Introduction

Lately a friend and I have started designing a cryptocurrency trading bot. Our idea is to have a bot that takes into account both quantitative analysis, but also fundamental analysis. The quantitative analysis is quite straight forward, but how to design an algorithm that analyzes the public view of a good? One way we thought of is to use web scraping to get the source code of websites containing cryptocurrency information, discussions, etcetera. Then we could parse these sites in order to obtain just the paragraph text from the webpages. Websites of course use a lot of HTML, CSS, Javascript and so on to make the sites look fancy, but we do not care about that, we just want the data. One way to parse the source code is to use regular expressions (regex). When the text data is parsed from the source code it can be used in a natural language processing framework, e.g. in which a sentimental words-database could be used.

This blog post will focus on getting the source code from a website and the next post will be about parsing it into text.

Resources

Requets module’s documentation:

Quickstart Advanced usages

For all other documentation such as api doc etc see link

Sentdex:

Python 3 Programming Tutorial - urllib module

Related blogpost

Python 3 Programming Tutorial - Parsing Websites with re and urllib

Related blogpost

YouTube:

HTML Post/Get explained

Note: It is usually recommended to use the requests library instead of urllib because of the api differences. Note also that there is a urllib2 at present time. Although Sentdex videos are a bit outdated they do a great job in explaining the concepts.

Wikipedia:

Web scraping

Web crawler

Other resources:

GET and POST requests using Python Using the Requests Module in Python

What is Hypertext Transfer Protocol?

Hypertext Transfer Protocol also known as HTTP is a request-response protocol that enables users to communicate with servers, also known as client-server model. A web browser could be a user and a website that the browser is trying to access might be a server. When the user wants a response from the server two main methods are commonly used. GET requests data from the server and POST submits data to be processed to the server.

The request module

To install the requests module:

pip install requests

If this doesn’t work and you’re on Windows you can try to use the unofficial Windows binaries, here you can download the wheel (.whl) file and run pip install with it.

A simple example:

If you go to your browser and type in “https://www.google.com/search?q=python programming”, and press enter you will come to the Google search for “python programming”. Notice that the “space” has been changed to “%20” due to url encoding.

Now we will try to visit the same webpage using the requests module in Python.

import requests

# Example 1
q = 'python programming'

url = 'https://google.com/search'

response = requests.get(url + '?' + 'q=' + q)

print(response.text)

When printing the response from the server the source code of that website is simply shown.

The requests module has the ability to send data with the url to query. In example 2 where are trying to visit the same webpage but instead using the params argument of the requests.get() function.

# Example 2
# Passing params in URLs

url = 'https://google.com/search'

query = {'q': 'python programming'}

response = requests.get(url, params = query)

print(response.text)

The requests module automatically decodes the response with some educated guess depending on the HTTPS headers. From the requests module’s documentation, you can see that you can find and change the encoding.

>>> r.encoding
'utf-8'
>>> r.encoding = 'ISO-8859-1'

Note:

>>> url = 'https://api.github.com/some/endpoint'
>>> headers = {'user-agent': 'my-app/0.0.1'}

>>> r = requests.get(url, headers=headers)
import requests
req = requests.get('path/to/image.jpg', stream=True)
req.raise_for_status()
with open('Forest.jpg', 'wb') as fd:
    for chunk in req.iter_content(chunk_size=50000):
        print('Received a Chunk')
        fd.write(chunk)

A good quote from this blogpost:

“The concepts discussed in this tutorial should help you make basic requests to a server by passing specific headers, cookies, or query strings. This will be very handy when you are trying to scrape some webpages for information. Now, you should also be able to automatically download music files and wallpapers from different websites once you have figured out a pattern in the URLs.”