--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/lib/python3.8/site-packages/pip/_internal/pyproject.py	Mon Jul 27 03:47:31 2020 -0400
@@ -0,0 +1,196 @@
+from __future__ import absolute_import
+
+import io
+import os
+import sys
+from collections import namedtuple
+
+from pip._vendor import pytoml, six
+from pip._vendor.packaging.requirements import InvalidRequirement, Requirement
+
+from pip._internal.exceptions import InstallationError
+from pip._internal.utils.typing import MYPY_CHECK_RUNNING
+
+if MYPY_CHECK_RUNNING:
+    from typing import Any, Optional, List
+
+
+def _is_list_of_str(obj):
+    # type: (Any) -> bool
+    return (
+        isinstance(obj, list) and
+        all(isinstance(item, six.string_types) for item in obj)
+    )
+
+
+def make_pyproject_path(unpacked_source_directory):
+    # type: (str) -> str
+    path = os.path.join(unpacked_source_directory, 'pyproject.toml')
+
+    # Python2 __file__ should not be unicode
+    if six.PY2 and isinstance(path, six.text_type):
+        path = path.encode(sys.getfilesystemencoding())
+
+    return path
+
+
+BuildSystemDetails = namedtuple('BuildSystemDetails', [
+    'requires', 'backend', 'check', 'backend_path'
+])
+
+
+def load_pyproject_toml(
+    use_pep517,  # type: Optional[bool]
+    pyproject_toml,  # type: str
+    setup_py,  # type: str
+    req_name  # type: str
+):
+    # type: (...) -> Optional[BuildSystemDetails]
+    """Load the pyproject.toml file.
+
+    Parameters:
+        use_pep517 - Has the user requested PEP 517 processing? None
+                     means the user hasn't explicitly specified.
+        pyproject_toml - Location of the project's pyproject.toml file
+        setup_py - Location of the project's setup.py file
+        req_name - The name of the requirement we're processing (for
+                   error reporting)
+
+    Returns:
+        None if we should use the legacy code path, otherwise a tuple
+        (
+            requirements from pyproject.toml,
+            name of PEP 517 backend,
+            requirements we should check are installed after setting
+                up the build environment
+            directory paths to import the backend from (backend-path),
+                relative to the project root.
+        )
+    """
+    has_pyproject = os.path.isfile(pyproject_toml)
+    has_setup = os.path.isfile(setup_py)
+
+    if has_pyproject:
+        with io.open(pyproject_toml, encoding="utf-8") as f:
+            pp_toml = pytoml.load(f)
+        build_system = pp_toml.get("build-system")
+    else:
+        build_system = None
+
+    # The following cases must use PEP 517
+    # We check for use_pep517 being non-None and falsey because that means
+    # the user explicitly requested --no-use-pep517.  The value 0 as
+    # opposed to False can occur when the value is provided via an
+    # environment variable or config file option (due to the quirk of
+    # strtobool() returning an integer in pip's configuration code).
+    if has_pyproject and not has_setup:
+        if use_pep517 is not None and not use_pep517:
+            raise InstallationError(
+                "Disabling PEP 517 processing is invalid: "
+                "project does not have a setup.py"
+            )
+        use_pep517 = True
+    elif build_system and "build-backend" in build_system:
+        if use_pep517 is not None and not use_pep517:
+            raise InstallationError(
+                "Disabling PEP 517 processing is invalid: "
+                "project specifies a build backend of {} "
+                "in pyproject.toml".format(
+                    build_system["build-backend"]
+                )
+            )
+        use_pep517 = True
+
+    # If we haven't worked out whether to use PEP 517 yet,
+    # and the user hasn't explicitly stated a preference,
+    # we do so if the project has a pyproject.toml file.
+    elif use_pep517 is None:
+        use_pep517 = has_pyproject
+
+    # At this point, we know whether we're going to use PEP 517.
+    assert use_pep517 is not None
+
+    # If we're using the legacy code path, there is nothing further
+    # for us to do here.
+    if not use_pep517:
+        return None
+
+    if build_system is None:
+        # Either the user has a pyproject.toml with no build-system
+        # section, or the user has no pyproject.toml, but has opted in
+        # explicitly via --use-pep517.
+        # In the absence of any explicit backend specification, we
+        # assume the setuptools backend that most closely emulates the
+        # traditional direct setup.py execution, and require wheel and
+        # a version of setuptools that supports that backend.
+
+        build_system = {
+            "requires": ["setuptools>=40.8.0", "wheel"],
+            "build-backend": "setuptools.build_meta:__legacy__",
+        }
+
+    # If we're using PEP 517, we have build system information (either
+    # from pyproject.toml, or defaulted by the code above).
+    # Note that at this point, we do not know if the user has actually
+    # specified a backend, though.
+    assert build_system is not None
+
+    # Ensure that the build-system section in pyproject.toml conforms
+    # to PEP 518.
+    error_template = (
+        "{package} has a pyproject.toml file that does not comply "
+        "with PEP 518: {reason}"
+    )
+
+    # Specifying the build-system table but not the requires key is invalid
+    if "requires" not in build_system:
+        raise InstallationError(
+            error_template.format(package=req_name, reason=(
+                "it has a 'build-system' table but not "
+                "'build-system.requires' which is mandatory in the table"
+            ))
+        )
+
+    # Error out if requires is not a list of strings
+    requires = build_system["requires"]
+    if not _is_list_of_str(requires):
+        raise InstallationError(error_template.format(
+            package=req_name,
+            reason="'build-system.requires' is not a list of strings.",
+        ))
+
+    # Each requirement must be valid as per PEP 508
+    for requirement in requires:
+        try:
+            Requirement(requirement)
+        except InvalidRequirement:
+            raise InstallationError(
+                error_template.format(
+                    package=req_name,
+                    reason=(
+                        "'build-system.requires' contains an invalid "
+                        "requirement: {!r}".format(requirement)
+                    ),
+                )
+            )
+
+    backend = build_system.get("build-backend")
+    backend_path = build_system.get("backend-path", [])
+    check = []  # type: List[str]
+    if backend is None:
+        # If the user didn't specify a backend, we assume they want to use
+        # the setuptools backend. But we can't be sure they have included
+        # a version of setuptools which supplies the backend, or wheel
+        # (which is needed by the backend) in their requirements. So we
+        # make a note to check that those requirements are present once
+        # we have set up the environment.
+        # This is quite a lot of work to check for a very specific case. But
+        # the problem is, that case is potentially quite common - projects that
+        # adopted PEP 518 early for the ability to specify requirements to
+        # execute setup.py, but never considered needing to mention the build
+        # tools themselves. The original PEP 518 code had a similar check (but
+        # implemented in a different way).
+        backend = "setuptools.build_meta:__legacy__"
+        check = ["setuptools>=40.8.0", "wheel"]
+
+    return BuildSystemDetails(requires, backend, check, backend_path)