How to simplify a lot of if... Elif... Else code?

In everyday code , We always face a lot of if…elif…else The problem of conditional branch selection . Tell the truth , In most cases, my first recommendation is to write honestly
if…elif, And try to extract the content under each branch into independent functions . The structure is clear , Intention understanding , It is very convenient for writing and reading . however , In some special circumstances , You can also use other more elegant ways , Like we shared before 《 Don't use
if-elif sentence , How to judge the rank of a number gracefully ？》, And this article to share today , You can broaden your thinking of coding .

In today's Github read EdgeDB[1] Code for , Found it dealing with a lot of if…elif…else When judging , Using a very clever decorator . Let's see what this method is .

Today is double eleven , Suppose we want to do a function , Judge the discount he can get according to the user's level . The conventional if … elif… This is how it is written ：

def get_discount(level):
    if level == 1:
        " A lot of calculation code "
        discount = 0.1
    elif level == 2:
        " A lot of calculation code "
        discount = 0.2
    elif level == 3:
        discount = 0.3
    elif level == 4:
        discount = 0.4
    elif level == 5:
        discount = 0.5
    elif level == 6:
        discount = 3 + 2 - 5 * 0.1
    else:
         return ' Wrong level '
    return discount

Everybody knows , So many if … elif… The code is very ugly , It's hard to maintain . And each if There is a lot of code inside . This function will be pulled very long .

Some students know , You can use a dictionary to rewrite this too long if Judge ：

def parse_level_1():
    " A lot of calculation code "
    discount = 0.1
    return discount

def parse_level_2():
    " A lot of calculation code "
    discount = 0.2
    return discount

def parse_level_3():
    " A lot of calculation code "
    discount = 0.3
    return discount

def parse_level_4():
    " A lot of calculation code "
    discount = 0.4
    return discount

def parse_level_5():
    " A lot of calculation code "
    discount = 0.5
    return discount

def parse_level_6():
    " A lot of calculation code "
    discount = 3 + 2 - 5 * 0.1
    return discount

discount_map = {
    
 1: parse_level_1,
  2: parse_level_2,
  3: parse_level_3,
  4: parse_level_4,
  5: parse_level_5,
  6: parse_level_6,
}

discount = discount_map.get(level, ' Wrong level ')

But the method I learned today , It's easier than using a dictionary . Let's first look at its effect ：

@value_dispatch
def get_discount(level):
    return ' Wrong level '

@get_discount.register(1)
def parse_level_1(level):
    " A lot of calculation code "
    discount = 0.1
    return discount

@get_discount.register(2)
def parse_level_2(level):
    " A lot of calculation code "
    discount = 0.2
    return discount

@get_discount.register(3)
def parse_level_3(level):
    " A lot of calculation code "
    discount = 0.3
    return discount

@get_discount.register(4)
def parse_level_4(level):
    " A lot of calculation code "
    discount = 0.4
    return discount

@get_discount.register(5)
def parse_level_5(level):
    " A lot of calculation code "
    discount = 0.5
    return discount

@get_discount.register(6)
def parse_level_1(level):
    " A lot of calculation code "
    discount = 3 + 2 - 5 * 0.1
    return discount


discount = get_discount(3)
print(f' Grade 3 Users of , The discount received is ：{discount}')

The operation effect is shown in the figure below ：

Write it like this , More intuitive than using a dictionary , It's better than using it directly if … elif… More concise .

that , This decorator value_dispatch How to achieve it ？ The password is hidden in this open source project EdgeDB Of Source code [2] in , The core code is just 20 Multiple lines ：

also , It can also implement or query . For example, the user level is 2 perhaps 3 When , Discounts are 0.2, Then the code can be written as ：

@get_discount.register(2)
@get_discount.register(3)
def parse_level_2(level):
    " A lot of calculation code "
    discount = 0.2
    return discount

The operation effect is shown in the figure below ：

Its code can only implement equal queries at present . But in fact, as long as the code is slightly modified , We can achieve greater than 、 Less than 、 Greater than or equal to 、 Less than or equal to 、 It's not equal to 、in Wait, judge . If you are interested , Please leave a message at the bottom of the article , Let's talk about how to modify this code tomorrow , Realize more logical judgment .

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