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String Conversion

String Conversion

TL;DR

  • Control to-string conversion in your own classes using the __str__ and __repr__ “dunder” methods
  • The result of __str__ should be readable. The result of __repr__ should be unambiguous.
  • Always add a __repr__ to your classes. The default implemen- tation for __str__ just calls __repr__.

When you define a custom class in Python and then try to print one of its instances to the console (or inspect it in an interpreter session), by default you will get a string containing the class name and the id of the object instance (which is the object’s memory address in CPython.) → This default “to string” conversion behavior is basic and lacks detail.

Example

class Car:
    def __init__(self, color, mileage):
        self.color = color 
        self.mileage = mileage
        
>>> my_car = Car('red', 37281)
>>> print(my_car)
<__console__.Car object at 0x109b73da0> 
>>> my_car
<__console__.Car object at 0x109b73da0>

nstead of building your own to-string conversion machinery, you’ll be better off adding the __str__ and __repr__ “dunder” methods to your class. They are the Pythonic way to control how objects are converted to strings in different situations

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __str__(self):
        # return f"a {self.color} car"
		return f'a {self.color} car'

>>> my_car = Car('red', 37281) 
>>> print(my_car)
'a red car'
>>> my_car
<__console__.Car object at 0x109ca24e0>

__str__ vs. __repr__

  • When print() an object, or involving string, the __str__ method will be called.
  • inspecting an object in a Python inter- preter session simply prints the result of the object’s __repr__.

Example:

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __str__(self):
        # return f"a {self.color} car"
        return "__str__ for Car"

    def __repr__(self):
        return "__repr__ for Car"

>>> my_car = Car('red', 37281) 

>>> print(my_car)
__str__ for Car

>>> '{}'.format(my_car) 
'__str__ for Car'

>>> my_car 
__repr__ for Car

Interestingly, containers like lists and dicts always use the result of __repr__ to represent the objects they contain. Even if you call str on the container itself:

>>> str([my_car])
'[__repr__ for Car]'

To express your code’s intent more clearly, it’s best to use the built-in str() and repr() functions. Using them is preferable over calling the object’s __str__ or __repr__ directly, as it looks nicer and gives the same result:

>>> str(my_car) 
'__str__ for Car' 

>>> repr(my_car) 
'__repr__ for Car'

Difference of usage of __str__ and __repr__:

  • The result of the date object’s __str__ function should primarily be readable.

    • It’s meant to return a concise textual representation for human consumption—something you’d feel comfortable displaying to a user.

  • With __repr__, the idea is that its result should be, above all, unambiguous.

    • The resulting string is intended more as a debugging aid for developers.

      → It needs to be as explicit as possible about what this object is.

    • Rule of thumb: make __repr__ strings unambiguous and helpful for developers 💪

Example:

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

>>> str(today)
'2022-12-04'

>>> repr(today)
'datetime.date(2022, 12, 4)'

Why Every Class Needs a __repr__

If you don’t add a __str__ method, Python falls back on the result of __repr__ when looking for __str__. Therefore, it is recommended to always add at least a __repr__ method to your classes. This will guarantee a useful string conversion result in almost all cases, with a minimum of implementation work 👏.

Example:

class Car:
    def __init__(self, color, mileage):
        self.color = color
        self.mileage = mileage

    def __str__(self):
        return f"a {self.color} car"

    def __repr__(self):
        return f"{self.__class__.__name__}({self.color!r}, {self.mileage!r})"

Note: using the !r conversion flag makes sure the output string uses repr(self.color) and repr(self.mileage) instead of str(self.color) and str(self.mileage)