wrapture

Wrap anything, capture everything, change nothing.

wrapture (wrapt + capture) is a Python library for attaching bindings to arbitrary call sites, without modifying the code being observed, and doing something useful with what flows through them.

It is a sibling project to wrapt and autowrapt, building on the safe monkey-patching machinery wrapt provides.

Status: alpha, ahead of 1.0.0. Pre-releases are published to PyPI, and until 1.0.0 is final a plain pip install wrapture picks up the latest pre-release automatically, so there is no need to pin a specific version. The existing API is not foreseen to break: the alpha series is mainly adding functionality rather than reworking what is already here, so code written against it today is expected to carry forward to 1.0.0.

Installation

wrapture is on PyPI:

$ pip install wrapture

or with uv:

$ uv add wrapture

Documentation

Full documentation is at wrapture.readthedocs.io. Start with the getting started page: everything on it can be pasted into a Python interpreter. Coming from unittest.mock? There is a comparison page mapping each mock idiom to its wrapture counterpart. After that, the worked examples, starting with testing code that calls external services, each take one question you might arrive with and answer it end to end.

At a glance

None of the classes below import wrapture or know they are observed:

place = wrapture.binding(OrderService, "place")
charge = wrapture.binding(Gateway, "charge")
record = wrapture.binding(Ledger, "record")

with wrapture.timeline(place, charge, record) as tape:
    OrderService().place(500)

print(tape.tree())

OrderService.place(amount=500)  -> {'id': 'ch_500', 'amount': 500}
  Gateway.charge(amount=500, currency='USD')  -> {'id': 'ch_500', 'amount': 500}
  Ledger.record(entry={'id': 'ch_500', 'amount': 500})  -> 'led_ch_500'

The same bindings intervene as well as observe: stub a result, inject a failure, or transform one argument while the real code keeps running.

What it does

One mechanism, three uses, in increasing order of machinery:

1. Monkey patching. A clean lifecycle and behaviour vocabulary over wrapt's wrap_object(). Point at a method by name and stub it, fail it, transform its arguments or result, or wrap it with a decorator, then remove it again, with honest reporting if something else displaced the patch in the meantime. Useful entirely on its own, with nothing else switched on.

2. Unit testing. Observe and assert on how calls actually flowed through a real call graph (nesting, ordering, arguments and return values) and optionally intervene (stub, transform, fail-inject). Unlike a unittest.mock Mock, which fabricates values and cannot see calls an object makes to itself, wrapture watches the real code run, and when a test must supply a stand-in it provides strict, recorded ones: stub() for a callable, spec-required mock() for a collaborator. This makes it possible to test code with no injectable seams at all, and to assert on what didn't happen on an error path: inject a gateway timeout, then verify the ledger was not written, the receipt was not sent, and the compensating refund was issued.

3. Ad-hoc tracing. Attach bindings to a running application, including one you cannot modify or redeploy, and emit a structured, nested trace to process or chart elsewhere. Name a handful of methods and a call tree appears; no code changes required: with a wrapture.toml naming the methods and a sink, python -m wrapture manage.py runserver traces the application untouched. With autowrapt installed, not even the launcher is needed: AUTOWRAPT_BOOTSTRAP=wrapture in the environment applies the same config at interpreter startup, so the program starts with plain python.

On top of the tracing layer sits OpenTelemetry export: the same recorded events sent to any OTLP backend as traces, metrics and correlated logs, switched on by one [otel] table in the config, with the trace identity arriving and leaving in W3C traceparent headers so two observed services join one distributed trace. These are layers of one mechanism, not separate products: the binding vocabulary that stubs a method in a test is the same one that traces it in production, and the config that names methods for a printed call tree is the config that exports spans, so what starts as a monkey patch or a test assertion can grow into full observability without the code being rewritten along the way.

The distinction that matters: most instrumentation, OpenTelemetry's own included, either has to be written into the code as SDK calls, or arrives as auto-instrumentation covering only the frameworks it already knows. wrapture needs neither: you point at your own methods by name and a trace appears, and the same pointing is how it lands in a test, a terminal, or a backend.

Why

No single existing tool covers "point at arbitrary methods, get a structured nested trace, assert on it or export it, in tests or in production":

• unittest.mock records a flat call list, with no nesting and no return values, and a patched call returns a fabricated MagicMock rather than running the real code.
• Span-assertion tools (logfire.testing, OpenTelemetry's InMemorySpanExporter) require the code to already be instrumented.
• sys.settrace tools (hunter, snoop) give a firehose with no assertion API.
• APM agents are all-or-nothing products rather than a toolkit.

wrapture fills that gap: a targeted call tree with normalized arguments and return values, produced by naming the methods you care about, usable as a testing assertion library, a tracing tool, or both at once.

What it is not

• Not a fabrication tool. There is no spec-less Mock() here by design; stand-ins are strict and built from named specs, and unittest.mock remains the tool for invented objects.
• Not a production APM. It is a toolkit that APM-like things could be built on.
• Not an OpenTelemetry competitor. It should emit to OTel, not replace it.

How it was built

wrapture's code and documentation were written by an AI assistant under the direction of Graham Dumpleton, the author of wrapt, through a long process of specification, layered implementation, and validation against real-world test suites. How wrapture was built explains the process and the thinking.

Requirements

• Python 3.12+
• wrapt 2.4.0+

Issues

Bug reports and feature requests go to the issue tracker. Include the wrapture and Python versions and, where you can, a small binding that reproduces the problem.

License

BSD 2-Clause. See LICENSE.