npm install エラー

truffle-hdwallet-provider を npmでインストールしようとすると、以下のようなエラー文が出ます。
python versionは、Python 2.7.13 :: Anaconda, Inc.を指定しています。
OSは、macOS, Sierra, 10.12.6です。
どのように対応すればよろしいでしょうか。ご教示よろしくお願いします。

$ npm install truffle-hdwallet-provider

scrypt@6.0.3 preinstall /Users/Documents/token/test/node_modules/scrypt
node node-scrypt-preinstall.js

scrypt@6.0.3 install /Users/Documents/token/test/node_modules/scrypt
node-gyp rebuild

gyp ERR! configure error
gyp ERR! stack Error: Can't find Python executable "/path/to/executable/python2.7", you can set the PYTHON env variable.
gyp ERR! stack at PythonFinder.failNoPython (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:483:19)
gyp ERR! stack at PythonFinder.<anonymous> (/usr/local/lib/node_modules/npm/node_modules/node-gyp/lib/configure.js:397:16)
gyp ERR! stack at F (/usr/local/lib/node_modules/npm/node_modules/which/which.js:68:16)
gyp ERR! stack at E (/usr/local/lib/node_modules/npm/node_modules/which/which.js:80:29)
gyp ERR! stack at /usr/local/lib/node_modules/npm/node_modules/which/which.js:89:16
gyp ERR! stack at /usr/local/lib/node_modules/npm/node_modules/which/node_modules/isexe/index.js:42:5
gyp ERR! stack at /usr/local/lib/node_modules/npm/node_modules/which/node_modules/isexe/mode.js:8:5
gyp ERR! stack at FSReqWrap.oncomplete (fs.js:166:21)
gyp ERR! System Darwin 16.7.0
gyp ERR! command "/usr/local/Cellar/node/9.5.0/bin/node" "/usr/local/lib/node_modules/npm/node_modules/node-gyp/bin/node-gyp.js" "rebuild"
gyp ERR! cwd /Users/Documents/token/test/node_modules/scrypt
gyp ERR! node -v v9.5.0
gyp ERR! node-gyp -v v3.6.2
gyp ERR! not ok
npm WARN test@1.0.0 No description
npm WARN test@1.0.0 No repository field.

npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! scrypt@6.0.3 install: node-gyp rebuild
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the scrypt@6.0.3 install script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.

npm ERR! A complete log of this run can be found in:
npm ERR! /Users/.npm/_logs/2018-03-01T06_05_21_944Z-debug.log