What is the difference between __str__ and __repr__ in Python

from

https://www.pythoncentral.io/what-is-the-difference-between-__str__-and-__repr__-in-python/

 

目的

官方解釋：

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.
object.__str__(self): called by the str() build-in function and by the print statement to compute the "informal" string representation of an object.

Quote from Python's Data Model

二者都是對對象的表述， repr是正式的解釋， str是信息形式的解釋

 

基本類型的的默認實現

>>> x = 1

>>> repr(x)

'1'

>>> str(x)

'1'

>>> y = 'a string'

>>> repr(y)

"'a string'"

>>> str(y)

'a string'

      對於整數沒有差異， 對於字符串， 咱們能夠看到差異

While the return of repr() and str() are identical for int x, you should notice the difference between the return values for str y. It is important to realize the default implementation of __repr__ for a str object can be called as an argument to eval and the return value would be a valid str object

      repr的返回值能夠被eval再還原回字符串。

>>> repr(y)

"'a string'"

>>> y2 = eval(repr(y))

>>> y == y2

True

複雜對象

Therefore, a "formal" representation of an object should be callable by eval() and return the same object, if possible. If not possible, such as in the case where the object's members are referring itself that leads to infinite circular reference, then __repr__ should be unambiguous and contain as much information as possible.

     對於非基本類型的複雜對象， 則不能被eval計算還原， 這種狀況必須遵照 意思清楚的 ， 包括信息儘可能多的原則。

>>> class ClassB(object):

...     def __init__(self, a=None):

...         self.a = a

...

...     def __repr__(self):

...         return '%s(a=a)' % (self.__class__)

...

 

>>> a = ClassA()

>>> b = ClassB(a=a)

>>> a.b = b

>>> repr(a)

"<class '__main__.ClassA'>(<class '__main__.ClassB'>(a=a))"

>>> repr(b)

"<class '__main__.ClassB'>(a=a)"

 

內置對象的可讀性

The __str__ representation of now looks cleaner and easier to read than the formal representation generated from __repr__. Sometimes, being able to quickly grasp what's stored in an object is valuable to grab the "big" picture of a complex program.

      str更加註重可讀性， 一眼就能夠獲取對象內容，不關注實現的細節。

>>> from datetime import datetime

>>> now = datetime.now()

>>> repr(now)

'datetime.datetime(2013, 2, 5, 4, 43, 11, 673075)'

>>> str(now)

'2013-02-05 04:43:11.673075'

 

Tips and Suggestions between __str__ and __repr__ in Python

• Implement __repr__ for every class you implement. There should be no excuse.
• Implement __str__ for classes which you think readability is more important of non-ambiguity.