Move InstallRequirement.from_req to constructors module

src/pip/_internal/req/constructors.py

@@ -22,6 +22,7 @@ from pip._internal.download import (
     is_archive_file, is_url, path_to_url, url_to_path,
 )
 from pip._internal.exceptions import InstallationError
+from pip._internal.models.index import PyPI, TestPyPI
 from pip._internal.models.link import Link
 from pip._internal.req.req_install import InstallRequirement
 from pip._internal.utils.misc import is_installable_dir
@@ -270,3 +271,28 @@ def install_req_from_line(
         constraint=constraint,
         extras=extras,
     )
+
+
+def install_req_from_req(
+    req, comes_from=None, isolated=False, wheel_cache=None
+):
+    try:
+        req = Requirement(req)
+    except InvalidRequirement:
+        raise InstallationError("Invalid requirement: '%s'" % req)
+
+    domains_not_allowed = [
+        PyPI.file_storage_domain,
+        TestPyPI.file_storage_domain,
+    ]
+    if req.url and comes_from.link.netloc in domains_not_allowed:
+        # Explicitly disallow pypi packages that depend on external urls
+        raise InstallationError(
+            "Packages installed from PyPI cannot depend on packages "
+            "which are not also hosted on PyPI.\n"
+            "%s depends on %s " % (comes_from.name, req)
+        )
+
+    return InstallRequirement(
+        req, comes_from, isolated=isolated, wheel_cache=wheel_cache
+    )

src/pip/_internal/req/req_install.py

@@ -9,7 +9,7 @@ import zipfile
 from distutils.util import change_root
 
 from pip._vendor import pkg_resources, six
-from pip._vendor.packaging.requirements import InvalidRequirement, Requirement
+from pip._vendor.packaging.requirements import Requirement
 from pip._vendor.packaging.utils import canonicalize_name
 from pip._vendor.packaging.version import Version
 from pip._vendor.packaging.version import parse as parse_version
@@ -21,7 +21,6 @@ from pip._internal.exceptions import InstallationError
 from pip._internal.locations import (
     PIP_DELETE_MARKER_FILENAME, running_under_virtualenv,
 )
-from pip._internal.models.index import PyPI, TestPyPI
 from pip._internal.models.link import Link
 from pip._internal.pyproject import load_pyproject_toml
 from pip._internal.req.req_uninstall import UninstallPathSet
@@ -125,29 +124,6 @@ class InstallRequirement(object):
         # but after loading this flag should be treated as read only.
         self.use_pep517 = None
 
-    # Constructors
-    #   TODO: Move these out of this class into custom methods.
-    @classmethod
-    def from_req(cls, req, comes_from=None, isolated=False, wheel_cache=None):
-        try:
-            req = Requirement(req)
-        except InvalidRequirement:
-            raise InstallationError("Invalid requirement: '%s'" % req)
-
-        domains_not_allowed = [
-            PyPI.file_storage_domain,
-            TestPyPI.file_storage_domain,
-        ]
-        if req.url and comes_from.link.netloc in domains_not_allowed:
-            # Explicitly disallow pypi packages that depend on external urls
-            raise InstallationError(
-                "Packages installed from PyPI cannot depend on packages "
-                "which are not also hosted on PyPI.\n"
-                "%s depends on %s " % (comes_from.name, req)
-            )
-
-        return cls(req, comes_from, isolated=isolated, wheel_cache=wheel_cache)
-
     def __str__(self):
         if self.req:
             s = str(self.req)

src/pip/_internal/resolve.py

@@ -18,7 +18,7 @@ from pip._internal.exceptions import (
     BestVersionAlreadyInstalled, DistributionNotFound, HashError, HashErrors,
     UnsupportedPythonVersion,
 )
-from pip._internal.req.req_install import InstallRequirement
+from pip._internal.req.constructors import install_req_from_req
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.misc import dist_in_usersite, ensure_dir
 from pip._internal.utils.packaging import check_dist_requires_python
@@ -268,7 +268,7 @@ class Resolver(object):
         more_reqs = []
 
         def add_req(subreq, extras_requested):
-            sub_install_req = InstallRequirement.from_req(
+            sub_install_req = install_req_from_req(
                 str(subreq),
                 req_to_install,
                 isolated=self.isolated,