title: No module named flask, despite having it installed
body: <p>New to flask and python here. I've written a simple python app, but when i run <code>python app.py</code>, I get <code>ImportError: No module named flask</code>. </p>
  
  <p>app.py:</p>
  
  <pre><code>from flask import Flask
  
  app = Flask(__name__)
  def hello():
      return "Hello World!"
  
  if __name__ == "__main__":
      app.run(debug=True)
  </code></pre>
  
  <p>Versions:</p>
  
  <pre><code>Flask 1.0.2
  Python 2.7.10 (default, Oct  6 2017, 22:29:07)
  </code></pre>
  
  <p>EDIT:</p>
  
  <p>Typing pip freeze gives me:</p>
  
  <pre><code>altgraph==0.10.2
  bdist-mpkg==0.5.0
  bonjour-py==0.3
  click==6.7
  Flask==1.0.2
  itsdangerous==0.24
  Jinja2==2.10
  macholib==1.5.1
  MarkupSafe==1.0
  matplotlib==1.3.1
  modulegraph==0.10.4
  numpy==1.8.0rc1
  py2app==0.7.3
  pyobjc-core==2.5.1
  pyobjc-framework-Accounts==2.5.1
  pyobjc-framework-AddressBook==2.5.1
  pyobjc-framework-AppleScriptKit==2.5.1
  pyobjc-framework-AppleScriptObjC==2.5.1
  pyobjc-framework-Automator==2.5.1
  pyobjc-framework-CFNetwork==2.5.1
  pyobjc-framework-Cocoa==2.5.1
  pyobjc-framework-Collaboration==2.5.1
  pyobjc-framework-CoreData==2.5.1
  pyobjc-framework-CoreLocation==2.5.1
  pyobjc-framework-CoreText==2.5.1
  pyobjc-framework-DictionaryServices==2.5.1
  pyobjc-framework-EventKit==2.5.1
  pyobjc-framework-ExceptionHandling==2.5.1
  pyobjc-framework-FSEvents==2.5.1
  pyobjc-framework-InputMethodKit==2.5.1
  pyobjc-framework-InstallerPlugins==2.5.1
  pyobjc-framework-InstantMessage==2.5.1
  pyobjc-framework-LatentSemanticMapping==2.5.1
  pyobjc-framework-LaunchServices==2.5.1
  pyobjc-framework-Message==2.5.1
  pyobjc-framework-OpenDirectory==2.5.1
  pyobjc-framework-PreferencePanes==2.5.1
  pyobjc-framework-PubSub==2.5.1
  pyobjc-framework-QTKit==2.5.1
  pyobjc-framework-Quartz==2.5.1
  pyobjc-framework-ScreenSaver==2.5.1
  pyobjc-framework-ScriptingBridge==2.5.1
  pyobjc-framework-SearchKit==2.5.1
  pyobjc-framework-ServiceManagement==2.5.1
  pyobjc-framework-Social==2.5.1
  pyobjc-framework-SyncServices==2.5.1
  pyobjc-framework-SystemConfiguration==2.5.1
  pyobjc-framework-WebKit==2.5.1
  pyOpenSSL==0.13.1
  pyparsing==2.0.1
  python-dateutil==1.5
  pytz==2013.7
  scipy==0.13.0b1
  six==1.4.1
  vboxapi==1.0
  Werkzeug==0.14.1
  xattr==0.6.4
  zope.interface==4.1.1
  </code></pre>
  
  <p>Also, FWIW, if I run the app using python3 app.py, then it seems to work. Why does the command python though seem to point to  python version 2 instead of version 3? I'm wondering if I messed up installation somewhere.</p>
  
  <p>Any ideas greatly appreciated!</p>