Bhuvansh

Posted on Aug 24

I built pyrift: a Python tool that catches runtime bugs linters can't see

#opensource #python

August 2026 update: pyrift v0.5.0 is now available with 90 rules and 316 tests. pip install --upgrade pyrift

The problem

Every Python developer has hit a bug that:

Passes all linting checks
Passes all type checking
Only appears at runtime, often in production

These are not syntax errors. They are not CVEs. They are silent behaviour differences — code that runs without errors but produces wrong results, leaks resources, or crashes only in certain environments.

I got frustrated with this pattern while working on CPython contributions and maintaining a PyPy review toolkit. So I built pyrift.

What pyrift does

pip install pyrift
pyrift scan .

It statically analyses your Python code and flags patterns that behave differently across CPython versions or between CPython and PyPy.

Example output

[ERROR] src/server.py:42 CPY002: Exception.add_note() requires Python 3.11+
→ Guard with: if sys.version_info >= (3, 11): e.add_note(...)

[ERROR] src/compat.py:17 CPY007: Module removed in Python 3.13
→ Find a third-party replacement for 'cgi' on PyPI

[WARNING] src/resource.py:88 PPY001: Relying on del for resource cleanup breaks on PyPy
→ Use context managers (with statement) or try/finally blocks

Scanned 23 file(s). Found 2 error(s), 1 warning(s). Score: 77/100

Real examples of what it catches

CPython version differences

Exception.add_note() — silent AttributeError on Python 3.10

try:
    connect()
except ConnectionError as e:
    e.add_note("Check your network")  # AttributeError on 3.10
    raise

add_note() was added in Python 3.11 (PEP 678). On 3.10 it raises AttributeError inside the except block — the worst possible place to get an error.

asyncio.TaskGroup — AttributeError on Python 3.10

async with asyncio.TaskGroup() as tg:  # AttributeError on 3.10
    tg.create_task(fetch())

TaskGroup was added in Python 3.11. This crashes silently on 3.10 with no hint of why.

datetime.utcnow() — deprecated since Python 3.12

# Bad — deprecated, will be removed
now = datetime.datetime.utcnow()

# Good — timezone-aware
now = datetime.datetime.now(datetime.timezone.utc)

Returns a naive datetime with no timezone info. Deprecated in 3.12, will be removed in a future version. Causes timezone bugs in logging and APIs.

asyncio.get_event_loop() — raises RuntimeError in Python 3.12+

# Bad — RuntimeError in 3.12+ if no loop exists
loop = asyncio.get_event_loop()

# Good
asyncio.run(main())

Before 3.12 it silently created a new event loop. Now it raises RuntimeError.

21 stdlib modules removed in Python 3.13

import cgi        # ModuleNotFoundError on Python 3.13
import telnetlib  # ModuleNotFoundError on Python 3.13
import asynchat   # ModuleNotFoundError on Python 3.13

PEP 594 removed 21 legacy modules. pyrift flags all of them.

PyPy runtime differences

__del__ cleanup silently leaks on PyPy

class DatabaseConnection:
    def __del__(self):
        self.conn.close()  # may never run on PyPy

CPython uses reference counting — __del__ runs immediately. PyPy uses a tracing GC — __del__ may run much later or never. This silently leaks database connections, file handles, and locks.

sys.getrefcount() returns a dummy value on PyPy

if sys.getrefcount(obj) == 1:
    cleanup(obj)  # always wrong on PyPy

PyPy has no reference counting. sys.getrefcount() always returns a dummy constant. Any logic based on it silently produces wrong results.

sys.settrace() disables the JIT on PyPy

sys.settrace(my_tracer)  # disables JIT — 10-100x slower on PyPy

Debuggers and coverage tools that use sys.settrace() will silently disable PyPy's JIT compiler, causing massive performance degradation.

sys.getsizeof() raises TypeError on PyPy

size = sys.getsizeof(obj)  # TypeError on PyPy — always

PyPy deliberately raises TypeError because memory sizing by object is meaningless on its GC.

Why existing tools miss these

Tool	What it catches	What it misses
`pylint` / `ruff`	Style, common bugs	Runtime behaviour differences
`mypy` / `pyright`	Type errors	Runtime behaviour differences
`pip-audit`	Known CVEs	Behaviour differences
`bandit`	Security patterns	Behaviour differences
pyrift	Silent runtime behaviour differences	(that's the whole point)

pyrift does not replace any of these tools. It catches what they all miss.

Use it in CI

- name: Run pyrift
  run: |
    pip install pyrift
    pyrift scan .

Exit code is 1 when errors are found — fails the build automatically.

Current state

90 rules — 45 CPython version compatibility + 45 PyPy runtime differences
316 tests — all passing on Python 3.10, 3.11, 3.12, 3.13
Zero dependencies — pure Python
MIT licensed

Try it

pip install pyrift
pyrift scan ./your_project

GitHub: https://github.com/BHUVANSH855/pyrift
PyPI: https://pypi.org/project/pyrift/
Rule docs: https://github.com/BHUVANSH855/pyrift/blob/main/docs/rules.md

Contributions very welcome — especially new rules for behaviour differences you have personally encountered. Rule IDs CPY046+ and PPY046+ are open for community contributions.

Built by a CPython contributor and PyPy toolkit author — Bhuvansh Kataria