locker: unify duplicate dependencies on export

(cherry picked from commit 733736cd1c2ea6189a77e34ab640059dbff2be23)

poetry/packages/locker.py

-import itertools
 import json
 import logging
 import os
@@ -6,6 +5,7 @@ import re
 
 from copy import deepcopy
 from hashlib import sha256
+from typing import Iterable
 from typing import Iterator
 from typing import List
 from typing import Optional
@@ -188,7 +188,7 @@ class Locker(object):
     @classmethod
     def get_project_dependencies(
         cls, project_requires, locked_packages, pinned_versions=False, with_nested=False
-    ):  # type: (List[Dependency], List[Package], bool, bool) -> List[Dependency]
+    ):  # type: (List[Dependency], List[Package], bool, bool) -> Iterable[Dependency]
         # group packages entries by name, this is required because requirement might use
         # different constraints
         packages_by_name = {}
@@ -296,11 +296,18 @@ class Locker(object):
 
         __walk_level(dependencies, 0)
 
-        return sorted(
-            itertools.chain(dependencies, nested_dependencies.values()),
-            key=lambda x: x.name.lower(),
+        # Merge same dependencies using marker union
+        for requirement in dependencies:
+            key = (requirement.name, requirement.pretty_constraint)
+            if key not in nested_dependencies:
+                nested_dependencies[key] = requirement
+            else:
+                nested_dependencies[key].marker = nested_dependencies[key].marker.union(
+                    requirement.marker
                 )
 
+        return sorted(nested_dependencies.values(), key=lambda x: x.name.lower())
+
     def get_project_dependency_packages(
         self, project_requires, dev=False, extras=None
     ):  # type: (List[Dependency], bool, Optional[Union[bool, Sequence[str]]]) -> Iterator[DependencyPackage]

tests/utils/test_exporter.py

@@ -256,8 +256,12 @@ def test_exporter_can_export_requirements_txt_with_nested_packages_and_markers(
     assert expected == {}
 
 
+@pytest.mark.parametrize(
+    "dev,lines",
+    [(False, ['a==1.2.3; python_version < "3.8"']), (True, ["a==1.2.3", "b==4.5.6"])],
+)
 def test_exporter_can_export_requirements_txt_with_nested_packages_and_markers_any(
-    tmp_dir, poetry
+    tmp_dir, poetry, dev, lines
 ):
     poetry.locker.mock_lock_data(
         {
@@ -295,24 +299,16 @@ def test_exporter_can_export_requirements_txt_with_nested_packages_and_markers_a
         Factory.create_dependency(
             name="b", constraint=dict(version="^4.5.6"), category="dev"
         ),
-        Factory.create_dependency(name="a", constraint=dict(version="^1.2.3")),
     ]
 
     exporter = Exporter(poetry)
 
-    exporter.export("requirements.txt", Path(tmp_dir), "requirements.txt", dev=True)
+    exporter.export("requirements.txt", Path(tmp_dir), "requirements.txt", dev=dev)
 
     with (Path(tmp_dir) / "requirements.txt").open(encoding="utf-8") as f:
         content = f.read()
 
-    assert (
-        content
-        == """\
-a==1.2.3
-a==1.2.3; python_version < "3.8"
-b==4.5.6
-"""
-    )
+    assert content.strip() == "\n".join(lines)
 
 
 def test_exporter_can_export_requirements_txt_with_standard_packages_and_hashes(