diff --git a/Library/Homebrew/utils/pypi.rb b/Library/Homebrew/utils/pypi.rb
index d85ecc7800..3193698b8a 100644
--- a/Library/Homebrew/utils/pypi.rb
+++ b/Library/Homebrew/utils/pypi.rb
@@ -286,6 +286,8 @@ module PyPI
   end
 
   def self.normalize_python_package(name)
+    # This normalization is defined in the PyPA packaging specifications;
+    # https://packaging.python.org/en/latest/specifications/name-normalization/#name-normalization
     name.gsub(/[-_.]+/, "-").downcase
   end