stackscope 0.2.2

Unusually detailed Python stack introspection

stackscope is a library that helps you tell what your running Python program is doing and how it got there. It can provide detailed stack traces, similar to what you get in an exception traceback, but without needing to throw an exception first. Compared to standard library facilities such as traceback.extract_stack(), it is far more versatile. It supports async tasks, generators, threads, and greenlets; provides information about active context managers in each stack frame; and includes a customization interface that library authors can use to teach it to improve the stack extraction logic for code that touches their library. (As an example of the latter, the stack of an async task blocked in a run_in_thread() function could be made to cover the code that’s running in the thread as well.)

stackscope is loosely affiliated with the Trio async framework, and shares Trio’s obsessive focus on usability and correctness. The context manager analysis is especially helpful with Trio since you can use it to understand where the nurseries are. You don’t have to use stackscope with Trio, though; it requires only the Python standard library, 3.8 or later, and the ExceptionGroup backport on versions below 3.11.

stackscope is mostly intended as a building block for other debugging and introspection tools. You can use it directly, but there’s only rudimentary support for end-user-facing niceties such as pretty-printed output. On the other hand, the core logic is (believed to be) robust and flexible, exposing customization points that third-party libraries can use to help stackscope make better tracebacks for their code. stackscope ships out of the box with such “glue” for Trio, greenback, and some of their lower-level dependencies.

stackscope requires Python 3.8 or later. It is fully type-annotated and is tested with CPython (every minor version through 3.12) and PyPy, on Linux, Windows, and macOS. It will probably work on other operating systems. Basic features will work on other Python implementations, but the context manager decoding will be less intelligent, and won’t work at all without a usable gc.get_referents().

Quickstart

Call stackscope.extract() to obtain a stackscope.Stack describing the stack of a coroutine object, generator iterator (sync or async), greenlet, or thread. If you want to extract part of the stack that led to the extract() call, then either pass a stackscope.StackSlice or use the convenience aliases stackscope.extract_since() and stackscope.extract_until().

Trio users: Try print(stackscope.extract(trio.lowlevel.current_root_task(), recurse_child_tasks=True)) to print the entire task tree of your Trio program.

Once you have a Stack, you can:

• Format it for human consumption: str() obtains a tree view as shown in the example below, or use stack.format() to customize it or stack.format_flat() to get an alternate format that resembles a standard Python traceback.

• Iterate over it (or equivalently, its frames attribute) to obtain a series of stackscope.Frames for programmatic inspection. Each frame represents one function call. In addition to the interpreter-level frame object, it lets you access information about the active context managers in that function.

• Look at its leaf attribute to see what’s left once you peel away all the frames. For example, this might be some atomic awaitable such as an asyncio.Future. It will be None if the frames tell the whole story.

• Use its as_stdlib_summary() method to get a standard library traceback.StackSummary object (with some loss of information), which can be pickled or passed to non-stackscope-aware tools.

See the documentation for more details.

Example

This code uses a number of context managers:

from contextlib import contextmanager, ExitStack

@contextmanager
def null_context():
    yield

def some_cb(*a, **kw):
    pass

@contextmanager
def inner_context():
    stack = ExitStack()
    with stack:
        stack.enter_context(null_context())
        stack.callback(some_cb, 10, "hi", answer=42)
        yield "inner"

@contextmanager
def outer_context():
    with inner_context() as inner:
        yield "outer"

def example():
    with outer_context():
        yield

def call_example():
    yield from example()

gen = call_example()
next(gen)

You can use stackscope to inspect the state of the partially-consumed generator gen, showing the tree structure of all of those context managers:

$ python3 -i example.py
>>> import stackscope
>>> stack = stackscope.extract(gen)
>>> print(stack)
stackscope.Stack (most recent call last):
╠ call_example in __main__ at [...]/stackscope/example.py:28
║ └ yield from example()
╠ example in __main__ at [...]/stackscope/example.py:25
║ ├ with outer_context():  # _: _GeneratorContextManager (line 24)
║ │ ╠ outer_context in __main__ at [...]/stackscope/example.py:21
║ │ ║ ├ with inner_context() as inner:  # inner: _GeneratorContextManager (line 20)
║ │ ║ │ ╠ inner_context in __main__ at [...]/stackscope/example.py:16
║ │ ║ │ ║ ├ with stack:  # stack: ExitStack (line 13)
║ │ ║ │ ║ ├── stack.enter_context(null_context(...))  # stack[0]: _GeneratorContextManager
║ │ ║ │ ║ │   ╠ null_context in __main__ at [...]/stackscope/example.py:5
║ │ ║ │ ║ │   ║ └ yield
║ │ ║ │ ║ ├── stack.callback(__main__.some_cb, 10, 'hi', answer=42)  # stack[1]: function
║ │ ║ │ ║ └ yield "inner"
║ │ ║ └ yield "outer"
║ └ yield

That full tree structure is exposed for programmatic inspection as well:

>>> print(stack.frames[1].contexts[0].inner_stack.frames[0].contexts[0])
inner_context(...)  # inner: _GeneratorContextManager (line 20)
╠ inner_context in __main__ at /Users/oremanj/dev/stackscope/example.py:16
║ ├ with stack:  # stack: ExitStack (line 13)
║ ├── stack.enter_context(null_context(...))  # stack[0]: _GeneratorContextManager
║ │   ╠ null_context in __main__ at /Users/oremanj/dev/stackscope/example.py:5
║ │   ║ └ yield
║ ├── stack.callback(__main__.some_cb, 10, 'hi', answer=42)  # stack[1]: function
║ └ yield "inner"

Of course, if you just want a “normal” stack trace without the added information, you can get that too:

>>> print("".join(stack.format_flat()))
stackscope.Stack (most recent call last):
  File "/Users/oremanj/dev/stackscope/example.py", line 28, in call_example
    yield from example()
  File "/Users/oremanj/dev/stackscope/example.py", line 25, in example
    yield

Development status

While stackscope is a young project that deals with some obscure Python internals, it is written quite defensively including 99%+ test coverage and type hints. The author is using it (cautiously) in production and thinks you might want to as well.

License

stackscope is licensed under your choice of the MIT or Apache 2.0 license. See LICENSE for details.