Fallback to pyproject.toml-based builds if setuptools cannot be imported (#10717)

This fallback is only triggered if the project has a `setup.py` file.

Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>
Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

news/10717.bugfix.rst

@@ -0,0 +1 @@
+Fallback to pyproject.toml-based builds if ``setup.py`` is present in a project, but ``setuptools`` cannot be imported.

src/pip/_internal/cli/cmdoptions.py

@@ -10,6 +10,7 @@ pass on state. To be consistent, all options will follow this design.
 # The following comment should be removed at some point in the future.
 # mypy: strict-optional=False
 
+import importlib.util
 import logging
 import os
 import textwrap
@@ -770,6 +771,12 @@ def _handle_no_use_pep517(
         """
         raise_option_error(parser, option=option, msg=msg)
 
+    # If user doesn't wish to use pep517, we check if setuptools is installed
+    # and raise error if it is not.
+    if not importlib.util.find_spec("setuptools"):
+        msg = "It is not possible to use --no-use-pep517 without setuptools installed."
+        raise_option_error(parser, option=option, msg=msg)
+
     # Otherwise, --no-use-pep517 was passed via the command-line.
     parser.values.use_pep517 = False
 

src/pip/_internal/pyproject.py

@@ -1,3 +1,4 @@
+import importlib.util
 import os
 from collections import namedtuple
 from typing import Any, List, Optional
@@ -89,9 +90,15 @@ def load_pyproject_toml(
 
     # 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.
+    # we do so if the project has a pyproject.toml file
+    # or if we cannot import setuptools.
+
+    # We fallback to PEP 517 when without setuptools,
+    # so setuptools can be installed as a default build backend.
+    # For more info see:
+    # https://discuss.python.org/t/pip-without-setuptools-could-the-experience-be-improved/11810/9
     elif use_pep517 is None:
-        use_pep517 = has_pyproject
+        use_pep517 = has_pyproject or not importlib.util.find_spec("setuptools")
 
     # At this point, we know whether we're going to use PEP 517.
     assert use_pep517 is not None