explr 0.1.3

Trace any Python process and generate a clean call graph diagram

explr

Trace any Python process and generate a clean call graph diagram.

Best suited for debugging small-to-medium synchronous Python programs (for now).

---
title: test_files/branching.py
---
flowchart LR
	__start__(("S"))
	__end__(("E"))
	__main____run["run"]
	__main____process["process"]
	__main____validate["validate"]
	__main____handle_even["handle_even"]
	__main____handle_odd["handle_odd"]
	__start__ --> __main____run
	__main____run --> __end__
	__main____run --> __main____process
	__main____process --> __main____validate
	__main____process --> __main____handle_even
	__main____process --> __main____handle_odd

Example PNG Diagram

Example Mermaid Diagram

How it works

explr injects Python's sys.settrace at runtime, records every function call, filters out noise (stdlib, dunders, private functions), and renders a flow diagram showing how control moves through your code.

The diagram has a horizontal spine of entry points in execution order, with each node's sub-calls hanging below it:

(S) → run → (E)
       ├── auth.register → db.save_user
       │                 → db.get_user
       ├── auth.login    → db.get_user
       └── report        → db.all_users

• S / E = start and end of execution
• Green nodes = entry points (called from top-level code), in the order they ran
• Blue nodes = sub-calls

Installation

Prerequisites

The default PNG output requires Graphviz. Install it for your OS:

OS	Command
macOS (Homebrew)	brew install graphviz
Ubuntu / Debian	sudo apt install graphviz
Fedora / RHEL	sudo dnf install graphviz
Windows	Download installer — make sure dot is added to PATH

If you only want Mermaid (.mmd) output, Graphviz is not required.

Install explr

pip install explr

CLI usage

explr [options] <target> [target-args ...]

Examples

# Trace a .py file
explr myscript.py

# Trace with the python prefix (same result)
explr python myscript.py
explr python3 myscript.py

# Pass arguments through to your script
explr myscript.py --config dev

# Trace a module-style tool (e.g. pytest, flask)
explr pytest tests/
explr python -m mypackage

# Trace any shell command that resolves to Python
# (PATH scripts, shell aliases, shell functions)
explr my_tool --some-arg

Resolving shell commands

explr automatically resolves commands that aren't directly a Python file or interpreter. It walks the following chain until it finds a Python target:

1. PATH scripts — executable files with a #!/usr/bin/env python3 shebang
2. Shell aliases — e.g. alias my_tool='python3 /path/to/tool.py'
3. Shell functions — defined in your .bashrc / .zshrc
4. Wrapper scripts — shell scripts that exec python3 ... internally

# If 'mtgs_viewer' is a shell alias for 'python3 viewer.py --theme dark':
explr mtgs_viewer

# explr prints what it resolved to:
# [explr] resolved 'mtgs_viewer' → viewer.py (extra args: ['--theme', 'dark'])

Options

Flag	Description
--depth N	Limit call depth (default: unlimited)
--no-stdlib	Skip tracing stdlib frames (faster, same visual result)
--output NAME	Override output filename (no extension needed)
--mermaid / --mmd	Output a Mermaid flowchart (.mmd) instead of a PNG

explr --depth 5 myscript.py
explr --no-stdlib myscript.py
explr --output my_graph myscript.py
explr --mermaid myscript.py

Output formats

Default — PNG (requires Graphviz):

explr_diagrams/
  myscript_diagram.png

Mermaid (--mermaid):

explr_diagrams/
  myscript_diagram.mmd

The .mmd file contains a standard Mermaid flowchart that renders in:

• VS Code (Markdown preview / Mermaid extension)
• GitHub Markdown (fenced ```mermaid blocks)
• mermaid.live — paste and view instantly

Python API

Trace a specific function from within your own code using explr.trace(). Works with both sync and async functions.

import explr

# Sync function
explr.trace(my_function, args=(1, 2))

# Async function — explr handles the event loop automatically
explr.trace(my_async_function, kwargs={"url": "...", "headers": {}})

# With keyword args
explr.trace(my_function, args=(x,), kwargs={"flag": True})

# All options
explr.trace(
    my_function,
    args=(x,),
    output="my_graph",   # custom output filename (no extension)
    depth=5,             # limit call depth
    no_stdlib=True,      # skip stdlib frames (faster)
)

# Returns the path to the generated PNG (or None if nothing was captured)
path = explr.trace(my_function, args=(x,))

Diagrams are written to ./explr_diagrams/<func_name>_diagram.png (or your output name).

explr.trace() runs entirely in-process using sys.settrace — no subprocess or temp files. Any existing trace hook is saved and restored around the call.

Async functions

For async functions, explr.trace() automatically runs the coroutine via asyncio.run(). Mock out any network/IO calls so the function executes without side effects:

import explr

# Mock the network call
async def fetch(url, headers):
    return b"mock response"

async def my_pipeline(url: str, headers: dict):
    raw = await fetch(url=url, headers=headers)
    result = process(raw)
    return result

explr.trace(
    my_pipeline,
    kwargs={"url": "https://example.com", "headers": {}},
    output="my_pipeline",
    no_stdlib=True,
)

Jupyter / running event loop: asyncio.run() cannot be called from inside an already-running loop. Install nest_asyncio to work around this:

pip install nest_asyncio

import nest_asyncio
nest_asyncio.apply()
explr.trace(my_async_function, kwargs={...})

---

What gets shown

Included	Excluded
User-defined functions	stdlib functions
Cross-module calls	Dunder methods (__init__, etc.)
Recursive calls (self-loops)	Private functions/modules (leading _)
Class methods	Synthetic names (<listcomp>, <lambda>, etc.)

If a function has no user-defined sub-calls, it still appears on the spine as S → fn → E.

---

Test files

The test_files/ directory contains examples covering common patterns:

explr test_files/simple.py             # linear call chain
explr test_files/recursive.py          # recursive functions
explr test_files/classes.py            # class methods
explr test_files/branching.py          # conditional branches
explr test_files/multi_module/main.py  # calls across multiple files
explr test_files/no_calls.py           # no sub-calls (spine only)

---

Project structure

explr/
  __init__.py   # explr.trace() Python API
  cli.py        # entry point, argument parsing, command resolution
  tracer.py     # sys.settrace bootstrap (CLI), in-process tracer (API), shell resolution
  renderer.py   # graphviz PNG and Mermaid diagram rendering
  models.py     # CallNode / CallEdge / CallGraph dataclasses
test_files/     # example scripts for testing
pyproject.toml