純函數(shù)在Python中是指給定相同輸入始終返回相同輸出且沒有副作用的函數(shù)。其特點包括：1. 確定性，即相同輸入總是產(chǎn)生相同輸出；2. 無副作用，即不修改外部變量、不改變輸入數(shù)據(jù)、不與外界交互。例如，def add(a, b): return ab 是純函數(shù)，因為無論調(diào)用多少次add(2, 3)，都始終返回5，且不更改程序中的其他內(nèi)容。相較而言，修改全局變量或改變輸入?yún)?shù)的函數(shù)則是非純函數(shù)。純函數(shù)的優(yōu)勢有：更容易測試、更適合併發(fā)執(zhí)行、可緩存結(jié)果提升性能，並能良好配合函數(shù)式編程工具如map()和filter()。編寫純函數(shù)的建議包括避免使用或修改全局變量、不更改輸入?yún)?shù)、將I/O操作與邏輯分離。雖然並非所有函數(shù)都必須是純函數(shù)，但在構(gòu)建複雜應(yīng)用時，應(yīng)在核心邏輯中優(yōu)先採用純函數(shù)，而將副作用用於日誌記錄、保存磁盤等場景。

A pure function in Python is a function that, given the same inputs, will always return the same output and has no side effects. That means it doesn't modify variables outside its scope, mutate input data, or interact with the outside world (like printing to the screen or writing to a file).

This might sound abstract at first, but once you get the idea, it makes your code more predictable and easier to test.

What Makes a Function "Pure"?

There are two main characteristics of a pure function:

• Deterministic : The same input always gives the same output.
• No Side Effects : It doesn't change anything outside itself — not global variables, not the input arguments, nothing.

Here's an example of a pure function:

 def add(a, b):
    return ab

No matter how many times you call add(2, 3) , it'll always return 5 . And it doesn't touch anything else in your program.

Now compare that to an impure function:

 count = 0

def increment():
    global count
    count = 1

This function changes a variable outside of itself ( count ), so it's not pure.

Why Should You Care About Pure Functions?

Pure functions help reduce bugs and make code easier to reason about. Here are a few practical benefits:

• Easier to test : You don't have to worry about setup or cleanup because they don't rely on or change external state.
• Better for concurrency : Since they don't share state, they're safer to use in parallel or multi-threaded environments.
• Cacheable : Because the output only depends on inputs, you can cache results for performance.

They also play well with functional programming tools like map() , filter() , and libraries like functools .

How to Write More Pure Functions

It's not always possible (or necessary) to write all your code this way, but here are some habits to lean into:

• Avoid using or modifying global variables.
• Don't change input parameters — especially mutable ones like lists or dicts. Instead, create new values and return them.
• Keep I/O operations (like reading/writing files or printing) separate from logic.

For example, instead of this:

 def add_item(lst, item):
    lst.append(item)

Do this:

 def add_item(lst, item):
    return lst [item]

The second version doesn't change the original list — it returns a new one.

When It's Okay to Break the Rules

Let's be real — not every function needs to be pure. Sometimes you need to update state or work with external systems. That's totally fine. Just be aware of when and why you're doing it.

If you're building complex apps or working with large data pipelines, aim for purity where it matters most — core logic, transformations, and business rules. Save side effects for things like logging, saving to disk, or user interaction.

基本上就這些。

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