如何编写更新关键字参数的 python 装饰器？

Question

目标是编写一个装饰器来更新包装函数的一个关键字参数。在以下代码包装器中尝试更新 kwarg1:

import inspect                                                                                                                                                                                         [0/300678]
from functools import wraps

def override_me(arg, kwarg1="default kwarg1", kwarg2="default kwarg2"):
    print(f"override_me {arg} kwarg1={kwarg1} kwarg2={kwarg2}")

def append_kwarg1(func):
    original_kwarg1_default = (
        inspect.signature(func).parameters["kwarg1"].default
    )

    @wraps(func)
    def wrapper(*args, kwarg1=original_kwarg1_default, **kwargs):
        func(*args, kwarg1=kwarg1 + "_patched!", **kwargs)

    return wrapper

override_me = append_kwarg1(override_me)


override_me("passed_arg")
override_me("passed_arg", kwarg1="passed_kwarg1_named")
override_me("passed_arg", "passed_kwarg1_as_arg") # TypeError: override_me() got multiple values for argument 'kwarg1'

但是，当 kwarg1 作为位置参数传递时，这会失败。

编辑：注释中指出的说明：override_me 签名无法更改（想想：外部模块）。

Answer 1

基于即使关键字参数也有索引这一事实的有效解决方案。然后根据调用者传递的 args 的长度，我们可以确定感兴趣的参数是按位置传递还是按关键字传递，并在 args 或 kwargs 中更新它：


import inspect                                                                                                                                                                                         
from functools import wraps

def override_me(arg, kwarg1="default kwarg1", kwarg2="default kwarg2"):
    print(f"override_me {arg} kwarg1={kwarg1} kwarg2={kwarg2}")

def append_kwarg1(func):
    params = inspect.signature(func).parameters
    kwarg1_index = next(
        x[0] for x in zip(range(len(params)), params.items()) if x[1][0] == "kwarg1"
    )

    def update(v):
        return v + "_patched!"

    @wraps(func)
    def wrapper(*args, **kwargs):
        if len(args) > kwarg1_index:
            args = (
                args[:kwarg1_index]
                + (update(args[kwarg1_index]),)
                + args[kwarg1_index + 1 :]
            )
            func(*args, **kwargs)
        else:
            kwargs["kwarg1"] = update(kwargs.get("kwarg1", params["kwarg1"].default))
            func(*args, **kwargs)

    return wrapper


override_me = append_kwarg1(override_me)

override_me("passed_arg", kwarg1="passed_kwarg1_named")
override_me("passed_arg")
override_me("passed_arg", "passed_kwarg1_as_arg")
override_me("passed_arg", "passed_kwarg1_as_arg", "passed_kwarg2_as_arg")

如何编写更新关键字参数的 python 装饰器？

How to write python decorator that updates keyword argument?

python

decorator

wrapper