title: After I installed flask by pip3, why can't I run it?
body: <p>After I installed flask by pip3, why can't I run it? How can I run it then? Thanks.</p>
  
  <pre><code>$ pip3 install Flask
  Collecting Flask
    Using cached https://files.pythonhosted.org/packages/7f/e7/08578774ed4536d3242b14dacb4696386634607af824ea997202cd0edb4b/Flask-1.0.2-py2.py3-none-any.whl
  Collecting Werkzeug&gt;=0.14 (from Flask)
    Using cached https://files.pythonhosted.org/packages/20/c4/12e3e56473e52375aa29c4764e70d1b8f3efa6682bef8d0aae04fe335243/Werkzeug-0.14.1-py2.py3-none-any.whl
  Collecting click&gt;=5.1 (from Flask)
    Using cached https://files.pythonhosted.org/packages/fa/37/45185cb5abbc30d7257104c434fe0b07e5a195a6847506c074527aa599ec/Click-7.0-py2.py3-none-any.whl
  Collecting Jinja2&gt;=2.10 (from Flask)
    Using cached https://files.pythonhosted.org/packages/7f/ff/ae64bacdfc95f27a016a7bed8e8686763ba4d277a78ca76f32659220a731/Jinja2-2.10-py2.py3-none-any.whl
  Collecting itsdangerous&gt;=0.24 (from Flask)
    Using cached https://files.pythonhosted.org/packages/76/ae/44b03b253d6fade317f32c24d100b3b35c2239807046a4c953c7b89fa49e/itsdangerous-1.1.0-py2.py3-none-any.whl
  Collecting MarkupSafe&gt;=0.23 (from Jinja2&gt;=2.10-&gt;Flask)
    Using cached https://files.pythonhosted.org/packages/08/04/f2191b50fb7f0712f03f064b71d8b4605190f2178ba02e975a87f7b89a0d/MarkupSafe-1.1.0-cp36-cp36m-manylinux1_x86_64.whl
  Installing collected packages: Werkzeug, click, MarkupSafe, Jinja2, itsdangerous, Flask
  Successfully installed Flask-1.0.2 Jinja2-2.10 MarkupSafe-1.1.0 Werkzeug-0.14.1 click-7.0 itsdangerous-1.1.0
  
  $ export FLASK_APP=hello.py
  
  $ python3 flask run
  python3: can't open file 'flask': [Errno 2] No such file or directory
  </code></pre>