inspection/info: pep517 fallback on invalid version

This change ensures that invalid versions in setup files raises an 
error and gracefully fallsback to PEP 517 builds to discover package
metadata.

Resolves: #2761

poetry/inspection/info.py

@@ -436,7 +436,7 @@ class PackageInfo:
         info = None
         try:
             info = cls.from_setup_files(path)
-            if info.requires_dist is not None:
+            if all([info.version, info.name, info.requires_dist]):
                 return info
         except PackageInfoError:
             pass

poetry/utils/setup_reader.py

@@ -8,6 +8,8 @@ from typing import Optional
 from typing import Tuple
 from typing import Union
 
+from poetry.core.semver import Version
+
 from ._compat import PY35
 from ._compat import Path
 from ._compat import basestring
@@ -109,7 +111,7 @@ class SetupReader(object):
             name = parser.get("metadata", "name")
 
         if parser.has_option("metadata", "version"):
-            version = parser.get("metadata", "version")
+            version = Version.parse(parser.get("metadata", "version")).text
 
         install_requires = []
         extras_require = {}

tests/inspection/test_info.py

@@ -213,6 +213,30 @@ def test_info_setup_complex_disable_build(mocker, demo_setup_complex):
     assert info.requires_dist is None
 
 
+@pytest.mark.skipif(not PY35, reason="Parsing of setup.py is skipped for Python < 3.5")
+@pytest.mark.parametrize("missing", ["version", "name", "install_requires"])
+def test_info_setup_missing_mandatory_should_trigger_pep517(
+    mocker, source_dir, missing
+):
+    setup = "from setuptools import setup; "
+    setup += "setup("
+    setup += 'name="demo", ' if missing != "name" else ""
+    setup += 'version="0.1.0", ' if missing != "version" else ""
+    setup += 'install_requires=["package"]' if missing != "install_requires" else ""
+    setup += ")"
+
+    setup_py = source_dir / "setup.py"
+    setup_py.write_text(decode(setup))
+
+    spy = mocker.spy(VirtualEnv, "run")
+    try:
+        PackageInfo.from_directory(source_dir)
+    except PackageInfoError:
+        assert spy.call_count == 3
+    else:
+        assert spy.call_count == 2
+
+
 def test_info_prefer_poetry_config_over_egg_info():
     info = PackageInfo.from_directory(
         FIXTURE_DIR_INSPECTIONS / "demo_with_obsolete_egg_info"

tests/utils/fixtures/setups/with-setup-cfg-attr/setup.cfg

+[metadata]
+name = with-setup-cfg-attr
+version = attr: with_setup_cfg_attr.__version__
+
+[options]
+zip_safe = true
+python_requires = >=2.6,!=3.0,!=3.1,!=3.2,!=3.3
+setup_requires = setuptools>=36.2.2
+install_requires =
+    six
+    tomlkit
+
+[options.extras_require]
+validation =
+    cerberus
+tests =
+    pytest
+    pytest-xdist
+    pytest-cov

tests/utils/fixtures/setups/with-setup-cfg-attr/setup.py

+from setuptools import setup
+
+setup()

tests/utils/test_setup_reader.py

@@ -2,6 +2,7 @@ import os
 
 import pytest
 
+from poetry.core.semver.exceptions import ParseVersionError
 from poetry.utils._compat import PY35
 from poetry.utils.setup_reader import SetupReader
 
@@ -117,6 +118,11 @@ def test_setup_reader_read_setup_cfg(setup):
     assert expected_python_requires == result["python_requires"]
 
 
+def test_setup_reader_read_setup_cfg_with_attr(setup):
+    with pytest.raises(ParseVersionError):
+        SetupReader.read_from_directory(setup("with-setup-cfg-attr"))
+
+
 @pytest.mark.skipif(not PY35, reason="AST parsing does not work for Python <3.4")
 def test_setup_reader_read_setup_kwargs(setup):
     result = SetupReader.read_from_directory(setup("pendulum"))