Number of arguments was 3 and is now 3 -- Strange Pylint(W0221:arguments-differ) on overriding log_message of BaseHTTPRequestHandler

Question

When I try to override the logging behaviour in my BaseHTTPRequestHandler-subclass like this:

from http.server import BaseHTTPRequestHandler

class MyHandler(BaseHTTPRequestHandler):

    def log_message(self, fmt, *args):
        pass

I get this warning

"Number of parameters was 3 in 'BaseHTTPRequestHandler.log_message' and is now 3 in overriding 'MyHandler.log_message' method Pylint(W0221:arguments-differ)"

This is strange because the number of arguments is obviously correct, as the message itself states. Is the varargs (*args) causing a problem with inheritance here?

jonrsharpe · Accepted Answer · 2025-08-26 09:18:47Z

1

The error message is misleading, but this is not a false positive. There is a problem with the code (and it has nothing to do with *varargs)

In stdlib\http\server.pyi of typeshed we find this signature for the super-method:

def log_message(self, format: str, *args: Any) -> None: ...

The argument name is format and not fmt. That is a problem because a caller of the method may pass the second argument as a keyword argument: server.log_message(format="something") and this code-snippet ought to work on a BaseHTTPRequestHandler as well as a MyHandler by the Liskov substitution principle.

The wrong argument name triggers Pylint(W0221:arguments-differ) even though the message and documentation hint only at the number of arguments.

edited Aug 26 at 9:18

jonrsharpe

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answered Aug 26 at 9:03

julaine

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