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import csv
import sys


class Test:
    def __init__(self, a, b):
        self.a = a
        self.b = b

    def __repr__(self):
        return f"Test({self.a}, {self.b})"


# Das hier klappt
x1 = Test(1, 2)
x2 = Test(5, 6)
tests = (x1, x2)
print(tests)

# Das hier leider nicht
csv_writer = csv.writer(sys.stdout)
csv_writer.writerows(tests)  # Hier kracht es

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#!/usr/bin/env python3
import csv
import sys

from attr import astuple, attrib, attrs


@attrs(frozen=True)
class Test:
    a = attrib()
    b = attrib()


def main():
    tests = [Test(1, 2), Test(5, 6)]
    print(tests)
    csv.writer(sys.stdout).writerows(map(astuple, tests))


if __name__ == "__main__":
    main()

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[Test(a=1, b=2), Test(a=5, b=6)]
1,2
5,6

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pip freeze | grep attr
attr==0.3.1

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python
Python 3.9.5 (default, May 11 2021, 08:20:37) 
[GCC 10.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import attr
>>> import inspect
>>> inspect.getmembers(attr, predicate=inspect.ismethod)
[]

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>>> print(attr)
<module 'attr' from '/home/chris/devel/test/venv/lib/python3.9/site-packages/attr.py'>

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class Test(Base):
    __tablename__ = "test"
    a = Column(Integer, primary_key=True)
    b = Column(Integer)

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@attrs(frozen=True)
class Test(Base):
    __tablename__ = "test"
    a = attrib(Column(Integer, primary_key=True))
    b = attrib(Column(Integer))