Python 中的默认值是什么？

悠悠 2024-03-27 08:48 115阅读 0赞

Python 语言具有表示函数参数的语法和默认值的不同方式。

默认值指示如果在函数调用期间未给出参数值，则函数参数将采用该值。默认值是使用表单关键字名称=值的赋值 **（=）** 运算符分配**的。**

#### 例 ####

# creating a function by giving default values
    def tutorialspoint(website, author ='XYZ', language ='Python'):
       print(website, "website article is written by the author", author,"of language", language)

### 不带关键字参数的函数调用 ###

#### 例 ####

\# creating a function by giving default values def tutorialspoint(website, author ='XYZ', language ='Python'): print(website, "website article is written by the author", author,"of the language", language) \# calling only 1 positional argument(website) \# here it takes the given default values for author(XYZ) and language(Python) tutorialspoint('tutorialspoint') \# calling 3 positional arguments (website, author, language) tutorialspoint('tutorialspoint', 'Alex', 'Java') \# calling 2 positional arguments (website, author) \# here it takes the given default value for the language(Python) tutorialspoint('tutorialspoint', 'Gayle') \# here it takes the given default value for author(XYZ) tutorialspoint('tutorialspoint', 'C++')

#### 输出 ####

在执行时，上述程序将生成以下输出 –

tutorialspoint website article is written by the author XYZ of the language
    Python
    tutorialspoint website article is written by the author Alex of the language
    Java
    tutorialspoint website article is written by the author Gayle of the language
    Python
    tutorialspoint website article is written by the author C++ of language Python

#### 解释 ####

*  在第一种情况下，第一次调用中只有一个必需的参数，其余参数设置为默认值。
 *  在第二个函数调用中，我们调用了一个具有 3 个位置参数（网站、作者、语言）的函数。**作者**和**标准**参数的值从默认值更改为新的传递值。

### 使用关键字参数调用函数 ###

#### 例 ####

\# creating a function by giving default values def tutorialspoint(website, author ='XYZ', language ='Python'): print(website, "website article is written by the author", author,"of language", language) \# calling only 1 positional argument(website) with keywords \# here it takes the given default values for author(XYZ) and language(Python) tutorialspoint(website= 'tutorialspoint') \# calling 2 positional arguments (website, language) with keywords tutorialspoint(website= 'tutorialspoint', language = 'Java') \# calling 2 positional arguments (author, website) with keywords \# the order of the keywords is not mandatory \# here it takes the given default value for language(Python) tutorialspoint(author= 'Gayle', website= 'tutorialspoint')

#### 输出 ####

在执行时，上述程序将生成以下输出 –

tutorialspoint website article is written by the author XYZ of language Python
    tutorialspoint website article is written by the author XYZ of language Java
    tutorialspoint website article is written by the author Gayle of language Python

#### 解释 ####

*  第一次调用中只需要一个关键字参数。
 *  在第二次调用中，一个参数是必需的，另一个是可选的（**语言**），其值从默认值更改为新的传递值。
 *  我们可以从第三次调用中看到，关键字参数的**顺序**不重要/不是强制性的。

### 无效的函数调用（引发错误） ###

现在我们介绍一些抛出错误的函数调用的无效情况。

#### 例 ####

#### 输出 ####

在执行时，上述程序将生成以下输出 –

Traceback (most recent call last):
    File "main.py", line 5, in <module>
    tutorialspoint()
    TypeError: tutorialspoint() missing 1 required positional argument: 'website'

没有参数传递给函数，因此会发生错误

#### 例 ####

\# creating a function by giving default values def tutorialspoint(website, author ='XYZ', language ='Python'): print(website, "website article is written by the author", author,"of language", language) \# There is a non-keyword argument for author(Alex) after \# a keyword argument website(tutorialspoint). Hence an error occurs tutorialspoint(website= 'tutorialspoint', 'Alex')

#### 输出 ####

在执行时，上述程序将生成以下输出 –

File "main.py", line 6
        tutorialspoint(website= 'tutorialspoint', 'Alex')
                                                  ^
    SyntaxError: positional argument follows keyword argument

在关键字参数之后，有一个作者（Alex）（tutorialspoint）的非关键字参数。因此，会发生错误。

#### 例 ####

\# creating a function by giving default values def tutorialspoint(website, author ='XYZ', language ='Python'): print(website, "website article is written by the author", author,"of language", language) \# The keyword address is not defined in the function(unknown keyword argument) \# hence it raises an error tutorialspoint(address ='Hyderabad')

#### 输出 ####

在执行时，上述程序将生成以下输出 –

Traceback (most recent call last):
      File "main.py", line 6, in <module>
    tutorialspoint(address ='Hyderabad')
    TypeError: tutorialspoint() got an unexpected keyword argument 'address'

由于函数（未知关键字参数）中未定义关键字地址，因此会引发错误。

### 使用可变对象作为默认参数 ###

必须非常小心地进行。原因是当控件到达函数时，参数的默认值仅计算一次。

第一次，一个定义。之后，在后续函数调用中引用相同的值（或可变对象）。

#### 例 ####

\# Creating a function by passing the list element, new list as arguments \# the function returns a new list after appending elements to it def appendingElement(listElement, newList = \[\]): newList.append(listElement) \# returning a new list return newList \# calling function by passing the list element as an argument \# and printing the result new list print(appendingElement('hello')) print(appendingElement('tutorialspoint')) print(appendingElement('python'))

#### 输出 ####

['hello']
    ['hello', 'tutorialspoint']
    ['hello', 'tutorialspoint', 'python']

### 结论 ###

我们在本文中了解了 Python 函数中的默认值。我们通过一些示例了解了默认参数和参数。