Raise exception in the case extension is not installed

There are situations that the user tries to update a previously
generated project with PyScaffold, but the extensions are not installed
in the current venv (like #506).

These changes try to make this situation obvious and ask for the user to
install the missing extensions.

I believe that it is better to panic and raise an exception instead of
proceed running PyScaffold, because there are cases that the extensions
completely change the desired behaviour (e.g. namespace indirectly via
custom_extension).

src/pyscaffold/exceptions.py

@@ -6,7 +6,7 @@
 import logging
 import sys
 import traceback
-from typing import Optional, cast
+from typing import Optional, Sequence, cast
 
 if sys.version_info[:2] >= (3, 8):
     # TODO: Import directly (no need for conditional) when `python_requires = >= 3.8`
@@ -141,6 +141,18 @@ def __init__(self, message=None, *args, **kwargs):
         super().__init__(message, *args, **kwargs)
 
 
+class ExtensionNotFound(ImportError):
+    """The following extensions were not found: {extensions}.
+    Please make sure you have the required versions installed.
+    You can look for official extensions at https://github.com/pyscaffold.
+    """
+
+    def __init__(self, extensions: Sequence[str]):
+        message = cast(str, self.__doc__)
+        message = message.format(extensions=extensions, version=pyscaffold_version)
+        super().__init__(message)
+
+
 class ErrorLoadingExtension(RuntimeError):
     """There was an error loading '{extension}'.
     Please make sure you have installed a version of the extension that is compatible

src/pyscaffold/extensions/__init__.py

@@ -19,6 +19,11 @@
 
 ENTRYPOINT_GROUP = "pyscaffold.cli"
 
+NO_LONGER_NEEDED = {"pyproject", "tox"}
+"""Extensions that are no longer needed and are now part of PyScaffold itself"""
+
+# TODO: NO_LONGER_SUPPORTED = {"no_pyproject"}
+
 
 class Extension:
     """Base class for PyScaffold's extensions

src/pyscaffold/info.py

@@ -9,7 +9,7 @@
 from enum import Enum
 from operator import itemgetter
 from pathlib import Path
-from typing import Optional, cast, overload
+from typing import Optional, Set, cast, overload
 
 import appdirs
 from configupdater import ConfigUpdater
@@ -18,6 +18,7 @@
 from . import __name__ as PKG_NAME
 from . import shell, toml
 from .exceptions import (
+    ExtensionNotFound,
     GitNotConfigured,
     GitNotInstalled,
     ImpossibleToFindConfigDir,
@@ -170,6 +171,7 @@ def project(
         :class:`~.NoPyScaffoldProject`: when project was not generated with PyScaffold
     """
     # Lazily load the following function to avoid circular dependencies
+    from .extensions import NO_LONGER_NEEDED  # TODO: NO_LONGER_SUPPORTED
     from .extensions import list_from_entry_points as list_extensions
 
     opts = copy.deepcopy({k: v for k, v in opts.items() if not callable(v)})
@@ -191,7 +193,7 @@ def project(
         "author": metadata.get("author"),
         "email": metadata.get("author_email") or metadata.get("author-email"),
         "url": metadata.get("url"),
-        "description": metadata.get("description"),
+        "description": metadata.get("description", "").strip(),
         "license": license and best_fit_license(license),
     }
     existing = {k: v for k, v in existing.items() if v}  # Filter out non stored values
@@ -201,14 +203,21 @@ def project(
     opts = {**existing, **opts}
 
     # Complement the cli extensions with the ones from configuration
+    not_found_ext: Set[str] = set()
     if "extensions" in pyscaffold:
         cfg_extensions = parse_extensions(pyscaffold.pop("extensions", ""))
         opt_extensions = {ext.name for ext in opts.setdefault("extensions", [])}
         add_extensions = cfg_extensions - opt_extensions
 
         other_ext = list_extensions(filtering=lambda e: e.name in add_extensions)
+        not_found_ext = add_extensions - {e.name for e in other_ext} - NO_LONGER_NEEDED
         opts["extensions"] = deterministic_sort(opts["extensions"] + other_ext)
 
+    if not_found_ext:
+        raise ExtensionNotFound(list(not_found_ext))
+
+    # TODO: NO_LONGER_SUPPORTED => raise Exception(use older PyScaffold)
+
     # The remaining values in the pyscaffold section can be added to opts
     # if not specified yet. Useful when extensions define other options.
     for key, value in pyscaffold.items():

tests/test_info.py

@@ -5,8 +5,9 @@
 from unittest.mock import MagicMock as Mock
 
 import pytest
+from configupdater import ConfigUpdater
 
-from pyscaffold import cli, exceptions, info, repo, structure, templates
+from pyscaffold import actions, cli, exceptions, info, repo, structure, templates
 
 
 def test_username_with_git(git_mock):
@@ -147,6 +148,16 @@ def test_project_old_setupcfg(tmpfolder):
         info.project({}, config_path=demoapp)
 
 
+def test_project_extensions_not_found(tmpfolder):
+    _, opts = actions.get_default_options({}, {})
+    cfg = ConfigUpdater().read_string(templates.setup_cfg(opts))
+    cfg["pyscaffold"]["extensions"] = "x_foo_bar_x"
+    (tmpfolder / "setup.cfg").write(str(cfg))
+    with pytest.raises(exceptions.ExtensionNotFound) as exc:
+        info.project(opts)
+    assert "x_foo_bar_x" in str(exc.value)
+
+
 @pytest.mark.no_fake_config_dir
 def test_config_dir_error(monkeypatch):
     # no_fake_config_dir => avoid previous mock of config_dir