Fix error due to blank lines in exported requirements (#320)

* test: Add failing test for blank lines

Co-authored-by: Matteo Santamaria <[email protected]>

* fix: Ignore blank lines in requirements.txt

Co-authored-by: Matteo Santamaria <[email protected]>

* refactor: Check for blank lines before invoking to_constraint

Co-authored-by: Matteo Santamaria <[email protected]>

src/nox_poetry/sessions.py

@@ -79,11 +79,12 @@ def to_constraints(requirements: str) -> str:
     """Convert requirements to constraints."""
 
     def _to_constraints() -> Iterator[str]:
-        lines = requirements.strip().splitlines()
+        lines = requirements.splitlines()
         for line, requirement in enumerate(lines, start=1):
-            constraint = to_constraint(requirement, line)
-            if constraint is not None:
-                yield constraint
+            if requirement.strip():
+                constraint = to_constraint(requirement, line)
+                if constraint is not None:
+                    yield constraint
 
     return "\n".join(_to_constraints())
 

tests/unit/test_sessions.py

@@ -1,4 +1,5 @@
 """Unit tests for the sessions module."""
+from textwrap import dedent
 from typing import Callable
 from typing import Iterator
 
@@ -153,9 +154,16 @@ def test_session_build_package(proxy: nox_poetry.Session) -> None:
             'regex==2020.10.28; python_version == "3.5"',
         ),
         ("-e ../lib/foo", ""),
+        ("--extra-index-url https://example.com/pypi/simple", ""),
         (
-            "--extra-index-url https://example.com/pypi/simple",
-            "",
+            dedent(
+                """
+                --extra-index-url https://example.com/pypi/simple
+
+                boltons==20.2.1
+                """
+            ),
+            "boltons==20.2.1",
         ),
     ],
 )