pyfollow 0.1.2

Lightweight Python tracing utility — trace function calls, variables, expressions, and code blocks via console and webhook

pyfollow

Lightweight Python tracing utility. Trace function calls, variables, expressions, and code blocks — output to console, webhook, or both.

Zero dependencies. Pure stdlib.

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• Installation
• Quick Start
• API Reference
• Configuration
• Webhook
• Project Structure
• Tests
• Contributing
• License

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Installation

pip install pyfollow

Or copy the pyfollow/ directory into your project.

from pyfollow import follow

Quick Start

Console tracing

from pyfollow import follow

t = follow.init()

@t.function
def add(a, b):
    return a + b

x = 1
t.var(x)

x = x + 5
r = add(3, 7)

t.flush()

Output:

[pyfollow.function] add:
[pyfollow.function] call add(a=3, b=7)
[pyfollow.var] x = 1
[pyfollow.var] x change: 6 [changed from 1]
[pyfollow.function] add -> 10

Webhook tracing

from pyfollow import follow

t = follow.init({"webhook_url": "http://localhost:8000/webhook"})

@t.function(usecode="batch1")
def compute(n):
    return n * 2

compute(5)
t.sync("batch1")
t.flush()

Variable watching

from pyfollow import follow

t = follow.init()

x = 10
t.var(x)          # trace init: x = 10

x = 20            # auto-detect: x changed from 10 to 20

print(repr(x))    # auto-detect: x read

t.unvar("x")      # stop watching

Block tracing

from pyfollow import follow

t = follow.init()

with t:
    a = 42
    b = a + 1
    print(b)

Expression tracing

from pyfollow import follow

t = follow.init()

t(1 + 2)          # [pyfollow] t(1 + 2) -> 3
t(sorted([3,1,2])) # [pyfollow] t(sorted([3,1,2])) -> [1, 2, 3]

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API Reference

follow.init(config=None)

Create a new Follow instance.

t = follow.init()                                        # default
t = follow.init({"webhook_url": "http://...", "uselogs": False})

t.config(**kwargs)

Update configuration at runtime.

t.config(uselogs=False)
t.config(webhook_url="http://localhost:8000/webhook")

@t.function

Decorator that traces function calls and returns.

@t.function
def my_func(x):
    return x + 1

@t.function(usecode="tag")

Decorator with deferred sync — events are buffered under a tag until t.sync("tag") is called.

@t.function(usecode="batch1")
def compute(n):
    return n * 2

compute(5)         # buffered, not sent
t.sync("batch1")   # flush only batch1 events

t.var(value, name=None)

Watch a variable. Detects init, changes, and reads (via repr()/str()).

x = 1
t.var(x)    # log: x = 1
x = 2       # log: x changed from 1 to 2

t.unvar(name)

Stop watching a variable.

t.unvar("x")

t(expr)

Trace an expression — logs the source code line and its result.

t(1 + 2)    # log: t(1 + 2) -> 3

with t:

Context manager that traces every line inside the block.

with t:
    x = 1
    y = x + 1

t.sync(code=None)

Flush buffered events to webhook.

• t.sync() — flush all untagged events
• t.sync("tag") — flush only events with matching tag

t.flush()

Block until all pending webhook sends complete.

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Configuration

Key	Type	Default	Description
webhook_url	str	None	URL for HTTP POST event delivery
uselogs	bool	True	Print trace events to console

# Console only (default)
t = follow.init()

# Webhook only
t = follow.init({"webhook_url": "http://localhost:8000/webhook", "uselogs": False})

# Both
t = follow.init({"webhook_url": "http://localhost:8000/webhook"})

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Webhook

Event types

type	Trigger
var_init	t.var(x) called
var (change)	Variable reassigned
var (read)	Variable accessed via repr()/str()
function_call	Traced function called
function_return	Traced function returned
block_start	with t: entered
block_line	Line executed inside block
block_end	with t: exited
expression	t(expr) evaluated

Event payload example

{
  "type": "function_call",
  "name": "add",
  "args": "a=2, b=3",
  "source": "def add(a, b):\n    return a + b"
}

Server example (FastAPI)

cd examples/server
uvicorn main:app --reload

# examples/server/main.py
from fastapi import FastAPI, Request

app = FastAPI()

@app.post("/webhook")
async def webhook(request: Request):
    events = await request.json()
    for event in events:
        print(event)
    return {"status": "ok"}

Client example

python examples/webhook_client.py

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Project Structure

trace-module-python/
├── pyproject.toml              # Build config
├── README.md                   # Documentation
├── LICENSE                     # MIT License
├── TODO.md                     # Task tracking
├── pyfollow/                   # Package
│   ├── __init__.py             # Public API + metadata
│   ├── follow.py               # Follow class (core orchestrator)
│   ├── watcher.py              # _FollowedVar proxy + VarWatcher (bytecode)
│   └── sender.py               # WebhookSender (async queue + retry)
├── tests/
│   └── test_trace.py           # 17 pytest tests
└── examples/
    ├── simple.py               # Console demo
    ├── webhook_client.py       # Webhook demo
    └── server/
        └── main.py             # FastAPI webhook receiver

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Tests

pip install pytest
python -m pytest tests/ -v

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How It Works

pyfollow hooks into Python's sys.settrace machinery to intercept line execution events. For variable watching, it uses dis (bytecode disassembly) to detect LOAD_FAST/LOAD_GLOBAL instructions that read watched variables.

The _FollowedVar proxy wraps values transparently — all dunder methods are forwarded to the original object, making the wrapper invisible to user code. Reads are detected when repr() or str() is called on the proxy.

Webhook delivery runs on a background daemon thread with a queue, retrying failed requests up to 2 times with 1-second backoff.

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Contributing

Contributions welcome. Open an issue or submit a PR on GitHub.

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Author

Rayan Rav — GitHub · Website

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License

MIT © 2026 Rayan Rav