Stack Overflow Asked by Bows on January 20, 2021
I have a class:
class Competition:
def __init__(self, year = int, distance = int, stages = int, winner = None , cyclist = list):
self.year = year
self.distance = distance
self.stages = stages
self.winner = winner
self.cyclist = cyclist
def __str__(self):
return "Competition({}, {}, {}, {}, {}".format(self.year, self.distance, self.stages, self.winner, self.cyclist)
def __repr__(self):
return "{} {} {} {} {}".format(self.year, self.distance, self.stages, self.winner, self.cyclist)
And I want to add the class to a dictionary as the value, and as key the years.
The information I get is from a file.
So I have this
with open('tour-de-france.csv') as file:
reader = csv.reader(file)
no_header = next(reader, None)
new_dict = {}
for x in reader:
new_dict[int(x[0])] = Competition(int(x[0]), x[3], x[11], None,[])
print(new_dict)
Part of the output is:
{1903: 1903 2428 6 None [], 1904: 1904 2428 6 None [], 1905: 1905 2994 11 None [],
But I would like it as:
{1903: Competition(1903, 2428, 6, None, []), 1904: Competition(1904, 2428, 6, None, []), 1905: Competition(1905, 2994, 11, None, []),
Is that possible?
When printing dictionaries with classes in them, it will print the return value of the __repr__
function. Therefore, you can simply make the __repr__
function as follows (since the __str__
function is already returning the value that you want):
def __repr__(self):
return self.__str__()
Correct answer by Seth on January 20, 2021
You cannot have the class as a value in this way. You sure can pass a reference to the class itself, but having it this way (Competition(1903, 2428, 6, None, [])
) what you actually want is to print an object - an instance to a class. So, if you want to print a string that is like the desired output you should do something like you already did with __str__
, but it will be a string (enclosed in quotes) instead of the declarative class form.
Answered by pr3 on January 20, 2021
When you print a container, it includes the repr of its elements. The problem is that you've swapped the __str__
and __repr__
methods.
Although, there's not much point in having a separate __str__
if it's practically the same as __repr__
just written a bit different. I would add some additional info to make it more helpful to an end user.
As well, you've mixed up type annotations and default values in __init__
. All the ones that have a type as their default value are incorrect. Only winner
is correct, but because it has a default and the others don't, it needs to go at the end.
class Competition:
def __init__(self, year: int, distance: int, stages: int, cyclist: list, winner=None):
self.year = year
self.distance = distance
self.stages = stages
self.cyclist = cyclist
self.winner = winner
def __repr__(self):
s = "Competition({}, {}, {}, {}, {})".format(
self.year,
self.distance,
self.stages,
self.cyclist,
self.winner
)
return s
def __str__(self):
s = "{}: {} km, {} stages, winner {} out of {})".format(
self.year,
self.distance,
self.stages,
self.winner,
self.cyclist,
)
return s
(You were also missing a closing parenthesis inside the format string.)
And then instantiation would look like this:
year = int(x[0])
new_dict[year] = Competition(year, x[3], x[11], [])
For example:
c = Competition(1903, 2428, 6, [])
print(c) # -> 1903: 2428 km, 6 stages, winner None out of [])
print(repr(c)) # -> Competition(1903, 2428, 6, [], None)
Answered by wjandrea on January 20, 2021
As @Seth Peace said, the thing is that in order to get a string value for your class, Python calls __repr__
if it is defined, not __str__
.
However, a cleaner way to do this can be to use a dataclass:
from dataclasses import dataclass
from typing import Optional
@dataclass
class Competition:
year: int
distance: int
stages: int
winner: Optional[str]
cyclist: list
print(Competition(2019, 3, 5, "John Doe", []))
By doing so, __init__
, __str__
and __repr__
will already have been defined in a meaningful way.
Answered by g2i on January 20, 2021
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