---
title: Print Overridden
fid: 20240929-171359
tags:
---

# Print Overridden



Override 重载（改写，覆盖）用于类，它使得新类在使用 Python 内置函数（或符号）时，功能可以定制。

例如，希望 print(object) 时，按照特别的格式打印 object 的关键属性 :

``` {.python}
def __srt__(self,):
    return "formatted string  {}".format(self.an_attribute)
```

 
`object.__str__`, `object.__repr__` 是为了解决程序运行过程中在屏幕上打印字符的形式

-   `__repr__()` goal is to be unambiguous
-   `__str__()` goal is to be readable


## Point Example

``` python
>>> class Point:
...   def __init__(self, x, y):
...     self.x, self.y = x, y
...   def __repr__(self):
...     return 'Point(x=%s, y=%s)' % (self.x, self.y)


>>> p = Point(1, 2)
>>> p
Point(x=1, y=2)
```

[(ref)](https://www.nltk.org/_modules/nltk/stem/wordnet.html)

## Return a  Name

```python
class WordNetLemmatizer(object):       
    def __init__(self):
        pass

    def __repr__(self):
        return "<WordNetLemmatizer>"
        # 仅仅只返回一个名字，有提示作用
```



-   `__repr__()` 为程序员提供了对象的官方字符串表示法。
-   `__str__()` 为用户提供了对象的非正式字符串表示。

## datetime example

``` python
>>> import datetime
>>> today = datetime.datetime.now()

>>> today
datetime.datetime(2023, 2, 18, 18, 40, 2, 160890)

>>> print(today)
2023-02-18 18:40:02.160890
```

换句话说： `__repr__()` 的格式可以用来直接作为类的实例化

``` python
>>> new_date = datetime.datetime(2023, 2, 18, 18, 40, 2, 160890)
>>> new_date == today
True

>>> new_date = 2023-02-18 18:40:02.160890
Traceback (most recent call last):
  ...
  File "<input>", line 1
    new_date = 2023-02-18 18:40:02.160890
                          ^
SyntaxError: leading zeros in decimal integer literals are not permitted ...
```


## Reference

[what-is-the-difference-between-str-and-repr](https://stackoverflow.com/questions/1436703/what-is-the-difference-between-str-and-repr)