Flask.route()触发TypeError：意外关键字参数问题解惑

问题背景 #

编写了一个猜数字的Flask应用，根路由可正常加载，但访问带数字的路由（如/1）时抛出TypeError: wrapper_function() got an unexpected keyword argument 'number'错误。

原Flask代码 #

from flask import Flask
from random import randint

rand_n = randint(0, 9)
app = Flask(__name__)


def decorator_header(function):
    def wrapper():
        return f"<h1>{function()}</h1>"
    return wrapper


def check_decorator(function):
    def wrapper_function(*args):
        result = function(args[0])
        if result == "You found me!":
            return f"<b style='color:green;'>{result}</b>"
        if result == "Too low, try again!":
            return f"<b style='color:red;'>{result}</b>"
        return f"<b style='color:purple;'>{result}</b>"
    return wrapper_function


@app.route('/')
@decorator_header
def guess_number():
    return 'Guess a number between 0 and 9'


@app.route('/<number>', endpoint='check_number')
@check_decorator
def check_number(number):
    if int(number) < rand_n:
        return "Too low, try again!"
    if int(number) > rand_n:
        return "Too high, try again!"
    return "You found me!"


if __name__ == "__main__":
    app.run()

错误信息 #

[2023-02-19 21:16:23,710] ERROR in app: Exception on /1 [GET]
Traceback (most recent call last):
  File "C:\Users\pauLo\venv\lib\site-packages\flask\app.py", line 2528, in wsgi_app
    response = self.full_dispatch_request()
  File "C:\Users\pauLo\venv\lib\site-packages\flask\app.py", line 1825, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "C:\Users\pauLo\venv\lib\site-packages\flask\app.py", line 1823, in full_dispatch_request
    rv = self.dispatch_request()
  File "C:\Users\pauLo\venv\lib\site-packages\flask\app.py", line 1799, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
TypeError: wrapper_function() got an unexpected keyword argument 'number'
127.0.0.1 - - [19/Feb/2023 21:16:23] "GET /1 HTTP/1.1" 500 -

特殊现象 #

• 将装饰器的wrapper_function参数改为number后可正常运行，用其他名称仍报错；
• 脱离Flask环境运行原装饰器代码无问题：

@decorator_header
def guess_number():
    return 'Guess a number between 0 and 9'


@check_decorator
def check_number(number):
    if int(number) < rand_n:
        return "Too low, try again!"
    if int(number) > rand_n:
        return "Too high, try again!"
    return "You found me!"


guess = input(guess_number())
print(check_number(guess))

原因分析 #

没错，Flask确实会以关键字参数的形式传递路由参数。当访问/1时，Flask解析出路由参数number=1，调用视图函数时会执行check_number(number=1)。

原装饰器的wrapper_function只定义了*args，这只能处理位置参数，无法接收关键字参数，因此抛出“意外关键字参数”的错误。而脱离Flask环境时，你是用位置参数调用check_number(guess)，*args能正常捕获参数，所以不会报错。

当你把wrapper_function的参数改为number时，它就可以接收number=1这个关键字参数，自然能正常运行。

正确解决方案 #

要让装饰器兼容Flask的关键字参数传递，需要让包装函数同时支持位置参数和关键字参数，修改check_decorator为：

def check_decorator(function):
    def wrapper_function(*args, **kwargs):
        result = function(*args, **kwargs)
        if result == "You found me!":
            return f"<b style='color:green;'>{result}</b>"
        if result == "Too low, try again!":
            return f"<b style='color:red;'>{result}</b>"
        return f"<b style='color:purple;'>{result}</b>"
    return wrapper_function

这种写法直接把接收到的所有参数原封不动传给原函数，不管Flask用位置参数还是关键字参数传递，装饰器都能正常处理，不需要硬编码参数名number。

内容的提问来源于stack exchange，提问作者Miqueias Brikalski