attr.ib converter function args issue

[wandth]

hello, when I try to converter some class members such as that

import attr

def new_converter(value):
    return " -rl " + value

@attr.s()
class Test(object):
    test_mem = attr.ib(default=None, converter = new_converter)

info = Test(test_mem="2222")

print("--->", info.test_mem)

that was all right

but when I do wrote those code

def new_converter(value):
    return " -rl " + value

@attr.s()
class Test(object):
    test_mem = attr.ib(default=None, converter = new_converter)

# info = Test(test_mem="2222")
info = Test()
info.test_mem = "2222"  # ----------> this is wrong

print("--->", info.test_mem)

the code was wrong

is that problem a requirement for code or a bug ?

[Tinche]

Your converter can't handle your default value, so it raises an exception.

You can change your default value to "" instead of None, or fix your converter so it can handle value being None.

[wandth]

@Tinche thanks

[wandth]

@Tinche

also wrong

[Tinche]

You're using old-style classes (attr.s instead of attrs.define), and I believe old-style classes do not run converters on setting the attribute, only on instance construction.

What happens if you rewrite your class like this:

from attrs import define, field
@define
class Test:
    test_mem = field(default="", converter=new_converter)

?

[wandth]

Thanks，I test define function in python 3, it works well