Why is `flake8` Flagging Boolean Comparisons in SQLAlchemy Filter Clauses?

flake8 Flagging Boolean Comparison in Filter Clause

When attempting to filter query results based on a boolean comparison in SQL, developers may encounter warnings from flake8 regarding the use of "==". While it's generally recommended to use "if cond is False:" or "if not cond:" for boolean comparisons elsewhere, this doesn't apply to filter clauses in SQLAlchemy.

In the example provided, a boolean field (obsoleted) in a database table is used to determine the count of non-obsoleted test cases. The code uses TestCase.obsoleted == False in the filter clause.

caseNum = session.query(TestCase).filter(TestCase.obsoleted == False).count()<br>

However, flake8 reports the warning "E712: Comparison to False should be 'if cond is False:' or 'if not cond:'."

To address this warning, developers may be tempted to change the code to use "is False" or "is not" operators:

caseNum = session.query(TestCase).filter(TestCase.obsoleted is False).count()<br>

or

caseNum = session.query(TestCase).filter(not TestCase.obsoleted).count()<br>

Unfortunately, these modifications result in incorrect results, returning 0 instead of the expected count. This is because filter clauses in SQLAlchemy do not support "is" or "is not" operators.

To resolve this issue without disabling flake8, developers can use a # noqa comment on the offending line:

caseNum = session.query(TestCase).filter(TestCase.obsoleted == False)  # noqa: E712<br>

Alternatively, the sqlalchemy.sql.expression.false function can be used:

from sqlalchemy.sql.expression import false<p>caseNum = session.query(TestCase).filter(TestCase.obsoleted == false())<br></p>

where false() returns the appropriate value for the SQL dialect in use.

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