`__repr__` 函数相对于普通函数的意义是什么 [英] what is the significance of `__repr__` function over normal function

问题描述

我正在尝试用自己的方式学习 Python，但我一直坚持使用 __repr__ 函数.虽然我已经阅读了很多关于 __repr__ 的帖子以及 python 文档.所以我决定在这里问这个问题.下面的代码解释了我的困惑.

class 要点:def __init__(self,x,y):self.x, self.y = x,ydef __repr__(self):return 'Point(x=%s, y=%s)'%(self.x, self.y)def print_class(self):return 'Point(x=%s, y=%s)'%(self.x, self.y)p = 点(1,2)打印打印 p.print_class()点(x=1，y=2)点(x=1，y=2)

如果一个普通函数也可以执行类似的任务，那么 __repr__ 相对于 print_class()(在我的例子中是一个普通函数)函数的额外优势是什么?

解决方案

__repr__ 函数由 repr() 内部调用.repr() 在直接打印对象时被调用，并且该类没有定义 __str__() .来自 文档 -

<块引用>

object.__repr__(self)

由 repr() 内置函数和字符串转换(反引号)调用以计算对象的官方"字符串表示.如果可能的话，这应该看起来像一个有效的 Python 表达式，可用于重新创建具有相同值的对象(给定适当的环境).如果这是不可能的，则应返回形式为 <...一些有用的描述...> 的字符串.返回值必须是字符串对象.如果一个类定义了 __repr__() 但没有定义 __str__()，那么 __repr__() 也用于实例的非正式"字符串表示需要那个班级的.

对于 print_class() ，您必须在打印对象时专门调用该方法.但是在 __repr__() 的情况下，它会被 print 内部调用.

当您混合不同的类/类型时，这尤其有用.例如，让我们使用一个列表，它可以包含 point 类的数字和对象，现在您要打印列表的元素.

如果你没有定义 __repr__() 或 __str__() ，你必须首先检查实例，它的类型是否为 Pointcode> 如果是这样调用 print_class() ，或者如果不是直接打印数字.

但是当您的类定义了 __repr__() 或 __str__() 时，您可以直接在所有元素上调用 printlist，print 语句会在内部处理打印正确的值.

示例，让我们假设一个类具有 print_class() 方法，但没有 __repr__() 或 __str__() ，代码 -

<预><代码>>>>CA类:... def __init__(self,x):... self.x = x... def print_class(self):...返回 self.x...>>>l = [1,2,3,CA(4),CA(5)]>>>对于我在 l:...打印(一)...123<__main__.CA 对象在 0x00590F10><__main__.CA 对象在 0x005A5070>语法错误:无效语法>>>对于我在 l:...如果 isinstance(i, CA):...打印(i.print_class())... 别的:...打印(一)...12345

如您所见，当我们在列表中混合数字和 CA 类型的对象时，然后当我们只执行 print(i) 时，它没有打印我们想要什么.为了使其正常工作，我们必须检查 i 的类型并调用适当的方法(如第二种情况中所做的那样).

现在让我们假设一个类实现了 __repr__() 而不是 print_class() -

<预><代码>>>>CA类:... def __init__(self,x):... self.x = x... def __repr__(self):...返回 str(self.x)...>>>>>>l = [1,2,3,CA(4),CA(5)]>>>对于我在 l:...打印(一)...12345

正如你在第二种情况下看到的，简单的打印工作，因为 print 在内部首先调用 __str__() ，并且因为它不存在回退到 __repr__() .

不仅如此，当我们执行 str(list) 时，内部每个列表元素的 __repr__() 都会被调用.示例 -

第一种情况(没有 __repr__() )-

<预><代码>>>>字符串(l)'[1, 2, 3, <__main__.CA 对象在 0x005AB3D0>, <__main__.CA 对象在 0x005AB410>]'

第二种情况(使用 __repr__() )-

<预><代码>>>>字符串(l)'[1, 2, 3, 4, 5]'

<小时>

此外，在交互式解释器中，当您直接使用对象时，它会显示repr() 函数的输出，示例 -

<预><代码>>>>CA类:... def __repr__(self):...返回CA实例"...>>>>>>c = CA()>>>CCA实例

I am trying to learn python with my own and i stucked at __repr__ function. Though i have read lots of post on __repr__ along with the python document. so i have decided to ask this Question here. The code bellow explains my confusion.

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)

    def print_class(self):
        return 'Point(x=%s, y=%s)'%(self.x, self.y)



p = Point(1,2)

print p
print p.print_class()


Point(x=1, y=2)
Point(x=1, y=2)

If a normal function can also perform similar task then what is the extra advantage of __repr__ over print_class() (in my case a normal function) function.

解决方案

The __repr__ function is called by repr() internally. repr() is called when you are printing the object directly , and the class does not define a __str__() . From documentation -

object.__repr__(self)

Called by the repr() built-in function and by string conversions (reverse quotes) to compute the "official" string representation of an object. If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object. If a class defines __repr__() but not __str__(), then __repr__() is also used when an "informal" string representation of instances of that class is required.

In your case for print_class() , you have to specifically call the method when printing the object. But in case of __repr__() , it gets internally called by print .

This is especially useful, when you are mixing different classes/types . For Example lets take a list which can have numbers and objects of your point class, now you want to print the elements of the list.

If you do not define the __repr__() or __str__() , you would have to first check the instance , whether its of type Point if so call print_class() , or if not directly print the number.

But when your class defines the __repr__() or __str__() , you can just directly call print on all the elements of the list, print statement would internally take care of printing the correct values.

Example , Lets assume a class which has print_class() method, but no __repr__() or __str__() , code -

>>> class CA:
...     def __init__(self,x):
...             self.x = x
...     def print_class(self):
...             return self.x
...
>>> l = [1,2,3,CA(4),CA(5)]
>>> for i in l:
...     print(i)
...
1
2
3
<__main__.CA object at 0x00590F10>
<__main__.CA object at 0x005A5070>
SyntaxError: invalid syntax
>>> for i in l:
...     if isinstance(i, CA):
...             print(i.print_class())
...     else:
...             print(i)
...
1
2
3
4
5

As you can see, when we mix numbers and objects of type CA in the list, and then when we just did print(i) , it did not print what we wanted. For this to work correctly, we had to check the type of i and call the appropriate method (as done in second case).

Now lets assume a class that implements __repr__() instead of print_class() -

>>> class CA:
...     def __init__(self,x):
...             self.x = x
...     def __repr__(self):
...             return str(self.x)
...
>>>
>>> l = [1,2,3,CA(4),CA(5)]
>>> for i in l:
...     print(i)
...
1
2
3
4
5

As you can see in second case, simply printing worked, since print internally calls __str__() first, and as that did not exist fell back to __repr__() .

And not just this, when we do str(list) , internally each list's element's __repr__() is called. Example -

First case (without __repr__() ) -

>>> str(l)
'[1, 2, 3, <__main__.CA object at 0x005AB3D0>, <__main__.CA object at 0x005AB410>]'

Second case (with __repr__() ) -

>>> str(l)
'[1, 2, 3, 4, 5]'

---

Also, in interactive interpreter, when you are directly using the object, it shows you the output of repr() function, Example -

>>> class CA:
...     def __repr__(self):
...             return "CA instance"
...
>>>
>>> c = CA()
>>> c
CA instance

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