Incompatible new and init

ID

python.incompatible_new_init

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Python

Tags

reliability, suspicious-construct

Description

Reports classes where new and init take different non-implicit parameters. Python calls both with the same arguments at instantiation time, so the call site needs a single argument list that satisfies both signatures; if the two methods do not agree, the resulting TypeError surfaces at the first construction.

class Bad:
    def __new__(cls, x):
        return super().__new__(cls)
    def __init__(self, x, y):     # FLAW — extra y not in __new__
        self.x = x
        self.y = y

class Good:
    def __new__(cls, x, y):
        return super().__new__(cls)
    def __init__(self, x, y):     # OK
        self.x = x
        self.y = y

Rationale

The pair new / init is one of Python’s data-model contracts that is easy to break and silent until runtime. Static detection turns the hidden mismatch into a code-review item.

The rule compares parameter names after dropping the implicit first argument (cls for new, self for init); args/*kwargs shapes are flagged when one side has them and the other does not.

Remediation

Make the parameter lists agree, or override only init and let object.new handle construction.

References