Load the pip version from the correct prefix

- Respect `--system` when it is supplied during install
- Fixes #4220

Signed-off-by: Dan Ryan <dan.ryan@canonical.com>

news/4220.bugfix.rst

@@ -0,0 +1 @@
+Fixed a bug which caused pipenv to search non-existent virtual environments for ``pip`` when installing using ``--system``.

pipenv/core.py

@@ -1335,8 +1335,10 @@ def get_pip_args(
     no_deps=False,  # type: bool,
     selective_upgrade=False,  # type: bool
     src_dir=None,  # type: Optional[str]
+    allow_global=False,  # type: bool
 ):
     # type: (...) -> List[str]
+    from .environment import Environment
     from .vendor.packaging.version import parse as parse_version
     arg_map = {
         "pre": ["--pre"],
@@ -1352,9 +1354,10 @@ def get_pip_args(
         ],
         "src_dir": src_dir,
     }
-    if project.environment.pip_version >= parse_version("19.0"):
+    environment = project.get_environment(allow_global=allow_global)
+    if environment.pip_version >= parse_version("19.0"):
         arg_map["no_use_pep517"].append("--no-use-pep517")
-    if project.environment.pip_version < parse_version("19.1"):
+    if environment.pip_version < parse_version("19.1"):
         arg_map["no_use_pep517"].append("--no-build-isolation")
     arg_set = []
     for key in arg_map.keys():
@@ -1495,7 +1498,7 @@ def pip_install(
     pip_args = get_pip_args(
         pre=pre, verbose=environments.is_verbose(), upgrade=True,
         selective_upgrade=selective_upgrade, no_use_pep517=not use_pep517,
-        no_deps=no_deps, require_hashes=not ignore_hashes
+        no_deps=no_deps, require_hashes=not ignore_hashes, allow_global=allow_global
     )
     pip_command.extend(pip_args)
     if r:

pipenv/project.py

@@ -25,7 +25,7 @@
 from .environments import (
     PIPENV_DEFAULT_PYTHON_VERSION, PIPENV_IGNORE_VIRTUALENVS, PIPENV_MAX_DEPTH,
     PIPENV_PIPFILE, PIPENV_PYTHON, PIPENV_TEST_INDEX, PIPENV_VENV_IN_PROJECT,
-    is_in_virtualenv, is_type_checking
+    PIPENV_USE_SYSTEM, is_in_virtualenv, is_type_checking
 )
 from .vendor.requirementslib.models.utils import get_default_pyproject_backend
 from .utils import (
@@ -328,21 +328,30 @@ def pipfile_package_names(self):
             "combined": dev_keys | default_keys
         }
 
+    def get_environment(self, allow_global=False):
+        # type: (bool) -> Environment
+        if allow_global:
+            prefix = sys.prefix
+        else:
+            prefix = self.virtualenv_location
+        is_venv = is_in_virtualenv()
+        sources = self.sources if self.sources else [DEFAULT_SOURCE]
+        environment = Environment(
+            prefix=prefix, is_venv=is_venv, sources=sources, pipfile=self.parsed_pipfile,
+            project=self
+        )
+        pipenv_dist = get_pipenv_dist(pkg="pipenv")
+        if pipenv_dist:
+            environment.extend_dists(pipenv_dist)
+        else:
+            environment.add_dist("pipenv")
+        return environment
+
     @property
     def environment(self):
         if not self._environment:
-            prefix = self.virtualenv_location
-            is_venv = is_in_virtualenv()
-            sources = self.sources if self.sources else [DEFAULT_SOURCE]
-            self._environment = Environment(
-                prefix=prefix, is_venv=is_venv, sources=sources, pipfile=self.parsed_pipfile,
-                project=self
-            )
-            pipenv_dist = get_pipenv_dist(pkg="pipenv")
-            if pipenv_dist:
-                self._environment.extend_dists(pipenv_dist)
-            else:
-                self._environment.add_dist("pipenv")
+            allow_global = os.environ.get("PIPENV_USE_SYSTEM", PIPENV_USE_SYSTEM)
+            self._environment = self.get_environment(allow_global=allow_global)
         return self._environment
 
     def get_outdated_packages(self):