fix: ignore installed when running pip (#1040)

Author: henryiii

docs/changelog/1040.bugfix.rst

@@ -0,0 +1,2 @@
+Add ``--ignore-installed`` to pip install command to prevent issues with packages already present in the isolated build
+environment - by :user:`henryiii` (:issue:`1037`)

pyproject.toml

@@ -92,6 +92,7 @@ test = [
   'setuptools >= 56.0.0; python_version == "3.11"',
   'setuptools >= 67.8.0; python_version >= "3.12"',
   "setuptools_scm >= 6",
+  "pip >= 22.3",
   { include-group = "extra" },
 ]
 typing = [

src/build/env.py

@@ -333,7 +333,7 @@ def install_dependencies(self, requirements: Collection[str], constraints: Colle
             if (verbosity := _ctx.verbosity) > 1:
                 cmd += [f'-{"v" * (verbosity - 1)}']
 
-            cmd += ['install', '--use-pep517', '--no-warn-script-location', '--no-compile', '--no-input']
+            cmd += ['install', '--ignore-installed', '--use-pep517', '--no-warn-script-location', '--no-compile', '--no-input']
 
             # pip does not honour environment markers in command line arguments
             # but it does from requirement files.

tests/constraints.txt

@@ -1,5 +1,7 @@
 importlib-metadata==4.6
 packaging==24.0
+pip==22.3; python_version < "3.12"
+pip==23.2; python_version >= "3.12"
 pyproject_hooks==1.0
 setuptools==42.0.0; python_version < "3.10"
 setuptools==56.0.0; python_version == "3.10"

tests/test_env.py

@@ -3,6 +3,7 @@
 
 import importlib.util
 import logging
+import pathlib
 import shutil
 import subprocess
 import sys
@@ -235,6 +236,7 @@ def test_default_impl_install_cmd_well_formed(
                 'pip',
                 *([f'-{"v" * (verbosity - 1)}'] if verbosity > 1 else []),
                 'install',
+                '--ignore-installed',
                 '--use-pep517',
                 '--no-warn-script-location',
                 '--no-compile',
@@ -573,3 +575,19 @@ def test_uv_install_respects_existing_keyring_env(  # pragma: no cover -- uv tes
 
     (install_call,) = run_subprocess.call_args_list
     assert install_call.kwargs['env']['UV_KEYRING_PROVIDER'] == 'disabled'
+
+
+@pytest.mark.network
+def test_pythonpath_does_not_interfere_with_outer_pip(
+    monkeypatch: pytest.MonkeyPatch,
+    tmp_path: pathlib.Path,
+) -> None:
+    flit_core = tmp_path.joinpath('flit_core-0.0.0.dist-info/')
+    flit_core.mkdir()
+
+    monkeypatch.setenv('PYTHONPATH', str(tmp_path))
+
+    with build.env.DefaultIsolatedEnv(installer='pip') as env:
+        env.install({'flit_core'})
+
+        assert subprocess.check_call([env.python_executable, '-c', 'import flit_core']) == 0

tox.ini

@@ -14,8 +14,6 @@ skip_missing_interpreters = true
 [testenv]
 description =
     run test suite with {basepython}
-deps =
-    pip
 pass_env =
     LC_ALL
     PIP_*