Improve the error message displayed on Python requirements conflict (#1681)

poetry/mixology/failure.py

@@ -31,18 +31,27 @@ class _Writer:
     def write(self):
     def write(self):
         buffer = []
         buffer = []
 
 
-        required_python_version = None
+        required_python_version_notification = False
         for incompatibility in self._root.external_incompatibilities:
         for incompatibility in self._root.external_incompatibilities:
             if isinstance(incompatibility.cause, PythonCause):
             if isinstance(incompatibility.cause, PythonCause):
-                required_python_version = incompatibility.cause.root_python_version
-                break
+                if not required_python_version_notification:
+                    buffer.append(
+                        "The current project's Python requirement ({}) "
+                        "is not compatible with some of the required "
+                        "packages Python requirement:".format(
+                            incompatibility.cause.root_python_version
+                        )
+                    )
+                    required_python_version_notification = True
 
 
-        if required_python_version is not None:
-            buffer.append(
-                "The current project must support the following Python versions: {}".format(
-                    required_python_version
+                buffer.append(
+                    "  - {} requires Python {}".format(
+                        incompatibility.terms[0].dependency.name,
+                        incompatibility.cause.python_version,
+                    )
                 )
                 )
-            )
+
+        if required_python_version_notification:
             buffer.append("")
             buffer.append("")
 
 
         if isinstance(self._root.cause, ConflictCause):
         if isinstance(self._root.cause, ConflictCause):

tests/mixology/version_solver/test_python_constraint.py

@@ -8,7 +8,9 @@ def test_dependency_does_not_match_root_python_constraint(root, provider, repo):
 
 
     add_to_repo(repo, "foo", "1.0.0", python="<3.5")
     add_to_repo(repo, "foo", "1.0.0", python="<3.5")
 
 
-    error = """The current project must support the following Python versions: ^3.6
+    error = """The current project's Python requirement (^3.6) \
+is not compatible with some of the required packages Python requirement:
+  - foo requires Python <3.5
 
 
 Because no versions of foo match !=1.0.0
 Because no versions of foo match !=1.0.0
  and foo (1.0.0) requires Python <3.5, foo is forbidden.
  and foo (1.0.0) requires Python <3.5, foo is forbidden.