The syntax format is as follows:

match subject:
    case <pattern_1>:
        <action_1>
    case <pattern_2>:
        <action_2>
    case <pattern_3>:
        <action_3>
    case _:
        <action_wildcard>

case _:Similar todefault:in C and Java, when other cases cannot match, this one matches, ensuring that a match always succeeds.

Example

def http_error(status):
    match status:
        case 400:
            return "Bad request"
        case 404:
            return "Not found"
        case 418:
            return "I'm a teapot"
        case _:
            return "Something's wrong with the internet"
 
print(http_error(400))
print(http_error(404))
print(http_error(418))
print(http_error(500))

The above is an example that outputs HTTP status codes. The output results for multiple status codes are:

Bad request
Not found
I'm a teapot
Something's wrong with the internet

A case can also set multiple matching conditions, separated by|, for example:

...
    case 401|403|404:
        return "Not allowed"
SVG: | multi-condition matching diagram status 401 403 404 | symbol | | Three lines converge Result box "Not allowed"

Example

def check_permission(status):
    match status:
        case 200:
            return "OK - request successful"
        case 301 | 302:
            return "Redirect - redirection"
        case 401 | 403 | 404:
            return "Not allowed - no permission or not found"
        case 500 | 502 | 503:
            return "Server Error - server error"
        case _:
            return "Unknown status - unknown status code"
 
for code in [200, 301, 403, 500, 418]:
    print(f"Status code {code}: {check_permission(code)}")

match...caseFor more content, refer to:Python match-case statement

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