Na Internetu ima više informacija nego što ih bilo koji čovjek može upiti u svom životu. Ono što vam treba nije pristup tim informacijama, već prilagodljiv način prikupljanja, organiziranja i analize.

Treba vam struganje s weba.

Web struganje automatski izdvaja podatke i prikazuje ih u formatu koji lako možete razumjeti. U ovom ćemo se vodiču usredotočiti na njegove primjene na financijskom tržištu, ali struganje weba može se koristiti u najrazličitijim situacijama.

Ako ste zagriženi investitor, svakodnevno zatvaranje cijena može vam naštetiti, posebno kada se potrebne informacije pronađu na nekoliko web stranica. Olakšat ćemo ekstrakciju podataka izgradnjom web strugača za automatsko preuzimanje indeksa dionica s Interneta.

Početak rada

Koristit ćemo Python kao naš jezik za struganje, zajedno s jednostavnom i moćnom bibliotekom BeautifulSoup.

Za korisnike Maca Python je unaprijed instaliran u OS X. Otvorite Terminal i upišite python --version. Trebali biste vidjeti da je vaša verzija pythona 2.7.x.
Za korisnike Windowsa instalirajte Python putem službenog web mjesta.

Dalje moramo nabaviti knjižnicu BeautifulSoup pomoću pipalata za upravljanje paketima za Python.

U terminal unesite:

easy_install pip pip install BeautifulSoup4

Napomena : Ako ne uspijete izvršiti gornji redak za naredbe, pokušajte dodati sudoispred svakog retka.

Osnove

Prije nego što počnemo skakati u kod, shvatimo osnove HTML-a i neka pravila struganja.

HTML oznake

Ako već razumijete HTML oznake, slobodno preskočite ovaj dio.

 First Scraping 
 Hello World

Ovo je osnovna sintaksa HTML web stranice. Svaki služi blok unutar web stranice:

1 .: HTML dokumenti moraju započeti deklaracijom tipa.

2. HTML dokument nalazi se između i .

3. Meta i skripta deklaracije HTML dokumenta nalazi se između i .

4. vidljivi dio HTML dokumenta je između te oznake.

5. Naslovi naslova definirani su s

Original text

Contribute a better translation

kroz

oznake.

6. Odlomci su definirani s

Other useful tags include for hyperlinks,

for tables, for table rows, and

za stupce tablice.

Također, HTML oznake ponekad dolaze sa idili s classatributima. idAtribut određuje jedinstveni ID za HTML oznake i vrijednost mora biti jedinstvena unutar HTML dokumenta. classAtribut se koristi za definiranje jednakih stilova za HTML oznake s istom razredu. Te se ID-ove i klase možemo koristiti kako bi nam pomogli da lociramo podatke koje želimo.

Za više informacija o HTML oznakama, id-u i klasi, pogledajte Vodiče za W3Schools.

Pravila struganja

Trebali biste provjeriti Uvjete i odredbe web stranice prije nego što je sastružete. Pazljivo pročitajte izjave o legalnoj upotrebi podataka. Podaci koje stružete obično se ne smiju koristiti u komercijalne svrhe.
Ne zahtijevajte podatke s web mjesta preagresivno sa svojim programom (poznatim i kao neželjena pošta), jer to može pokvariti web mjesto. Pobrinite se da se vaš program ponaša na razuman način (tj. Ponaša se poput čovjeka). Jedan zahtjev za jednom web stranicom u sekundi dobra je praksa.
Izgled web stranice može se s vremena na vrijeme promijeniti, pa svakako posjetite web mjesto i po potrebi prepišite kôd

Pregled stranice

Uzmimo za primjer jednu stranicu s web stranice Bloomberg Quote.

Kao netko tko prati burzu, željeli bismo s ove stranice dobiti naziv indeksa (S&P 500) i njegovu cijenu. Prvo kliknite desnu tipku miša i otvorite inspektor preglednika da biste pregledali web stranicu.

Pokušajte zadržati pokazivač na cijeni i trebali biste vidjeti plavi okvir koji ga okružuje. Ako ga kliknete, povezani će se HTML odabrati na konzoli preglednika.

From the result, we can see that the price is inside a few levels of HTML tags, which is → → .

Similarly, if you hover and click the name “S&P 500 Index”, it is inside and

.
Now we know the unique location of our data with the help of `class` tags.

Jump into the Code

Now that we know where our data is, we can start coding our web scraper. Open your text editor now!

First, we need to import all the libraries that we are going to use.

# import libraries import urllib2 from bs4 import BeautifulSoup

Next, declare a variable for the url of the page.

# specify the url quote_page = ‘//www.bloomberg.com/quote/SPX:IND'

Then, make use of the Python urllib2 to get the HTML page of the url declared.

# query the website and return the html to the variable ‘page’ page = urllib2.urlopen(quote_page)

Finally, parse the page into BeautifulSoup format so we can use BeautifulSoup to work on it.

# parse the html using beautiful soup and store in variable `soup` soup = BeautifulSoup(page, ‘html.parser’)

Now we have a variable, soup, containing the HTML of the page. Here’s where we can start coding the part that extracts the data.

Remember the unique layers of our data? BeautifulSoup can help us get into these layers and extract the content with find(). In this case, since the HTML class name is unique on this page, we can simply query .

# Take out the of name and get its value name_box = soup.find(‘h1’, attrs={‘class’: ‘name’})

After we have the tag, we can get the data by getting its text.

name = name_box.text.strip() # strip() is used to remove starting and trailing print name

Similarly, we can get the price too.

# get the index price price_box = soup.find(‘div’, attrs={‘class’:’price’}) price = price_box.text print price

When you run the program, you should be able to see that it prints out the current price of the S&P 500 Index.

Export to Excel CSV

Now that we have the data, it is time to save it. The Excel Comma Separated Format is a nice choice. It can be opened in Excel so you can see the data and process it easily.

But first, we have to import the Python csv module and the datetime module to get the record date. Insert these lines to your code in the import section.

import csv from datetime import datetime

At the bottom of your code, add the code for writing data to a csv file.

# open a csv file with append, so old data will not be erased with open(‘index.csv’, ‘a’) as csv_file: writer = csv.writer(csv_file) writer.writerow([name, price, datetime.now()])

Now if you run your program, you should able to export an index.csv file, which you can then open with Excel, where you should see a line of data.

So if you run this program everyday, you will be able to easily get the S&P 500 Index price without rummaging through the website!

Going Further (Advanced uses)

Multiple Indices

So scraping one index is not enough for you, right? We can try to extract multiple indices at the same time.

First, modify the quote_page into an array of URLs.

quote_page = [‘//www.bloomberg.com/quote/SPX:IND', ‘//www.bloomberg.com/quote/CCMP:IND']

Then we change the data extraction code into a for loop, which will process the URLs one by one and store all the data into a variable data in tuples.

# for loop data = [] for pg in quote_page: # query the website and return the html to the variable ‘page’ page = urllib2.urlopen(pg) # parse the html using beautiful soap and store in variable `soup` soup = BeautifulSoup(page, ‘html.parser’) # Take out the of name and get its value name_box = soup.find(‘h1’, attrs={‘class’: ‘name’}) name = name_box.text.strip() # strip() is used to remove starting and trailing # get the index price price_box = soup.find(‘div’, attrs={‘class’:’price’}) price = price_box.text # save the data in tuple data.append((name, price))

Also, modify the saving section to save data row by row.

# open a csv file with append, so old data will not be erased with open(‘index.csv’, ‘a’) as csv_file: writer = csv.writer(csv_file) # The for loop for name, price in data: writer.writerow([name, price, datetime.now()])

Rerun the program and you should be able to extract two indices at the same time!

Advanced Scraping Techniques

BeautifulSoup is simple and great for small-scale web scraping. But if you are interested in scraping data at a larger scale, you should consider using these other alternatives:

Scrapy, a powerful python scraping framework
Try to integrate your code with some public APIs. The efficiency of data retrieval is much higher than scraping webpages. For example, take a look at Facebook Graph API, which can help you get hidden data which is not shown on Facebook webpages.
Consider using a database backend like MySQL to store your data when it gets too large.

Adopt the DRY Method

DRY stands for “Don’t Repeat Yourself”, try to automate your everyday tasks like this person. Some other fun projects to consider might be keeping track of your Facebook friends’ active time (with their consent of course), or grabbing a list of topics in a forum and trying out natural language processing (which is a hot topic for Artificial Intelligence right now)!

If you have any questions, please feel free to leave a comment below.

References

//www.gregreda.com/2013/03/03/web-scraping-101-with-python/

//www.analyticsvidhya.com/blog/2015/10/beginner-guide-web-scraping-beautiful-soup-python/

This article was originally published on Altitude Labs’ blog and was written by our software engineer, Leonard Mok. Altitude Labs is a software agency that specializes in personalized, mobile-first React apps.