139 lines
3.7 KiB
Markdown
139 lines
3.7 KiB
Markdown
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---
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title: Functions
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localeTitle: 功能
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---
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## 功能
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函数允许您定义可在程序中多次执行的可重用代码块。
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功能允许您为复杂问题创建更多模块化和[DRY](https://en.wikipedia.org/wiki/Don%27t_repeat_yourself)解决方案。
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虽然Python已经提供了许多内置函数，例如`print()`和`len()` ，但您也可以定义自己的函数以在项目中使用。
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在代码中使用函数的一大优势是它减少了项目中代码行的总数。
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### 句法
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在Python中，函数定义具有以下功能：
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1. 关键字`def`
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2. 一个功能名称
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3. paranthesis'（）'，并在paranthesis输入参数内，尽管输入参数是可选的。
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4. 冒号'：'
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5. 一些要执行的代码块
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6. 返回语句（可选）
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```python
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# a function with no parameters or returned values
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def sayHello():
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print("Hello!")
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sayHello() # calls the function, 'Hello!' is printed to the console
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# a function with a parameter
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def helloWithName(name):
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print("Hello " + name + "!")
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helloWithName("Ada") # calls the function, 'Hello Ada!' is printed to the console
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# a function with multiple parameters with a return statement
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def multiply(val1, val2):
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return val1 * val2
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multiply(3, 5) # prints 15 to the console
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```
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函数是代码块，只需调用函数即可重用。这样可以实现简单，优雅的代码重用，而无需显式重写代码段。这使代码更易读，使调试更容易，并限制输入错误。
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Python中的函数是使用`def`关键字创建的，后跟括号内的函数名和函数参数。
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函数总是返回一个值，函数使用`return`关键字返回一个值，如果你不想返回任何值，则返回默认值`None` 。
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函数名称用于调用函数，在括号内传递所需的参数。
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```python
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# this is a basic sum function
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def sum(a, b):
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return a + b
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result = sum(1, 2)
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# result = 3
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```
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您可以为参数定义默认值，这样Python将解释该参数的值是默认值，如果没有给出。
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```python
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def sum(a, b=3):
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return a + b
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result = sum(1)
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# result = 4
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```
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您可以使用参数名称按所需顺序传递参数。
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```python
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result = sum(b=2, a=2)
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# result = 4
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```
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但是，无法在非关键字参数之前传递关键字参数
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```Python
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result = sum(3, b=2)
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#result = 5
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result2 = sum(b=2, 3)
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#Will raise SyntaxError
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```
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函数也是对象，因此您可以将它们分配给变量，并像函数一样使用该变量。
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```python
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s = sum
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result = s(1, 2)
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# result = 3
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```
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### 笔记
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* 如果函数定义包含参数，则在调用函数时必须提供相同数量的参数。
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```python
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print(multiply(3)) # TypeError: multiply() takes exactly 2 arguments (0 given)
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print(multiply('a', 5)) # 'aaaaa' printed to the console
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print(multiply('a', 'b')) # TypeError: Python can't multiply two strings
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```
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* 函数将运行的代码块包括函数内缩进的所有语句。
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```python
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def myFunc():
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print('this will print')
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print('so will this')
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x = 7
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# the assignment of x is not a part of the function since it is not indented
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```
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* 函数中定义的变量仅存在于该函数的范围内。
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```python
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def double(num):
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x = num * 2
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return x
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print(x) # error - x is not defined
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print(double(4)) # prints 8
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```
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\-Python仅在调用函数时解释函数块，而不是在定义函数时解释函数块。即使函数定义块包含某种错误，python解释器也只会在函数被调用时指出。
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### 更多信息：
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* [Python 3 Docs：定义函数](https://docs.python.org/3/tutorial/controlflow.html#defining-functions)
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