Signature mismatch in overriding method¶
ID: py/inheritance/signature-mismatch
Kind: problem
Security severity:
Severity: warning
Precision: very-high
Tags:
- quality
- reliability
- correctness
Query suites:
- python-security-and-quality.qls
Click to see the query in the CodeQL repository
When the signature of a method of a base class and a method of a subclass that overrides it don’t match, a call to the base class method may not be a valid call to the subclass method, and thus raise an exception if an instance of the subclass is passed instead. If following the Liskov Substitution Principle, in which an instance of a subclass should be usable in every context as though it were an instance of the base class, this behavior breaks the principle.
Recommendation¶
Ensure that the overriding method in the subclass accepts the same parameters as the base method.
Example¶
In the following example, Base.runsource takes an optional filename argument. However, the overriding method Sub.runsource does not. This means the run function will fail if passed an instance of Sub.
class Base:
def runsource(self, source, filename="<input>"):
...
class Sub(Base):
def runsource(self, source): # BAD: Does not match the signature of overridden method.
...
def run(obj: Base):
obj.runsource("source", filename="foo.txt")
References¶
Wikipedia: Liskov Substitution Principle, Method overriding.