socketserver.BaseRequestHandler inherited class

[matsievskiysv]

BPO	45818
Nosy	@matsievskiysv

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-11-16.16:21:51.670>
labels = ['type-bug', 'library', '3.9']
title = 'socketserver.BaseRequestHandler inherited class'
updated_at = <Date 2021-11-16.16:21:51.670>
user = 'https://github.com/matsievskiysv'

bugs.python.org fields:

activity = <Date 2021-11-16.16:21:51.670>
actor = 'matsievskiysv'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-11-16.16:21:51.670>
creator = 'matsievskiysv'
dependencies = []
files = []
hgrepos = []
issue_num = 45818
keywords = []
message_count = 1.0
messages = ['406413']
nosy_count = 1.0
nosy_names = ['matsievskiysv']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45818'
versions = ['Python 3.9']

[matsievskiysv]

Due to

try:
    self.handle()
finally:
    self.finish()

construct in socketserver.BaseRequestHandler.__init__() method inherited classes with overrided __init__() method may suffer from incomplete initialization.
For example, in the following snippet

def __init__(self, request, client_address, server):
    super().__init__(request, client_address, server)
    self.foo = 1

in some cases all the code after super() call will not be executed.

This is a MWE of the server with partially initialized Handler class

from socketserver import UnixStreamServer, StreamRequestHandler, ForkingMixIn


class Handler(StreamRequestHandler):
    def __init__(self, request, client_address, server):
        super().__init__(request, client_address, server)
        self.foo = 1

    def handle(self):
        print(self.foo)


class ThreadedUnixStreamServer(ForkingMixIn, UnixStreamServer):
    pass


with ThreadedUnixStreamServer("/tmp/test.socket", Handler) as server:
    server.serve_forever()

[vadmium]

I wonder why you wouldn’t override setup instead.

It may be worth documenting not to override __init__. (Or if necessary, I guess the rule is the overriding __init__ method should invoke the base method as the final step.)

[tdoan2010]

I wonder why you wouldn’t override setup instead.

Because it is a bad practice to define an instance attribute outside of __init__ method.

I'm also facing the same problem that I need to define instance attributes for my handler.

[picnixz]

Because it is a bad practice to define an instance attribute outside of init method.

setup() is a method meant to be called in __init__, so this rule shouldn't apply here.

[tdoan2010]

I'm against it. Declaring attributes outside of __init__ violates the POLS. In addition, it fails any style check, e.g. Pylint W0201.

A constructor should only initialize attributes, nothing more.

[picnixz]

The interface has been like that since then. It's the same as for a __post_init__ method of a dataclass and changing it is a breaking change.