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参数的默认值

让部分参数变为可选

参数的默认值 是 CoddyKit 上的免费 Mojo Academy 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Mojo Academy 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Mojo Academy 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Making a Parameter Optional

A default value lets a parameter stand in for itself. If the caller skips it, Mojo quietly uses the value you wrote in the definition.

fn greet(name: String, times: Int = 1):
    for _ in range(times):
        print(name)

Calling With and Without

Now both calls work. Omit times and you get the default of 1; pass it and you override the default for that one call.

greet("Mojo")        # times = 1
greet("Mojo", 3)     # times = 3

Why Defaults Are Handy

Defaults capture the common case. Most callers never touch the option, yet the power is there for the few who need it.

Defaults Go Last

Parameters with defaults must come after required ones. Otherwise Mojo could not tell which value you meant to skip.

fn open(path: String, mode: String = "r"): ...  # ok

A Required-Then-Optional Shape

Think of it as required-first, optional-last. Reading left to right, you supply what is needed, then accept defaults for the rest. This shape is predictable.

Overriding Just One Default

To change a later default while keeping earlier ones, use a keyword. You skip straight to the option you care about.

fn make(width: Int = 80, height: Int = 24): ...
make(height=40)  # width stays 80

Multiple Defaults Together

A function can give several parameters defaults at once. Callers fill only what they want and let the rest fall back.

fn box(w: Int = 10, h: Int = 10, fill: String = "x"): ...

Defaults Build Friendly APIs

Good defaults make a function easy to start with and powerful to grow into. Beginners call it plainly; experts tune the options.

Choosing a Sensible Default

Pick the value most callers would want anyway. A surprising default is worse than none, because it hides behavior the reader did not expect.

The Default Is Evaluated Once

The default expression belongs to the signature, not the body. Treat it as a fixed fallback that documents the normal value.

fn retry(count: Int = 3): ...  # 3 is the documented norm

Defaults Plus Keywords Shine

Defaults and keyword calls are a great pair. Together they let you write functions with many knobs that still feel simple to call.

request(url, timeout=30, retries=5)

Quick Check

Where must a parameter with a default value be placed in a Mojo function?

Recap: Smart Fallbacks

Default values make parameters optional and capture the common case. Put them last, choose them wisely, and override single ones with keywords.

常见问题解答

「参数的默认值」课时是免费的吗？

是的 — 「参数的默认值」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Mojo Academy 课程的其余内容，请升级到 CoddyKit PRO。 Mojo Academy 课程共包含 4 节课。

「参数的默认值」这节课中我会学到什么？

让部分参数变为可选你通过在浏览器中直接运行的动手代码来练习 Mojo Academy，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Mojo Academy 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Mojo Academy 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 2 节课，共 4 节。

「参数的默认值」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Mojo Academy 课中编写并运行代码吗？

能。每节 Mojo Academy 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

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