Given the InventoryItemexample from the dataclasses documentation.
from dataclasses import dataclass
@dataclass
class InventoryItem:
"""Class for keeping track of an item in inventory."""
name: str
unit_price: float
quantity_on_hand: int = 0
InventoryItem(name="Banana", unit_price=5, quantity_on_hand=3)
# OUTPUT:
# InventoryItem(name='Banana', unit_price=5, quantity_on_hand=3)
How to overwrite the standard output message such that the output string
"3 Banana(s) at a unit price of 5."
is displayed?
First of all, dataclasses are normal classes. There is no such a thing as a dataclass type.
There are two ways to override the standard output message of any Python class:
define the __repr__ method
In Python's data model this method is...
... 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.
The representation that you want is not a valid expression to recreate the object, so this is not a good choice.
define the __str__ method
Again from the data model:
Called by str(object) and the built-in functions format() and print() to compute the “informal” or nicely printable string representation of an object. [...] This method differs from object.__repr__() in that there is no expectation that __str__() return a valid Python expression.
This is what I would recommend using.
from dataclasses import dataclass
@dataclass
class InventoryItem:
"""Class for keeping track of an item in inventory."""
name: str
unit_price: float
quantity_on_hand: int = 0
def __str__(self):
return f"{self.quantity_on_hand} {self.name}(s) at a unit price of {self.unit_price}"
>>> item = InventoryItem(name="Banana", unit_price=5, quantity_on_hand=3)
>>> item # __repr__ being called
InventoryItem(name='Banana', unit_price=5, quantity_on_hand=3)
>>> print(item) # __str__ being called
3 Banana(s) at a unit price of 5