Repository: pypa/build

Issue:
API: Add a method to customise handling of build output
When calling the API (`builder.build()`) the backend output is sent directly to the terminal. There's no clear way to capture that output, either to redirect it to a log, or to save it for manual processing. This makes it very difficult to use the API without the caller's output being extremely cluttered.

Related: #188

For now, in my code, I am using

```python
build.pep517.default_subprocess_runner = build.pep517.quiet_subprocess_runner
```

to suppress output. Yes, I *know* this is an immense hack. Would a PR to allow use of a pep517 custom runner object at https://github.com/pypa/build/blob/main/src/build/__init__.py#L201 be accepted?

Comments:

Patch:
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
index 585e8d4..2f9858e 100644
--- a/CHANGELOG.rst
+++ b/CHANGELOG.rst
@@ -10,15 +10,18 @@ Unreleased
 - Set and test minimum versions of build's runtime dependencies (`PR #267`_, Fixes `#263`_)
 - Use symlinks on creating venv's when available (`PR #274`_, Fixes `#271`_)
 - Error sooner if pip upgrade is required and fails (`PR #288`_, Fixes `#256`_)
+- Add a ``runner`` argument to ``ProjectBuilder`` (`PR #290`_, Fixes `#289`_)
 
 .. _PR #260: https://github.com/pypa/build/pull/260
 .. _PR #267: https://github.com/pypa/build/pull/267
 .. _PR #274: https://github.com/pypa/build/pull/274
 .. _PR #288: https://github.com/pypa/build/pull/288
+.. _PR #290: https://github.com/pypa/build/pull/290
 .. _#256: https://github.com/pypa/build/issues/256
 .. _#259: https://github.com/pypa/build/issues/259
 .. _#263: https://github.com/pypa/build/issues/263
 .. _#271: https://github.com/pypa/build/issues/271
+.. _#289: https://github.com/pypa/build/issues/289
 
 Breaking Changes
 ----------------
diff --git a/src/build/__init__.py b/src/build/__init__.py
index 10e5c9d..4345623 100644
--- a/src/build/__init__.py
+++ b/src/build/__init__.py
@@ -20,6 +20,8 @@ import toml
 import toml.decoder
 
 
+RUNNER_TYPE = Callable[[Sequence[str], Optional[Union[bytes, Text]], Optional[Dict[str, str]]], None]
+
 if sys.version_info < (3,):
     FileNotFoundError = IOError
     PermissionError = OSError
@@ -139,12 +141,31 @@ class ProjectBuilder(object):
     The PEP 517 consumer API.
     """
 
-    def __init__(self, srcdir, python_executable=sys.executable, scripts_dir=None):
-        # type: (str, Union[bytes, Text], Optional[Union[bytes, Text]]) -> None
+    def __init__(
+        self,
+        srcdir,  # type: str
+        python_executable=sys.executable,  # type: Union[bytes, Text]
+        scripts_dir=None,  # type: Optional[Union[bytes, Text]]
+        runner=pep517.wrappers.default_subprocess_runner,  # type: RUNNER_TYPE
+    ):
+        # type: (...) -> None
         """
         :param srcdir: The source directory
         :param scripts_dir: The location of the scripts dir (defaults to the folder where the python executable lives)
         :param python_executable: The python executable where the backend lives
+        :param runner: An alternative runner for backend subprocesses
+
+        The 'runner', if provided, must accept the following arguments:
+
+        - cmd: a list of strings representing the command and arguments to
+          execute, as would be passed to e.g. 'subprocess.check_call'.
+        - cwd: a string representing the working directory that must be
+          used for the subprocess. Corresponds to the provided srcdir.
+        - extra_environ: a dict mapping environment variable names to values
+          which must be set for the subprocess execution.
+
+        The default runner simply calls the backend hooks in a subprocess, writing backend output
+        to stdout/stderr.
         """
         self.srcdir = os.path.abspath(srcdir)  # type: str
         _validate_source_directory(srcdir)
@@ -180,6 +201,7 @@ class ProjectBuilder(object):
         self._build_system = build_system
         self._backend = self._build_system['build-backend']
         self._scripts_dir = scripts_dir
+        self._hook_runner = runner
         self._hook = pep517.wrappers.Pep517HookCaller(
             self.srcdir,
             self._backend,
@@ -198,7 +220,7 @@ class ProjectBuilder(object):
                 paths.update((i, None) for i in os.environ['PATH'].split(os.pathsep))
             extra_environ = {} if extra_environ is None else extra_environ
             extra_environ['PATH'] = os.pathsep.join(paths)
-        pep517.default_subprocess_runner(cmd, cwd, extra_environ)
+        self._hook_runner(cmd, cwd, extra_environ)
 
     @property
     def python_executable(self):  # type: () -> Union[bytes, Text]


Test patch:
diff --git a/tests/test_projectbuilder.py b/tests/test_projectbuilder.py
index 1485b74..9c1f8d8 100644
--- a/tests/test_projectbuilder.py
+++ b/tests/test_projectbuilder.py
@@ -460,3 +460,12 @@ def test_prepare_not_dir_outdir(mocker, tmp_dir, test_flit_path):
         f.write('Not a directory')
     with pytest.raises(build.BuildException, match='Build path .* exists and is not a directory'):
         builder.prepare('wheel', out)
+
+
+def test_runner_user_specified(tmp_dir, test_flit_path):
+    def dummy_runner(cmd, cwd=None, env=None):
+        raise RuntimeError('Runner was called')
+
+    builder = build.ProjectBuilder(test_flit_path, runner=dummy_runner)
+    with pytest.raises(build.BuildBackendException, match='Runner was called'):
+        builder.build('wheel', tmp_dir)