Question about (properties `((python2-variant . ,(delay XXX)))) and name resolution

[Danny Milosavljevic]

Hi,

in the process of packaging ptpython I ran into the following problem:

   ;;; Failed to autoload canonical-package in (gnu packages base):
   ;;; ERROR: Unbound variable: ptpython-2

... while I did the following patch to gnu/packages/python.scm :

diff --git a/gnu/packages/python.scm b/gnu/packages/python.scm
index b9d61dd..5bb6b27 100644
--- a/gnu/packages/python.scm
+++ b/gnu/packages/python.scm
@@ -9750,3 +9750,34 @@ characters, mouse support, and auto suggestions.")
       (native-inputs
        `(("python2-setuptools" ,python2-setuptools)
          ,@(package-native-inputs base))))))
+
+(define-public ptpython
+  (package
+    (name "ptpython")
+    (version "0.34")
+    (source
+      (origin
+        (method url-fetch)
+        (uri (pypi-uri "ptpython" version))
+        (sha256
+          (base32
+            "1mmbiyzf0n8hm7z2a562x7w5cbl6jc0zsk6vp40q1z4cyblv1k13"))))
+    (build-system python-build-system)
+    (inputs
+      `(("python-docopt" ,python-docopt)
+        ("python-jedi" ,python-jedi)
+        ("python-prompt-toolkit" ,python-prompt-toolkit)
+        ("python-pygments" ,python-pygments)
+        ("python-setuptools" ,python-setuptools)))
+    (home-page
+      "https://github.com/jonathanslenders/ptpython";)
+    (synopsis
+      "Python REPL build on top of prompt_toolkit")
+    (description
+      "Python REPL build on top of prompt_toolkit")
+    (license bsd-3)
+    (properties `((python2-variant . ,(delay ptpython-2))))
+))
+
+(define-public ptpython-2
+  (package-with-python2 ptpython))

What does it mean?