Confusion with Boolean and Integer Values in the Expression Engine

[esnyder-rve]

What is the bug or the crash?

When evaluating a boolean expression such as "Field_A" IS NULL, the expression editor will show the result as either 1 or 0. This value appears to be an integer, but does not behave like one. Math, for instance, cannot be done against this value. These expressions will need to be converted with to_int() first (which is confusing because why would I need to convert 0 or 1 to an integer, it "already is one").

This is inconsistent with other aspects of the expression editor where some boolean operations will result in true or false explicitly (such as overlay_within())

I think the ideal solution would be to fix the integer-like returning expressions to instead return true or false explicitly.

Steps to reproduce the issue

1. Create an expression such as "Field" IS NULL + 10
2. Notice the value is not either 10 or 11.

Versions

QGIS version	3.44.0-Solothurn
QGIS code revision	5d9ba037df1
Libraries
Qt version	5.15.13
Python version	3.12.11
GDAL version	3.11.0 (Compiled) 3.11.3 (Running) — Eganville
PROJ version	9.6.2
EPSG Registry database version	v12.013 (2025-05-26)
GEOS version	3.13.1-CAPI-1.19.2
SQLite version	3.46.1
PDAL version	2.9.0
PostgreSQL client version	17.3
SpatiaLite version	5.1.0
QWT version	6.3.0
QScintilla2 version	2.14.1
OS version	Windows 11 Version 2009
Active Python plugins
db_manager	0.1.20
grassprovider	2.12.99
processing	2.12.99

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[agiudiceandrea]

@esnyder-rve, it seems to me you didn't take in consideration the precedence of the operators writing the expression. Usually + and - have higher precedence than is and is not (see e.g. https://docs.python.org/3/reference/expressions.html#operator-precedence). In fact, ( "Field" IS NULL ) + 10 returns 11 o 10 depending on whether the "Field" value is Null or not.

[esnyder-rve]

My issue regarding the integer conversion is resolved (order of operation issue). However, there still is an outstanding issue here. Why do some expressions (like the overlay functions) return true/false whereas others (such as IS NULL) return 0/1?

[DelazJ]

Why do some expressions (like the overlay functions) return true/false whereas others (such as IS NULL) return 0/1?

Good question. An inconsistency questioned years ago when we switched the help text from 0/1 to FALSE/TRUE.