Context Manager Defines Both enter and exit

ID

python.ctx_manager_enter_exit

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Python

Tags

reliability, suspicious-construct

Description

Reports classes that define one half of the context-manager protocol but not the other. The runtime requires both enter and exit (or both aenter and aexit for async context managers); a class that only defines one half cannot be used in a with statement.

class Bad:
    def __enter__(self):           # FLAW — __exit__ is missing
        return self

class AlsoBad:
    def __exit__(self, *exc):      # FLAW — __enter__ is missing
        pass

class Good:
    def __enter__(self):
        return self
    def __exit__(self, *exc):
        pass

Rationale

A half-implemented context manager throws AttributeError the first time someone tries to use it in a with block — usually well after the class was added. Static detection turns the runtime crash into a code review.

Remediation

Implement the missing half (or remove the existing one if the class was never meant to be a context manager). For a no-op cleanup, exit(self, *exc): pass is sufficient.

For async context managers, use aenter / aexit. Mixing the two protocols (enter plus aexit) does not make a usable manager.

References