Pytest/Warning/How To/Resolve AST-related Deprecation Warnings on Python 3.13

• Pytest AST-related Deprecation Warnings on Python 3.13 Pytest/v/7/3 and under

  • TL;DR

    • Running pytest < 7.4 under Python 3.13 (or 3.12) triggers a flood of warnings like

    • DeprecationWarning: ast.Str is deprecated and will be removed in Python 3.14; use ast.Constant instead

    • because pytest's assertion-rewriter still used the legacy ast.Str/ast.NameConstant nodes.
    • Upgrade to pytest ≥ 7.4.0 (or any 8.x release) to eliminate the warnings; those versions switch to the new ast.Constant API. (Avoid ast deprecation warnings on Python 3.12)

  • What you see

    • Example warning (truncated): _pytest/assertion/rewrite.py:958: DeprecationWarning: ast.Str is deprecated…
    • Where it comes from: pytest's assertion-rewriting pass instruments every assert in your code to produce richer failure messages.
    • If your project promotes warnings to errors (-W error, python -X dev, or pytest --strict-config) the test session may even fail.

  • Why it happens

    • Python 3.8 unified several constant node classes (ast.Str, ast.Num, ast.NameConstant, etc.) into a single ast.Constant. The old node types stayed around but became officially deprecated, and Python 3.13 now emits a DeprecationWarning each time they are referenced.
    • pytest versions prior to 7.4 rewrote assert statements by constructing those legacy nodes, so merely importing pytest on Python 3.13 raises the warnings.

  • How to fix it (root-cause, not suppression)

    • 1 Upgrade pytest

      • # Poetry
        poetry add --group dev "pytest>=7.4"
        
        # pip
        python -m pip install -U "pytest>=7.4"

      • 7.4.0 includes commit 9335a0b ("Avoid ast deprecation warnings on Python 3.12").
      • This commit aliases the old names to ast.Constant at runtime, eliminating the warnings. (Avoid ast deprecation warnings on Python 3.12)
      • Side-benefit: Newer pytest releases bring many bug-fixes and features; upgrading is the canonical solution.

    • 2 Pinned-Python fallback (not recommended long-term)

      • Use Python ≤ 3.11 in the project's virtualenv until you're ready to update pytest.
      • This merely postpones the issue.

    • 3 Temporary warning filter (stop-gap only)

      • Add to pyproject.toml or pytest.ini:

      • [tool.pytest.ini_options]
        filterwarnings = [
        "ignore::DeprecationWarning:_pytest.assertion.rewrite",
        ]

      • …but remember this hides the warning; it does not future-proof your code for Python 3.14, where the old nodes will disappear entirely.

  • References

    • Python docs – Deprecated AST node types (Pending removal in 3.14 section for ast.Str/ast.NameConstant)
    • Assertion rewriting triggers DeprecationWarning (pytest issue #10977)
    • pytest commit 9335a0b (part of 7.4.0) – fixes the problem. (Avoid ast deprecation warnings on Python 3.12)