Fix Python-Requires being invalid for single Python versions

CHANGELOG.md

@@ -8,6 +8,7 @@
 - Fixed possible errors when resolving dependencies for specific packages.
 - Fixed handling of Python versions compatibility.
 - Fixed the dependency resolver picking up unnecessary dependencies due to not using the `python_full_version` marker.
+- Fixed the `Python-Requires` metadata being invalid for single Python versions.
 
 
 ## [0.12.4] - 2018-10-21

poetry/version/helpers.py

@@ -21,7 +21,10 @@ def format_python_constraint(constraint):
     This helper will help in transforming
     disjunctive constraint into proper constraint.
     """
-    if isinstance(constraint, Version) and constraint.precision < 3:
+    if isinstance(constraint, Version):
+        if constraint.precision >= 3:
+            return "=={}".format(str(constraint))
+
         # Transform 3.6 or 3
         if constraint.precision == 2:
             # 3.6

tests/masonry/builders/fixtures/single_python/README.rst

+Single Python
+=============

tests/masonry/builders/fixtures/single_python/pyproject.toml

+[tool.poetry]
+name = "single-python"
+version = "0.1"
+description = "Some description."
+authors = [
+    "Sébastien Eustace <sebastien@eustace.io>"
+]
+license = "MIT"
+
+readme = "README.rst"
+
+homepage = "https://poetry.eustace.io/"
+
+
+[tool.poetry.dependencies]
+python = "2.7.15"

tests/masonry/builders/fixtures/single_python/single_python.py

+"""Example module"""
+
+__version__ = "0.1"

tests/masonry/builders/test_sdist.py

@@ -471,7 +471,7 @@ def test_default_with_excluded_data(mocker):
         assert "my-package-1.2.3/PKG-INFO" in names
 
 
-def test_proper_python_requires_if_single_version_specified():
+def test_proper_python_requires_if_two_digits_precision_version_specified():
     poetry = Poetry.create(project("simple_version"))
 
     builder = SdistBuilder(poetry, NullEnv(), NullIO())
@@ -480,3 +480,14 @@ def test_proper_python_requires_if_single_version_specified():
     parsed = p.parsestr(to_str(pkg_info))
 
     assert parsed["Requires-Python"] == ">=3.6,<3.7"
+
+
+def test_proper_python_requires_if_three_digits_precision_version_specified():
+    poetry = Poetry.create(project("single_python"))
+
+    builder = SdistBuilder(poetry, NullEnv(), NullIO())
+    pkg_info = builder.build_pkg_info()
+    p = Parser()
+    parsed = p.parsestr(to_str(pkg_info))
+
+    assert parsed["Requires-Python"] == "==2.7.15"