Improve the error message displayed on Python requirements conflict (p…

…ython-poetry#1681)

poetry/mixology/failure.py

@@ -31,18 +31,27 @@ def __init__(self, root): # type: (Incompatibility) -> None
  def write(self):
  buffer = []
 
- required_python_version = None
+ required_python_version_notification = False
  for incompatibility in self._root.external_incompatibilities:
  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("")
 
  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")
 
- 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
  and foo (1.0.0) requires Python <3.5, foo is forbidden.