计算机代考程序代写 python COMP3430 / COMP8430 Data wrangling – cscodehelp代写
COMP3430 / COMP8430 Data wrangling
Lecture 4: Web scraping and geocoding of data (Lecturer: )
Lecture outline
● Web scraping
– Extracting data from the Web – Web scraping with Python
● Forward and reverse geocoding – Geocoding with Python
● Summary
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Extracting data from the Web
Collection and cleaning of Web data is required in many Web data-driven projects
Examples: price comparison, product review, real estate listings, stock market, web mashup, web data integration, and research
Websites contain unstructured and semi-structured data (HTML format)
Need to extract data into structured format from unstructured data
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Ways to extract data from the Web
APIs (Application Programming Interface)
Some websites provide APIs to extract (structured) data Examples: Twitter, Google, Facebook
Some APIs are restricted by what data is available to be extracted and how frequent
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Web scraping techniques
Cannot always rely on APIs to access Web data
Web scraping techniques are developed to transform unstructured data in the Web to structured data
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Is Web scraping legal?
Several case studies around the world
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In Feist Publications, Inc. v. Rural Telephone Service Co., the United States Supreme Court decided that scraping and republishing facts, such as telephone listings, is allowed.
See: http://caselaw.findlaw.com/us-supreme-court/499/340.html
Then, a similar case in Australia, Telstra Corporation Limited v. Phone Directories Company Pty Ltd, demonstrated that only data with an identifiable author can be copyrighted. See: http://www.austlii.edu.au/au/cases/cth/FCA/2010/44.html
Also, the European Union case, ofir.dk vs home.dk, concluded that regular crawling is permissible.
(Web scraping with Python, , 2015)
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Is Web scraping legal? (2)
Scraped data used for personal use does not involve legal issues
However, when re-publishing scraped data the type of data scraped is important:
When scraped data contains facts, such as price, location, and contact details, it can be republished
Data containing opinions and reviews cannot be republished for copyright reasons
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Understanding the target Web site
Check restrictions about crawling – robots.txt Examine sitemap file – links to all Web pages
Estimate Web site size – determines efficiency of crawling (Web site with million of pages requires distributed downloading)
Technology used – static/dynamic content and interactive determine how we crawl
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HTML tags in Web pages
: type declaration Contained between and
and links with
| a | b | |
| c | d |
where rows start with
with
Web scraping in Python
Python provides several libraries for scraping Web data mechanize, Scrapemark, Scrapy, regular expressions, lxml,
BeautifulSoup
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BeautifulSoup is a popular module being used for Web scraping, since it is easy and intuitive
Urllib2 standard Python module can be used in combination with BeautifulSoup for fetching Web pages
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Web scraping using BeautifulSoup
Import libraries
import urllib2
from bs4 import BeautifulSoup (https://www.crummy.com/software/BeautifulSoup/)
Specify the URL of page to be scraped
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page = urllib2.urlopen(‘http://www.akc.org/content/news/articles/labrador- retriever-is-once-again-americas-most-popular-dog/’)
soup = BeautifulSoup(page)
Structure of the page – print soup.prettify()
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Web scraping using BeautifulSoup (2)
soup.
soup.title –
soup.a – Home
soup.
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soup.title.string – u`Breaking News: Is Once Again America’s Most Popular Dog – American Kennel Club’
soup.a.string – u`Home’
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Find all links in a page
soup.a – provides only one link soup.find_all(“a”) – provides all links in the page
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Extract information to Pandas DataFrame
for row in my_table.find_all(‘tr’): col = row.find_all(‘td’) A.append(col[0].find(text=True)) B.append(col[3].find(text=True)) import pandas as pd
df = pd.DataFrame(A, columns=[‘Breed’]) df[‘2013’] = B
print df
Web scraping using BeautifulSoup (3)
Find tables
soup.find_all(‘table’) – extract information from all tables my_table = soup.find_all(‘table’)[0]
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Breed
2013
Retrievers (Labrador)
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Dogs
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Geocoding of data
Extracting geographical data (longitude and latitude) from address data (text)
Required in spatial data analysis and mining
Examples: Health epidemiological research such as finding local clusters of a disease, or analysis of geographical health issues, tracking, and marketing
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Ways of geocoding
Using geocoded reference dataset, addresses are matched to corresponding coordinates
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Several APIs are available to enable geocoding (Example: Google maps, Yahoo maps, Geocoder, etc.)
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APIs
Python
Python geocoding libraries (GeoPy, geocoder)
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Address text
Multi-variate attribute
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Unit number, street number, street name, suburb name, state, postcode, country
Example: “Unit 20”, “18 North road”, “Acton ACT 2602” Exact matching
Exact location by one-to-one match
Fuzzy matching
Possible matches (one-to-many) in neighboring region
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Forward and reverse geocoding
Geocoding is a two-way process: forward and reverse mapping
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Transforming address (text) data to geographical coordinates (longitude and latitude)
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Forward geocoding
Reverse geocoding
Transforming geographical coordinates to address data
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Forward geocoding
g = geocoder.google(“108 North road, Acton ACT 2602”) print g.latlng, g.city, g.state, g.street, g.country
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Reverse geocoding
g = geocoder.google([-35.27,149.12],method=”reverse”) print g.city, g.country
Geocoding with Python
Using Google’s geocoded data
import geocoder (https://pypi.python.org/pypi/geocoder)
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Summary
Extracting and transforming complex and semi-structured data, such as Web data and geographical data, is required in many applications
Web scraping transforms Web data (unstructured) to structured data
Geocoding transforms address data (text format) to geographical coordinates or vice versa
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