I corrected as suggested by cyril and efraim.
I hope it is acceptable now.
--- a/guix/build-system/python.scm
+++ b/guix/build-system/python.scm
@@ -41,12 +41,15 @@
;;
;; Code:
-(define (pypi-uri name version)
+(define* (pypi-uri name version
+ #:optional
+ (extension ".tar.gz"))
"Return a URI string for the Python package hosted on the Python
Package
-Index (PyPI) corresponding to NAME and VERSION."
+Index (PyPI) corresponding to NAME, VERSION and EXTENSION which
defaults to
+'.tar.gz' as most packages in the archive has this extension."