title: Heroku not install anything listed in requirements.txt
body: <blockquote>
    <p>I'm not a native English user and I'm a Python beginner. Please excuse any grammar error.</p>
  </blockquote>
  
  <p>I am having trouble pushing a simple flask project to Heroku.</p>
  
  <p><a href="https://github.com/TomazWang/tomazwang-flask-site" rel="nofollow noreferrer">The project is on Github</a></p>
  
  <p>First, my requirements.txt is like below:
  </p>
  
  <pre><code>click==6.7
  Flask==0.12.2
  gunicorn==19.7.1
  itsdangerous==0.24
  Jinja2==2.9.6
  MarkupSafe==1.0
  Werkzeug==0.12.2
  </code></pre>
  
  
  
  <p>However, Heroku return the error code</p>
  
  <pre class="lang-html prettyprint-override"><code>remote:
  remote: -----&gt; Python app detected
  remote: -----&gt; Installing python-3.6.1
  remote: -----&gt; Installing pip
  remote: -----&gt; Installing requirements with pip
  remote:        Failed to import the site module
  remote:        Traceback (most recent call last):
  remote:          File "/tmp/build_b20ee4f76cefa5dfa20998cf2e3692c7/site.py", line 1, in &lt;module&gt;
  remote:            from flask import Flask, render_template
  remote:        ModuleNotFoundError: No module named 'flask'
  remote:  !     Push rejected, failed to compile Python app.
  remote:
  remote:  !     Push failed
  </code></pre>
  
  
  
  <p>I've run the project successfully on venv and global environment, and run the <code>./venv/bin/pip freeze</code> command several times both in venv activate and deactive mode. <code>./venv/bin/pip -r install requirements.txt</code> in a new venv can work just fine.</p>
  
  <p>I'm not sure which step I've done wrong. Any advise will help. </p>
  
  <p>Thanks.              </p>