Write a temporary setup.cfg to override any system distutils configur…

…ation (#18)

* Write a temporary setup.cfg to override any system distutils configuration

Previously, shiv could not install packages that did not provide wheels
using Homebrew's Pythons, because `--target` is incompatible with
Homebrew's `distutils.cfg`. We can work around that by creating a
`setup.cfg` in the same directory we run `pip install` containing:

```ini
[install]
prefix=
```

Fixes #16.

* Restore the previous directory after installing

* Move temporary setup.cfg to clean_pip_env()

src/shiv/constants.py

@@ -18,3 +18,4 @@
     ("-d", "--download"): "Shiv needs to actually perform an install, not merely a download.",
     ("--user", "--root", "--prefix"): "Which conflicts with Shiv's internal use of '--target'.",
 }
+SETUP_CFG_NO_PREFIX = "[install]\nprefix="

src/shiv/pip.py

@@ -3,9 +3,11 @@
 import subprocess
 import sys
 
+from pathlib import Path
+from tempfile import TemporaryDirectory
 from typing import Generator, List
 
-from .constants import PIP_REQUIRE_VIRTUALENV, PIP_INSTALL_ERROR
+from .constants import PIP_REQUIRE_VIRTUALENV, PIP_INSTALL_ERROR, SETUP_CFG_NO_PREFIX
 
 
 @contextlib.contextmanager
@@ -16,13 +18,27 @@ def clean_pip_env() -> Generator[None, None, None]:
 
     """
     require_venv = os.environ.pop(PIP_REQUIRE_VIRTUALENV, None)
-
-    try:
-        yield
-
-    finally:
-        if require_venv is not None:
-            os.environ[PIP_REQUIRE_VIRTUALENV] = require_venv
+    cwd = Path.cwd()
+
+    with TemporaryDirectory() as working_path:
+        # distutils doesn't support using --target if there's a config file
+        # specifying --prefix. Homebrew's Pythons include a distutils.cfg that
+        # breaks `pip install --target` with any non-wheel packages. We can
+        # work around that by creating a setup.cfg specifying an empty prefix
+        # in the directory we run `pip install` from.
+        with Path(working_path, "setup.cfg").open("w") as f:
+            f.write(SETUP_CFG_NO_PREFIX)
+        os.chdir(working_path)
+
+        try:
+            yield
+
+        finally:
+            if require_venv is not None:
+                os.environ[PIP_REQUIRE_VIRTUALENV] = require_venv
+
+            # return to the previous working directory
+            os.chdir(cwd)
 
 
 def install(interpreter_path: str, args: List[str]) -> None:

test/test_pip.py

@@ -0,0 +1,26 @@
+import os
+
+from pathlib import Path
+
+from shiv.constants import PIP_REQUIRE_VIRTUALENV
+from shiv.constants import SETUP_CFG_NO_PREFIX
+from shiv.pip import clean_pip_env
+
+
+def test_clean_pip_env(monkeypatch):
+    before_env_var = 'foo'
+    monkeypatch.setenv(PIP_REQUIRE_VIRTUALENV, before_env_var)
+
+    before_cwd = Path.cwd()
+
+    with clean_pip_env():
+        assert PIP_REQUIRE_VIRTUALENV not in os.environ
+
+        cwd = Path.cwd()
+        assert cwd != before_cwd
+
+        with cwd.joinpath("setup.cfg").open() as f:
+            assert f.read() == SETUP_CFG_NO_PREFIX
+
+    assert os.environ.get(PIP_REQUIRE_VIRTUALENV) == before_env_var
+    assert Path.cwd() == before_cwd