From 3a53732264d3c7905c31e67cdde3059abfb74325 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juan=20Luis=20Cano=20Rodr=C3=ADguez?=
 <juanlu@satellogic.com>
Date: Tue, 2 Feb 2021 16:39:48 +0100
Subject: [PATCH] Clarify intent of extra=None

---
 piptools/scripts/compile.py | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/piptools/scripts/compile.py b/piptools/scripts/compile.py
index 60d19730e..0638337f9 100755
--- a/piptools/scripts/compile.py
+++ b/piptools/scripts/compile.py
@@ -429,7 +429,11 @@ def cli(
     constraints = [
         req
         for req in constraints
-        if req.markers is None or req.markers.evaluate(dict(extra=None))
+        if req.markers is None
+        # We explicitly set extra=None to filter out optional requirements
+        # since evaluating an extra marker with no environment raises UndefinedEnvironmentName
+        # (see https://packaging.pypa.io/en/latest/markers.html#usage)
+        or req.markers.evaluate(dict(extra=None))
     ]
 
     log.debug("Using indexes:")