Building Objects¶

Once you know how to load a config (see Config), the next step is turning that configuration into live Python objects. confwire does this with a single function, build_from_config.

Note

This section builds on the tutorial. Everything here assumes you already have a config dictionary — whether hand-written or loaded from a file via Config.fromfile.

The idea¶

A config dictionary describes what to build using a special "type" key holding a dotted import path. Every other key becomes a keyword argument to that callable.

>>> from confwire import build_from_config
>>> cfg = {"type": "urllib.request.Request", "url": "https://example.com", "method": "GET"}
>>> req = build_from_config(cfg)
>>> req.full_url, req.method
('https://example.com', 'GET')

confwire imports urllib.request.Request, then calls it as Request(url="https://example.com", method="GET").

A dict without a "type" key is treated as plain data and returned unchanged:

>>> build_from_config({"a": 1, "b": 2})
{'a': 1, 'b': 2}

Nested objects¶

Any value that is itself a "type"-tagged dict is built first, then passed to its parent. This works at any depth:

config = {
    "type": "myproject.models.Detector",
    "backbone": {
        "type": "myproject.models.ResNet",
        "depth": 50,
    },
    "num_classes": 80,
}

detector = build_from_config(config)
# Equivalent to:
#   Detector(backbone=ResNet(depth=50), num_classes=80)

Relative type paths¶

If a "type" starts with a dot, it is resolved relative to base_package:

build_from_config(
    {"type": ".models.ResNet", "depth": 50},
    base_package="myproject",
)

Passing a relative type without a base_package raises ImportError.

From a config file¶

build_from_config pairs naturally with Config.fromfile:

request = dict(
    type="urllib.request.Request",
    url="https://example.com",
    method="GET",
)

request:
  type: urllib.request.Request
  url: https://example.com
  method: GET

{
  "request": {
    "type": "urllib.request.Request",
    "url": "https://example.com",
    "method": "GET"
  }
}

>>> from confwire import Config, build_from_config
>>> cfg = Config.fromfile("model.py")
>>> req = build_from_config(cfg["request"])
>>> req.full_url, req.method
('https://example.com', 'GET')

Safety: blocked types¶

Some import paths allow arbitrary code or command execution and are never built, regardless of your configuration:

>>> build_from_config({"type": "os.system", "command": "rm -rf /"})
Traceback (most recent call last):
    ...
PermissionError: Building objects of type 'os.system' is not allowed.

The default blocklist (confwire.build.DEFAULT_BLOCKED_TYPES) covers three categories of dangerous calls:

Process/command execution — os.system, os.popen, os.execv, os.execve, os.execvp, os.execvpe, os.spawnl, os.spawnv, subprocess.Popen, subprocess.call, subprocess.run, subprocess.check_call, subprocess.check_output
File/directory removal — os.remove, os.unlink, os.rmdir, shutil.rmtree
Arbitrary code execution — eval, exec, builtins.eval, builtins.exec

You can supply your own set via the blocked_types argument to override the default — note that this replaces the default blocklist rather than extending it, so include any of the entries above that you still want blocked.