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---
title: Declare App Dependencies
owner: Tobias Fuhrimann
---
<strong><%= modified_date %></strong>
<%= vars.product_short %> recognizes an app as Python by the existence of a `requirements.txt` file in the root directory. For your own apps, you can create one by running `pip freeze > requirements.txt`.
The demo app you deployed already has a `requirements.txt`, and it looks something like this:
```txt
Flask>=0.11
```
The `requirements.txt` file determines the dependencies that should be installed with your application. When an app is deployed, <%= vars.product_short %> reads this file and installs the appropriate dependencies using the `pip install -r requirements.txt` command.
To do this locally, create a virtualenv for the app:
<pre class="terminal">
$ virtualenv venv
</pre>
Then, activate the virtualenv.
If you are using Windows, run this command:
<pre class="terminal">
> venv\Scripts\activate.bat
</pre>
If you are not using Windows, run this command:
<pre class="terminal">
$ source venv/bin/activate
</pre>
Now run this command in your local directory to install the dependencies, preparing your system for running the app locally:
<pre class="terminal">
$ pip install -r requirements.txt
Collecting Flask>=0.11 (from -r requirements.txt (line 1))
Downloading Flask-0.11.tar.gz (544kB)
...
</pre>
Once dependencies are installed, you will be ready to run your app locally.
Upon running `cf push`, we will also push our whole virtual environment folder `venv`. Since Cloud Foundry runs `pip install -r requirements.txt` anyways, this is redundant and should be omitted to save bandwidth and even more importantly: time. You can create a `.cfignore` file which works just like a `.gitignore` file and which tells Cloud Foundry what files should be excluded when pushing. Let's create a `.cfignore` file and exclude the `venv` folder:
```txt
venv
```
<p class="note">
<strong>Note</strong>: Cloud Foundry uses the latest version of Python 2 by default. To change that, create a <code>runtime.txt</code> file and put the respective version number into it (e.g. <code>python-3.6.1</code>). Be aware however, that Cloud Foundry only accepts the version specified <a href="https://github.com/cloudfoundry/python-buildpack/blob/master/manifest.yml" target="_blank">here</a>.
</p>
<div style="text-align:center;margin:3em;">
<a href="./run-locally.html" class="btn btn-primary">I've installed the App dependencies locally</a>
</div>