From e2ede4717cdf9fa06805c065481aee65564ffa35 Mon Sep 17 00:00:00 2001
From: Vincent Biret <vincentbiret@hotmail.com>
Date: Thu, 5 Nov 2026 09:45:02 -0500
Subject: [PATCH] Python: treat await expressions as side-effecting

Prevent ineffectual-statement false positives when awaiting a task or coroutine without assigning its returned value. Add regression cases for both forms.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: f0800d30-e4f9-463c-a0d8-660fe1aceaf1
---
 .../change-notes/2026-11-05-await-side-effects.md |  4 ++++
 python/ql/lib/semmle/python/Exprs.qll             |  2 ++
 .../Statements/no_effect/await_task.py            | 15 +++++++++++++++
 3 files changed, 21 insertions(+)
 create mode 100644 python/ql/lib/change-notes/2026-11-05-await-side-effects.md
 create mode 100644 python/ql/test/query-tests/Statements/no_effect/await_task.py

diff --git a/python/ql/lib/change-notes/2026-11-05-await-side-effects.md b/python/ql/lib/change-notes/2026-11-05-await-side-effects.md
new file mode 100644
index 000000000000..9f84762e711d
--- /dev/null
+++ b/python/ql/lib/change-notes/2026-11-05-await-side-effects.md
@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* Await expressions are now treated as having side effects. This prevents false positives from the "Statement has no effect" query (`py/ineffectual-statement`) when an awaited task or coroutine's result is discarded.
diff --git a/python/ql/lib/semmle/python/Exprs.qll b/python/ql/lib/semmle/python/Exprs.qll
index 937c9e5ffd79..1d4b545909ae 100644
--- a/python/ql/lib/semmle/python/Exprs.qll
+++ b/python/ql/lib/semmle/python/Exprs.qll
@@ -623,6 +623,8 @@ class None extends NameConstant {
 class Await extends Await_ {
   /* syntax: await Expr */
   override Expr getASubExpression() { result = this.getValue() }
+
+  override predicate hasSideEffects() { any() }
 }
 
 /** A formatted string literal expression, such as `f'hello {world!s}'` */
diff --git a/python/ql/test/query-tests/Statements/no_effect/await_task.py b/python/ql/test/query-tests/Statements/no_effect/await_task.py
new file mode 100644
index 000000000000..6d464c954666
--- /dev/null
+++ b/python/ql/test/query-tests/Statements/no_effect/await_task.py
@@ -0,0 +1,15 @@
+import asyncio
+
+
+async def returns_value():
+    return 42
+
+
+async def await_task_result():
+    task = asyncio.create_task(returns_value())
+    await task
+
+
+async def await_coroutine_result():
+    coroutine = returns_value()
+    await coroutine