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Python: repr vs str dunder methods

 2 years ago
source link: https://proinsias.github.io/til/python-repr-vs-str-dunder-methods/
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RealPython.com has a great article on how to think about the __repr__() and __str__() methods for classes.

When should you use one method instead of the other?Permalink

In Short: Use .__repr__() for Programmers vs .__str__() for Users.

The reason there are two methods to display an object is that they have different purposes:

  • .__repr__() provides the official string representation of an object, aimed at the programmer.
  • .__str__() provides the informal string representation of an object, aimed at the user.

In general, [.__repr__()] provides detailed and unambiguous information about the object. Another important property of the official string representation is that a programmer can normally use it to re-create an object equal to the original one.

[The .__str__()] representation enables any user to understand the data contained in the object. Usually, it’s simpler and easier to read for a user.

As an example, consider an instance of the datetime class in the datetime module:

>>> 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

[The .__repr__()] representation shows the name of the data type and all the arguments needed to re-create the object. … [The .__str__() representation] shows the [International Organization for Standardization (ISO)] standard format for displaying dates and times.

You can also access the official and informal string representations using the built-in functions repr() and str():

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

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

>>> str(today)
'2023-02-18 18:40:02.160890'
>>> today.__str__()
'2023-02-18 18:40:02.160890'

How to use these methods with f-stringsPermalink

Note that:

f-strings display the informal string representation that .__str__() returns. You can get the official string representation from .__repr__() by using the string conversion flag "!r" in the f-string:

>>> f"{today}"
'2023-02-18 18:40:02.160890'

>>> f"{today!r}"
'datetime.datetime(2023, 2, 18, 18, 40, 2, 160890)'

You can also use f-strings with an equal sign (=) to show both the variable name and its value. You’ll use this option primarily for debugging purposes:

>>> f"{today = }"
'today = datetime.datetime(2023, 2, 18, 18, 40, 2, 160890)'

Note that when you use an equal sign, the f-string defaults to using the official string representation returned by .__repr__(). The representation aimed at the programmer is the most appropriate in this case since the equal sign in f-strings is normally used for debugging a program.

You can override this behavior using the conversion flag "!s":

>>> f"{today = !s}"
'today = 2023-02-18 18:40:02.160890'

An example class with these methodsPermalink

When you define a class, it’s a best practice to include the official string representation by defining .__repr__(). By including this method, you avoid the default representation, which isn’t very useful in most cases. This method will also provide a fallback option for the informal string representation, which comes in handy when you can use the same representation for both use cases.

If you need to display the object to a user, then you can also define .__str__(). This method will provide an output that’s easier for the user to understand.

Here’s an example class:

>>> class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

    def __repr__(self):
        class_name = type(self).__name__
        return f"{class_name}(title={self.title!r}, author={self.author!r})"

    def __str__(self):
        return f'"{self.title}" by {self.author}'

>>> odyssey = Book("The Odyssey", "Homer")

>>> print(repr(odyssey))
Book(title='The Odyssey', author='Homer')

>>> print(str(odyssey))
"The Odyssey" by Homer

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