I. Grammar

1.1 if (single branch structure)

'''
<Code Block 1>
If <condition>:
    <Code Block 2> # Execute Code Block 2 and then Execute Code Block 3 when the condition is True, otherwise execute Code Block 3 directly without Executing Code Block 2.  
Code Block 3 > # Run Code Block 3 directly when conditions do not hold
'''

light = 'green'
if light == 'red':
    print('wait for')

print('adopt')

1.2 if... else (double branching structure)

'''
<Code Block 1>
if <condition>:
    <Code Block 2>  # Execute Block 2 and then Block 3 when the condition is True  
else:
    <Code Block 4>  # When conditions do not hold, run block 4, and then block 3
<Code Block 3>  # When the condition does not hold, first run code block 4, then run code block 3.
'''

light = 'yellow'
if light == 'red':
    print('etc.')
else:
    print('adopt')
print('Aha ha')

1.3, if... elif... else (multi-branch structure)

'''
<Code Block 1>
if <Conditions 1>:
    <Code Block 2>  # When condition 1 is True, code block 2 is executed and code block 3 is executed. 
elif <Conditions 2>:
    <Code Block 5>  # When condition 1 does not hold, condition 2 holds, code block 5 is executed, and then code block 3 is executed.
...
elif <condition n>:
    <code block n>
else:
    <Code Block 4>  # When none of the conditions for if and elif are valid, code block 4 is executed, and then code block 3 is executed.
<Code Block 3>
'''

light = 'white'
if light == 'red':
    print("wait for")
elif light == 'green':
    print("adopt")
elif light == 'yellow':
    print("Be careful")
else:
    print("No signal!")
print("Make you laugh!")

2. Multiple if judgments and if...elif...else

2.1 Multiple if Judgments

# For age guessing applications, multiple IFS take more time

age = 18

inp_age = int(input('age:'))  # 17

if age > inp_age:  # a  # a is done as soon as it's established. It's not related to b, c.
    print('Guess big.')
if age < inp_age:  # b  # b is done as soon as it's established. It's not related to a and c.
    print('Guess it's small')
if age == inp_age:  # c  # c does it when it's set up. It's not related to a and b.
    print('Guess right.')

2.2 if...elif...else

Only one operation, less time-consuming

age = 18

inp_age = int(input('age:'))  # 17

if age >= inp_age:   # a
    if age > inp_age:  # b
        if 'e':
            print('Guess it's small') #  a, b, e, I'll do it.
    else:
        print('Guess right.') # a is founded c is founded before I do it.
else:  # a I'll do it if it doesn't work.
    print('Guess big.')