Draft: Build wheels for different Python interpreter

Fromager now support building wheels for a different Python interpreter:

```shell
fromager --python-interpreter=/path/to/python ...
```

Fixes: #431
Signed-off-by: Christian Heimes <[email protected]>

src/fromager/__main__.py

@@ -2,6 +2,7 @@
 
 import logging
 import pathlib
+import sys
 
 import click
 
@@ -127,6 +128,13 @@
     help="Build sdist and wheen with network isolation (unshare -cn)",
     show_default=True,
 )
+@click.option(
+    "--python-interpreter",
+    type=clickext.ClickPath(),
+    default=pathlib.Path(sys.executable),
+    help="Python interpreter to build wheels for",
+    show_default=True,
+)
 @click.pass_context
 def main(
     ctx: click.Context,
@@ -145,6 +153,7 @@ def main(
     variant: str,
     jobs: int | None,
     network_isolation: bool,
+    python_interpreter: pathlib.Path,
 ) -> None:
     # Set the overall logger level to debug and allow the handlers to filter
     # messages at their own level.
@@ -211,6 +220,7 @@ def main(
         network_isolation=network_isolation,
         max_jobs=jobs,
         settings_dir=settings_dir,
+        python_interpreter=python_interpreter,
     )
     wkctx.setup()
     ctx.obj = wkctx

src/fromager/build_environment.py

@@ -1,10 +1,8 @@
 import importlib.metadata
 import logging
 import pathlib
-import platform
 import re
 import subprocess
-import sys
 import typing
 
 from packaging.requirements import Requirement
@@ -75,7 +73,7 @@ def __init__(
         build_requirements: typing.Iterable[Requirement] | None,
     ):
         self._ctx = ctx
-        self.path = parent_dir / f"build-{platform.python_version()}"
+        self.path = parent_dir / f"build-{ctx.python_version()}"
         self._build_requirements = build_requirements
         self._createenv()
 
@@ -90,7 +88,7 @@ def _createenv(self) -> None:
 
         logger.debug("creating build environment in %s", self.path)
         external_commands.run(
-            [sys.executable, "-m", "virtualenv", str(self.path)],
+            [str(self._ctx.python_interpreter), "-m", "virtualenv", str(self.path)],
             network_isolation=False,
         )
         logger.info("created build environment in %s", self.path)
@@ -258,7 +256,7 @@ def _safe_install(
     logger.debug("installing %s %s", req_type, req)
     external_commands.run(
         [
-            sys.executable,
+            str(ctx.python_interpreter),
             "-m",
             "pip",
             "-vvv",

src/fromager/context.py

@@ -3,6 +3,7 @@
 import logging
 import os
 import pathlib
+import sys
 import typing
 from urllib.parse import urlparse
 
@@ -13,6 +14,7 @@
 from . import (
     constraints,
     dependency_graph,
+    external_commands,
     packagesettings,
 )
 
@@ -38,6 +40,7 @@ def __init__(
         network_isolation: bool = False,
         max_jobs: int | None = None,
         settings_dir: pathlib.Path | None = None,
+        python_interpreter: pathlib.Path = pathlib.Path(sys.executable),
     ):
         if active_settings is None:
             active_settings = packagesettings.Settings(
@@ -69,6 +72,8 @@ def __init__(
         self.variant = variant
         self.network_isolation = network_isolation
         self.settings_dir = settings_dir
+        self.python_interpreter = python_interpreter.absolute()
+        self._python_version: str | None = None
 
         self._build_order_filename = self.work_dir / "build-order.json"
         self._constraints_filename = self.work_dir / "constraints.txt"
@@ -165,6 +170,21 @@ def package_build_info(
             name = package
         return self.settings.package_build_info(name)
 
+    def python_version(self) -> str:
+        """Return Python version of `python_interpreter` (major.minor.patch)
+        """
+        if self._python_version is None:
+            out = external_commands.run(
+                [
+                    str(self.python_interpreter),
+                    "-c",
+                    "import platform; print(platform.python_version())",
+                ],
+                network_isolation=False,
+            )
+            self._python_version = out.split("\n", 1)[0].strip()
+        return self._python_version
+
     def setup(self) -> None:
         # The work dir must already exist, so don't try to create it.
         # Use os.makedirs() to create the others in case the paths