title: Yarn add node-sass fails
body: <p>I'm following a tutorial, nothing fancy. I'm trying to add <code>node-sass</code> and <code>sass-loader</code> via <code>yarn add</code> .</p>
  
  <p>The command I run (I also tried the latest versions available) : </p>
  
  <pre><code>yarn add sass-loader@6.0.6 node-sass@4.5.3
  </code></pre>
  
  <p>This fails, and this is the output:</p>
  
  <pre><code>yarn add v1.9.2
  [1/4] Resolving packages...
  [2/4] Fetching packages...
  warning Pattern ["colors@latest"] is trying to unpack in the same destination "C
  :\\Users\\sagil\\AppData\\Local\\Yarn\\Cache\\v1\\npm-colors-1.1.2-168a4701756b6
  a7f51a12ce0c97bfa28c084ed63" as pattern ["colors@~1.1.2"]. This could result in
  non-deterministic behavior, skipping.
  warning Pattern ["object-assign@latest"] is trying to unpack in the same destina
  tion "C:\\Users\\sagil\\AppData\\Local\\Yarn\\Cache\\v1\\npm-object-assign-4.1.1
  -2109adc7965887cfc05cbbd442cac8bfbb360863" as pattern ["object-assign@^4.1.0","o
  bject-assign@^4.1.0","object-assign@^4.0.1","object-assign@^4.0.1","object-assig
  n@^4.0.1","object-assign@^4.0.1","object-assign@^4.1.0","object-assign@^4","obje
  ct-assign@^4.1.1","object-assign@^4.1.0","object-assign@^4.1.1","object-assign@^
  4.1.1","object-assign@^4.0.1","object-assign@^4.0.1","object-assign@^4.0.1","obj
  ect-assign@^4.0.1","object-assign@^4.0.1"]. This could result in non-determinist
  ic behavior, skipping.
  info fsevents@1.1.2: The platform "win32" is incompatible with this module.
  info "fsevents@1.1.2" is an optional dependency and failed compatibility check.
  Excluding it from installation.
  [3/4] Linking dependencies...
  warning " &gt; react-modal@2.2.2" has incorrect peer dependency "react@^0.14.0 || ^
  15.0.0".
  warning " &gt; react-modal@2.2.2" has incorrect peer dependency "react-dom@^0.14.0
  || ^15.0.0".
  [4/4] Building fresh packages...
  [-/2] ? waiting...
  error C:\Users\sagil\Desktop\ReactTutorial\indecision-app\node_modules\node-sass
  : Command failed.
  Exit code: 1
  Command: node scripts/build.js
  Arguments:
  Directory: C:\Users\sagil\Desktop\ReactTutorial\indecision-app\node_modules\node
  -sass
  Output:
  Building: C:\Program Files\nodejs\node.exe C:\Users\sagil\Desktop\ReactTutorial\
  indecision-app\node_modules\node-gyp\bin\node-gyp.js rebuild --verbose --libsass
  _ext= --libsass_cflags= --libsass_ldflags= --libsass_library=
  gyp info it worked if it ends with ok
  gyp verb cli [ 'C:\\Program Files\\nodejs\\node.exe',
  gyp verb cli   'C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_m
  odules\\node-gyp\\bin\\node-gyp.js',
  gyp verb cli   'rebuild',
  gyp verb cli   '--verbose',
  gyp verb cli   '--libsass_ext=',
  gyp verb cli   '--libsass_cflags=',
  gyp verb cli   '--libsass_ldflags=',
  gyp verb cli   '--libsass_library=' ]
  gyp info using node-gyp@3.8.0
  gyp info using node@10.7.0 | win32 | x64
  gyp verb command rebuild []
  gyp verb command clean []
  gyp verb clean removing "build" directory
  gyp verb command configure []
  gyp verb check python checking for Python executable "python2" in the PATH
  gyp verb `which` failed Error: not found: python2
  gyp verb `which` failed     at getNotFoundError (C:\Users\sagil\Desktop\ReactTut
  orial\indecision-app\node_modules\which\which.js:13:12)
  gyp verb `which` failed     at F (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:68:19)
  gyp verb `which` failed     at E (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:80:29)
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\which\which.js:89:16
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\index.js:42:5
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\windows.js:36:5
  gyp verb `which` failed     at FSReqWrap.oncomplete (fs.js:158:21)
  gyp verb `which` failed  python2 { Error: not found: python2
  gyp verb `which` failed     at getNotFoundError (C:\Users\sagil\Desktop\ReactTut
  orial\indecision-app\node_modules\which\which.js:13:12)
  gyp verb `which` failed     at F (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:68:19)
  gyp verb `which` failed     at E (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:80:29)
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\which\which.js:89:16
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\index.js:42:5
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\windows.js:36:5
  gyp verb `which` failed     at FSReqWrap.oncomplete (fs.js:158:21)
  gyp verb `which` failed   stack:
  gyp verb `which` failed    'Error: not found: python2\n    at getNotFoundError (
  C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_modules\\which\\w
  hich.js:13:12)\n    at F (C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-a
  pp\\node_modules\\which\\which.js:68:19)\n    at E (C:\\Users\\sagil\\Desktop\\R
  eactTutorial\\indecision-app\\node_modules\\which\\which.js:80:29)\n    at C:\\U
  sers\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_modules\\which\\which.
  js:89:16\n    at C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_
  modules\\isexe\\index.js:42:5\n    at C:\\Users\\sagil\\Desktop\\ReactTutorial\\
  indecision-app\\node_modules\\isexe\\windows.js:36:5\n    at FSReqWrap.oncomplet
  e (fs.js:158:21)',
  gyp verb `which` failed   code: 'ENOENT' }
  gyp verb check python checking for Python executable "python" in the PATH
  gyp verb `which` failed Error: not found: python
  gyp verb `which` failed     at getNotFoundError (C:\Users\sagil\Desktop\ReactTut
  orial\indecision-app\node_modules\which\which.js:13:12)
  gyp verb `which` failed     at F (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:68:19)
  gyp verb `which` failed     at E (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:80:29)
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\which\which.js:89:16
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\index.js:42:5
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\windows.js:36:5
  gyp verb `which` failed     at FSReqWrap.oncomplete (fs.js:158:21)
  gyp verb `which` failed  python { Error: not found: python
  gyp verb `which` failed     at getNotFoundError (C:\Users\sagil\Desktop\ReactTut
  orial\indecision-app\node_modules\which\which.js:13:12)
  gyp verb `which` failed     at F (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:68:19)
  gyp verb `which` failed     at E (C:\Users\sagil\Desktop\ReactTutorial\indecisio
  n-app\node_modules\which\which.js:80:29)
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\which\which.js:89:16
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\index.js:42:5
  gyp verb `which` failed     at C:\Users\sagil\Desktop\ReactTutorial\indecision-a
  pp\node_modules\isexe\windows.js:36:5
  gyp verb `which` failed     at FSReqWrap.oncomplete (fs.js:158:21)
  gyp verb `which` failed   stack:
  gyp verb `which` failed    'Error: not found: python\n    at getNotFoundError (C
  :\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_modules\\which\\wh
  ich.js:13:12)\n    at F (C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-ap
  p\\node_modules\\which\\which.js:68:19)\n    at E (C:\\Users\\sagil\\Desktop\\Re
  actTutorial\\indecision-app\\node_modules\\which\\which.js:80:29)\n    at C:\\Us
  ers\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_modules\\which\\which.j
  s:89:16\n    at C:\\Users\\sagil\\Desktop\\ReactTutorial\\indecision-app\\node_m
  odules\\isexe\\index.js:42:5\n    at C:\\Users\\sagil\\Desktop\\ReactTutorial\\i
  ndecision-app\\node_modules\\isexe\\windows.js:36:5\n    at FSReqWrap.oncomplete
   (fs.js:158:21)',
  gyp verb `which` failed   code: 'ENOENT' }
  gyp verb could not find "python". checking python launcher
  gyp verb could not find "python". guessing location
  gyp verb ensuring that file exists: C:\Python27\python.exe
  gyp ERR! configure error
  gyp ERR! stack Error: Can't find Python executable "python", you can set the PYT
  HON env variable.
  gyp ERR! stack     at PythonFinder.failNoPython (C:\Users\sagil\Desktop\ReactTut
  orial\indecision-app\node_modules\node-gyp\lib\configure.js:484:19)
  gyp ERR! stack     at PythonFinder.&lt;anonymous&gt; (C:\Users\sagil\Desktop\ReactTuto
  rial\indecision-app\node_modules\node-gyp\lib\configure.js:509:16)
  gyp ERR! stack     at C:\Users\sagil\Desktop\ReactTutorial\indecision-app\node_m
  odules\graceful-fs\polyfills.js:284:29
  gyp ERR! stack     at FSReqWrap.oncomplete (fs.js:158:21)
  gyp ERR! System Windows_NT 6.1.7601
  gyp ERR! command "C:\\Program Files\\nodejs\\node.exe" "C:\\Users\\sagil\\Deskto
  p\\ReactTutorial\\indecision-app\\node_modules\\node-gyp\\bin\\node-gyp.js" "reb
  uild" "--verbose" "--libsass_ext=" "--libsass_cflags=" "--libsass_ldflags=" "--l
  ibsass_library="
  gyp ERR! cwd C:\Users\sagil\Desktop\ReactTutorial\indecision-app\node_modules\no
  de-sass
  gyp ERR! node -v v10.7.0
  gyp ERR! node-gyp -v v3.8.0
  gyp ERR! not ok
  Build failed with error code: 1
  info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this comm
  </code></pre>
  
  <p>why is this happening?</p>