• Gets a street address from the command line arguments or clipboard.

• Opens the web browser to the Google Maps page for the address.

• Read the command line arguments from sys.argv.

• Read the clipboard contents.

• Call the webbrowser.open() function to open the web browser.

Based on the instructions in Appendix B, set up mapIt.py so that when you run it from the command line, like so...

C:\> mapit 870 Valencia St, San Francisco, CA 94110

... the script will use the command line arguments instead of the clipboard. If there are no command line arguments, then the program will know to use the contents of the clipboard.

First you need to figure out what URL to use for a given street address. When you load http://maps.google.com/ in the browser and search for an address, the URL in the address bar looks something like this: https://www.google.com/maps/place/870+Valencia+St/@37.7590311,-122.4215096,17z/data=!3m1!4b1!4m2!3m1!1s0x808f7e3dadc07a37:0xc86b0b2bb93b73d8.

The address is in the URL, but there’s a lot of additional text there as well. Websites often add extra data to URLs to help track visitors or customize sites. But if you try just going to https://www.google.com/maps/place/870+Valencia+St+San+Francisco+CA/, you’ll find that it still brings up the correct page. So your program can be set to open a web browser to 'https://www.google.com/maps/place/your_address_string' (where your_address_string is the address you want to map).

Make your code look like this:

#! python3
# mapIt.py - Launches a map in the browser using an address from the
# command line or clipboard.

import webbrowser, sys
if len(sys.argv) > 1:
    # Get address from command line.
    address = ' '.join(sys.argv[1:])

# TODO: Get address from clipboard.

After the program’s #! shebang line, you need to import the webbrowser module for launching the browser and import the sys module for reading the potential command line arguments. The sys.argv variable stores a list of the program’s filename and command line arguments. If this list has more than just the filename in it, then len(sys.argv) evaluates to an integer greater than 1, meaning that command line arguments have indeed been provided.

Command line arguments are usually separated by spaces, but in this case, you want to interpret all of the arguments as a single string. Since sys.argv is a list of strings, you can pass it to the join() method, which returns a single string value. You don’t want the program name in this string, so instead of sys.argv, you should pass sys.argv[1:] to chop off the first element of the array. The final string that this expression evaluates to is stored in the address variable.

If you run the program by entering this into the command line...

mapit 870 Valencia St, San Francisco, CA 94110

... the sys.argv variable will contain this list value:

['mapIt.py', '870', 'Valencia', 'St, ', 'San', 'Francisco, ', 'CA', '94110']

The address variable will contain the string '870 Valencia St, San Francisco, CA 94110'.

Make your code look like the following:

#! python3
# mapIt.py - Launches a map in the browser using an address from the
# command line or clipboard.

import webbrowser, sys, pyperclip
if len(sys.argv) > 1:
    # Get address from command line.
    address = ' '.join(sys.argv[1:])
else:
    # Get address from clipboard.
    address = pyperclip.paste()

webbrowser.open('https://www.google.com/maps/place/' + address)

If there are no command line arguments, the program will assume the address is stored on the clipboard. You can get the clipboard content with pyperclip.paste() and store it in a variable named address. Finally, to launch a web browser with the Google Maps URL, call webbrowser.open().

While some of the programs you write will perform huge tasks that save you hours, it can be just as satisfying to use a program that conveniently saves you a few seconds each time you perform a common task, such as getting a map of an address. Table 11-1 compares the steps needed to display a map with and without mapIt.py.

See how mapIt.py makes this task less tedious?

As long as you have a URL, the webbrowser module lets users cut out the step of opening the browser and directing themselves to a website. Other programs could use this functionality to do the following:

The requests.get() function takes a string of a URL to download. By calling type() on requests.get()’s return value, you can see that it returns a Response object, which contains the response that the web server gave for your request. I’ll explain the Response object in more detail later, but for now, enter the following into the interactive shell while your computer is connected to the Internet:

   >>> import requests
   >>> res = requests.get('https://automatetheboringstuff.com/files/rj.txt')
   >>> type(res)
   <class 'requests.models.Response'>
❶ >>> res.status_code == requests.codes.ok
   True
   >>> len(res.text)
   178981
   >>> print(res.text[:250])
   The Project Gutenberg EBook of Romeo and Juliet, by William Shakespeare

   This eBook is for the use of anyone anywhere at no cost and with
   almost no restrictions whatsoever. You may copy it, give it away or
   re-use it under the terms of the Proje

The URL goes to a text web page for the entire play of Romeo and Juliet. You can tell that the request for this web page succeeded by checking the status_code attribute of the Response object. If it is equal to the value of requests.codes.ok, then everything went fine ❶. (Incidentally, the status code for “OK” in the HTTP protocol is 200. You may already be familiar with the 404 status code for “Not Found.”)

If the request succeeded, the downloaded web page is stored as a string in the Response object’s text variable. This variable holds a large string of the entire play; the call to len(res.text) shows you that it is more than 178,000 characters long. Finally, calling print(res.text[:250]) displays only the first 250 characters.

As you’ve seen, the Response object has a status_code attribute that can be checked against requests.codes.ok to see whether the download succeeded. A simpler way to check for success is to call the raise_for_status() method on the Response object. This will raise an exception if there was an error downloading the file and will do nothing if the download succeeded. Enter the following into the interactive shell:

>>> res = requests.get('http://inventwithpython.com/page_that_does_not_exist')
>>> res.raise_for_status()
Traceback (most recent call last):
  File "<pyshell#138>", line 1, in <module>
    res.raise_for_status()
  File "C:\Python34\lib\site-packages\requests\models.py", line 773, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found

The raise_for_status() method is a good way to ensure that a program halts if a bad download occurs. This is a good thing: You want your program to stop as soon as some unexpected error happens. If a failed download isn’t a deal breaker for your program, you can wrap the raise_for_status() line with try and except statements to handle this error case without crashing.

import requests
res = requests.get('http://inventwithpython.com/page_that_does_not_exist')
try:
    res.raise_for_status()
except Exception as exc:
    print('There was a problem: %s' % (exc))

This raise_for_status() method call causes the program to output the following:

There was a problem: 404 Client Error: Not Found

Always call raise_for_status() after calling requests.get(). You want to be sure that the download has actually worked before your program continues.

1. Call requests.get() to download the file.

2. Call open() with 'wb' to create a new file in write binary mode.

3. Loop over the Response object’s iter_content() method.

4. Call write() on each iteration to write the content to the file.

5. Call close() to close the file.

Hypertext Markup Language (HTML) is the format that web pages are written in. This chapter assumes you have some basic experience with HTML, but if you need a beginner tutorial, I suggest one of the following sites:

In case it’s been a while since you’ve looked at any HTML, here’s a quick overview of the basics. An HTML file is a plaintext file with the .html file extension. The text in these files is surrounded by tags, which are words enclosed in angle brackets. The tags tell the browser how to format the web page. A starting tag and closing tag can enclose some text to form an element. The text (or inner HTML) is the content between the starting and closing tags. For example, the following HTML will display Hello world! in the browser, with Hello in bold:

<strong>Hello</strong> world!

This HTML will look like Figure 11-1 in a browser.

Figure 11-1. Hello world! rendered in the browser

The opening <strong> tag says that the enclosed text will appear in bold. The closing </strong> tags tells the browser where the end of the bold text is.

There are many different tags in HTML. Some of these tags have extra properties in the form of attributes within the angle brackets. For example, the <a> tag encloses text that should be a link. The URL that the text links to is determined by the href attribute. Here’s an example:

Al's free <a href="http://inventwithpython.com">Python books</a>.

This HTML will look like Figure 11-2 in a browser.

Figure 11-2. The link rendered in the browser

Some elements have an id attribute that is used to uniquely identify the element in the page. You will often instruct your programs to seek out an element by its id attribute, so figuring out an element’s id attribute using the browser’s developer tools is a common task in writing web scraping programs.

You’ll need to look at the HTML source of the web pages that your programs will work with. To do this, right-click (or CTRL-click on OS X) any web page in your web browser, and select View Source or View page source to see the HTML text of the page (see Figure 11-3). This is the text your browser actually receives. The browser knows how to display, or render, the web page from this HTML.

Figure 11-3. Viewing the source of a web page

I highly recommend viewing the source HTML of some of your favorite sites. It’s fine if you don’t fully understand what you are seeing when you look at the source. You won’t need HTML mastery to write simple web scraping programs—after all, you won’t be writing your own websites. You just need enough knowledge to pick out data from an existing site.

In addition to viewing a web page’s source, you can look through a page’s HTML using your browser’s developer tools. In Chrome and Internet Explorer for Windows, the developer tools are already installed, and you can press F12 to make them appear (see Figure 11-4). Pressing F12 again will make the developer tools disappear. In Chrome, you can also bring up the developer tools by selecting View▸Developer▸Developer Tools. In OS X, pressing -OPTION-I will open Chrome’s Developer Tools.

Figure 11-4. The Developer Tools window in the Chrome browser

In Firefox, you can bring up the Web Developer Tools Inspector by pressing CTRL-SHIFT-C on Windows and Linux or by pressing ⌘-OPTION-C on OS X. The layout is almost identical to Chrome’s developer tools.

In Safari, open the Preferences window, and on the Advanced pane check the Show Develop menu in the menu bar option. After it has been enabled, you can bring up the developer tools by pressing -OPTION-I.

After enabling or installing the developer tools in your browser, you can right-click any part of the web page and select Inspect Element from the context menu to bring up the HTML responsible for that part of the page. This will be helpful when you begin to parse HTML for your web scraping programs.

Once your program has downloaded a web page using the requests module, you will have the page’s HTML content as a single string value. Now you need to figure out which part of the HTML corresponds to the information on the web page you’re interested in.

This is where the browser’s developer tools can help. Say you want to write a program to pull weather forecast data from http://weather.gov/. Before writing any code, do a little research. If you visit the site and search for the 94105 ZIP code, the site will take you to a page showing the forecast for that area.

What if you’re interested in scraping the temperature information for that ZIP code? Right-click where it is on the page (or CONTROL-click on OS X) and select Inspect Element from the context menu that appears. This will bring up the Developer Tools window, which shows you the HTML that produces this particular part of the web page. Figure 11-5 shows the developer tools open to the HTML of the temperature.

Figure 11-5. Inspecting the element that holds the temperature text with the developer tools

From the developer tools, you can see that the HTML responsible for the temperature part of the web page is <p class="myforecast-current -lrg">59°F</p>. This is exactly what you were looking for! It seems that the temperature information is contained inside a <p> element with the myforecast-current-lrg class. Now that you know what you’re looking for, the BeautifulSoup module will help you find it in the string.

The bs4.BeautifulSoup() function needs to be called with a string containing the HTML it will parse. The bs4.BeautifulSoup() function returns is a BeautifulSoup object. Enter the following into the interactive shell while your computer is connected to the Internet:

>>> import requests, bs4
>>> res = requests.get('http://nostarch.com')
>>> res.raise_for_status()
>>> noStarchSoup = bs4.BeautifulSoup(res.text)
>>> type(noStarchSoup)
<class 'bs4.BeautifulSoup'>

This code uses requests.get() to download the main page from the No Starch Press website and then passes the text attribute of the response to bs4.BeautifulSoup(). The BeautifulSoup object that it returns is stored in a variable named noStarchSoup.

You can also load an HTML file from your hard drive by passing a File object to bs4.BeautifulSoup(). Enter the following into the interactive shell (make sure the example.html file is in the working directory):

>>> exampleFile = open('example.html')
>>> exampleSoup = bs4.BeautifulSoup(exampleFile)
>>> type(exampleSoup)
<class 'bs4.BeautifulSoup'>

Once you have a BeautifulSoup object, you can use its methods to locate specific parts of an HTML document.

You can retrieve a web page element from a BeautifulSoup object by calling the select()method and passing a string of a CSS selector for the element you are looking for. Selectors are like regular expressions: They specify a pattern to look for, in this case, in HTML pages instead of general text strings.

A full discussion of CSS selector syntax is beyond the scope of this book (there’s a good selector tutorial in the resources at http://nostarch.com/automatestuff/), but here’s a short introduction to selectors. Table 11-2 shows examples of the most common CSS selector patterns.

The various selector patterns can be combined to make sophisticated matches. For example, soup.select('p #author') will match any element that has an id attribute of author, as long as it is also inside a <p> element.

The select() method will return a list of Tag objects, which is how Beautiful Soup represents an HTML element. The list will contain one Tag object for every match in the BeautifulSoup object’s HTML. Tag values can be passed to the str() function to show the HTML tags they represent. Tag values also have an attrs attribute that shows all the HTML attributes of the tag as a dictionary. Using the example.html file from earlier, enter the following into the interactive shell:

>>> import bs4
>>> exampleFile = open('example.html')
>>> exampleSoup = bs4.BeautifulSoup(exampleFile.read())
>>> elems = exampleSoup.select('#author')
>>> type(elems)
<class 'list'>
>>> len(elems)
1
>>> type(elems[0])
<class 'bs4.element.Tag'>
>>> elems[0].getText()
'Al Sweigart'
>>> str(elems[0])
'<span id="author">Al Sweigart</span>'
>>> elems[0].attrs
{'id': 'author'}

This code will pull the element with id="author" out of our example HTML. We use select('#author') to return a list of all the elements with id="author". We store this list of Tag objects in the variable elems, and len(elems) tells us there is one Tag object in the list; there was one match. Calling getText() on the element returns the element’s text, or inner HTML. The text of an element is the content between the opening and closing tags: in this case, 'Al Sweigart'.

Passing the element to str() returns a string with the starting and closing tags and the element’s text. Finally, attrs gives us a dictionary with the element’s attribute, 'id', and the value of the id attribute, 'author'.

You can also pull all the <p> elements from the BeautifulSoup object. Enter this into the interactive shell:

>>> pElems = exampleSoup.select('p')
>>> str(pElems[0])
'<p>Download my <strong>Python</strong> book from <a href="http://
inventwithpython.com">my website</a>.</p>'
>>> pElems[0].getText()
'Download my Python book from my website.'
>>> str(pElems[1])
'<p class="slogan">Learn Python the easy way!</p>'
>>> pElems[1].getText()
'Learn Python the easy way!'
>>> str(pElems[2])
'<p>By <span id="author">Al Sweigart</span></p>'
>>> pElems[2].getText()
'By Al Sweigart'

This time, select() gives us a list of three matches, which we store in pElems. Using str() on pElems[0], pElems[1], and pElems[2] shows you each element as a string, and using getText() on each element shows you its text.

The get() method for Tag objects makes it simple to access attribute values from an element. The method is passed a string of an attribute name and returns that attribute’s value. Using example.html, enter the following into the interactive shell:

>>> import bs4
>>> soup = bs4.BeautifulSoup(open('example.html'))
>>> spanElem = soup.select('span')[0]
>>> str(spanElem)
'<span id="author">Al Sweigart</span>'
>>> spanElem.get('id')
'author'
>>> spanElem.get('some_nonexistent_addr') == None
True
>>> spanElem.attrs
{'id': 'author'}

Here we use select() to find any <span> elements and then store the first matched element in spanElem. Passing the attribute name 'id' to get() returns the attribute’s value, 'author'.

• Gets search keywords from the command line arguments.

• Retrieves the search results page.

• Opens a browser tab for each result.

• Read the command line arguments from sys.argv.

• Fetch the search result page with the requests module.

• Find the links to each search result.

• Call the webbrowser.open() function to open the web browser.

Before coding anything, you first need to know the URL of the search result page. By looking at the browser’s address bar after doing a Google search, you can see that the result page has a URL like https://www.google.com/search?q=SEARCH_TERM_HERE. The requests module can download this page and then you can use Beautiful Soup to find the search result links in the HTML. Finally, you’ll use the webbrowser module to open those links in browser tabs.

Make your code look like the following:

#! python3
# lucky.py - Opens several Google search results.

import requests, sys, webbrowser, bs4

print('Googling...') # display text while downloading the Google page
res = requests.get('http://google.com/search?q=' + ' '.join(sys.argv[1:]))
res.raise_for_status()

# TODO: Retrieve top search result links.

# TODO: Open a browser tab for each result.

The user will specify the search terms using command line arguments when they launch the program. These arguments will be stored as strings in a list in sys.argv.

Now you need to use Beautiful Soup to extract the top search result links from your downloaded HTML. But how do you figure out the right selector for the job? For example, you can’t just search for all <a> tags, because there are lots of links you don’t care about in the HTML. Instead, you must inspect the search result page with the browser’s developer tools to try to find a selector that will pick out only the links you want.

After doing a Google search for Beautiful Soup, you can open the browser’s developer tools and inspect some of the link elements on the page. They look incredibly complicated, something like this: <a href="/url?sa =t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8& amp;ved=0CCgQFjAA&url=http%3A%2F%2Fwww.crummy.com%2Fsoftware%2FBeautifulSoup %2F&ei=LHBVU_XDD9KVyAShmYDwCw&usg=AFQjCNHAxwplurFOBqg5cehWQEVKi-TuLQ&a mp;sig2=sdZu6WVlBlVSDrwhtworMA" onmousedown="return rwt(this,'','','','1','AFQ jCNHAxwplurFOBqg5cehWQEVKi-TuLQ','sdZu6WVlBlVSDrwhtworMA','0CCgQFjAA','','',ev ent)" data-href="http://www.crummy.com/software/BeautifulSoup/"><em>Beautiful Soup</em>: We called him Tortoise because he taught us.</a>.

It doesn’t matter that the element looks incredibly complicated. You just need to find the pattern that all the search result links have. But this <a> element doesn’t have anything that easily distinguishes it from the nonsearch result <a> elements on the page.

Make your code look like the following:

#! python3
# lucky.py - Opens several google search results.

import requests, sys, webbrowser, bs4

--snip--

# Retrieve top search result links.
soup = bs4.BeautifulSoup(res.text)

# Open a browser tab for each result.
linkElems = soup.select('.r a')

If you look up a little from the <a> element, though, there is an element like this: <h3 class="r">. Looking through the rest of the HTML source, it looks like the r class is used only for search result links. You don’t have to know what the CSS class r is or what it does. You’re just going to use it as a marker for the <a> element you are looking for. You can create a BeautifulSoup object from the downloaded page’s HTML text and then use the selector '.r a' to find all <a> elements that are within an element that has the r CSS class.

Finally, we’ll tell the program to open web browser tabs for our results. Add the following to the end of your program:

#! python3
# lucky.py - Opens several google search results.

import requests, sys, webbrowser, bs4

--snip--

# Open a browser tab for each result.
linkElems = soup.select('.r a')
numOpen = min(5, len(linkElems))
for i in range(numOpen):
    webbrowser.open('http://google.com' + linkElems[i].get('href'))

By default, you open the first five search results in new tabs using the webbrowser module. However, the user may have searched for something that turned up fewer than five results. The soup.select() call returns a list of all the elements that matched your '.r a' selector, so the number of tabs you want to open is either 5 or the length of this list (whichever is smaller).

The built-in Python function min() returns the smallest of the integer or float arguments it is passed. (There is also a built-in max() function that returns the largest argument it is passed.) You can use min() to find out whether there are fewer than five links in the list and store the number of links to open in a variable named numOpen. Then you can run through a for loop by calling range(numOpen).

On each iteration of the loop, you use webbrowser.open() to open a new tab in the web browser. Note that the href attribute’s value in the returned <a> elements do not have the initial http://google.com part, so you have to concatenate that to the href attribute’s string value.

Now you can instantly open the first five Google results for, say, Python programming tutorials by running lucky python programming tutorials on the command line! (See Appendix B for how to easily run programs on your operating system.)

The benefit of tabbed browsing is that you can easily open links in new tabs to peruse later. A program that automatically opens several links at once can be a nice shortcut to do the following:

• Loads the XKCD home page.

• Saves the comic image on that page.

• Follows the Previous Comic link.

• Repeats until it reaches the first comic.

• Download pages with the requests module.

• Find the URL of the comic image for a page using Beautiful Soup.

• Download and save the comic image to the hard drive with iter_content().

• Find the URL of the Previous Comic link, and repeat.

If you open the browser’s developer tools and inspect the elements on the page, you’ll find the following:

Make your code look like the following:

#! python3
# downloadXkcd.py - Downloads every single XKCD comic.

import requests, os, bs4

url = 'http://xkcd.com'              # starting url
os.makedirs('xkcd', exist_ok=True)   # store comics in ./xkcd
while not url.endswith('#'):
    # TODO: Download the page.

    # TODO: Find the URL of the comic image.

    # TODO: Download the image.

    # TODO: Save the image to ./xkcd.

    # TODO: Get the Prev button's url.

print('Done.')

You’ll have a url variable that starts with the value 'http://xkcd.com' and repeatedly update it (in a for loop) with the URL of the current page’s Prev link. At every step in the loop, you’ll download the comic at url. You’ll know to end the loop when url ends with '#'.

You will download the image files to a folder in the current working directory named xkcd. The call os.makedirs() ensures that this folder exists, and the exist_ok=True keyword argument prevents the function from throwing an exception if this folder already exists. The rest of the code is just comments that outline the rest of your program.

Let’s implement the code for downloading the page. Make your code look like the following:

#! python3
# downloadXkcd.py - Downloads every single XKCD comic.

import requests, os, bs4

url = 'http://xkcd.com'              # starting url
os.makedirs('xkcd', exist_ok=True)   # store comics in ./xkcd
while not url.endswith('#'):
    # Download the page.
    print('Downloading page %s...' % url)
    res = requests.get(url)
    res.raise_for_status()

    soup = bs4.BeautifulSoup(res.text)

    # TODO: Find the URL of the comic image.

    # TODO: Download the image.

    # TODO: Save the image to ./xkcd.

    # TODO: Get the Prev button's url.

print('Done.')

First, print url so that the user knows which URL the program is about to download; then use the requests module’s request.get() function to download it. As always, you immediately call the Response object’s raise_for_status() method to throw an exception and end the program if something went wrong with the download. Otherwise, you create a BeautifulSoup object from the text of the downloaded page.

Make your code look like the following:

#! python3
# downloadXkcd.py - Downloads every single XKCD comic.

import requests, os, bs4

--snip--

    # Find the URL of the comic image.
    comicElem = soup.select('#comic img')
    if comicElem == []:
         print('Could not find comic image.')
    else:
         try:
             comicUrl = 'http:' + comicElem[0].get('src')
             # Download the image.
             print('Downloading image %s...' % (comicUrl))
             res = requests.get(comicUrl)
             res.raise_for_status()
         except requests.exceptions.MissingSchema:
             # skip this comic
             prevLink = soup.select('a[rel="prev"]')[0]
             url = 'http://xkcd.com' + prevLink.get('href')
             continue


    # TODO: Save the image to ./xkcd.

    # TODO: Get the Prev button's url.

print('Done.')

From inspecting the XKCD home page with your developer tools, you know that the <img> element for the comic image is inside a <div> element with the id attribute set to comic, so the selector '#comic img' will get you the correct <img> element from the BeautifulSoup object.

A few XKCD pages have special content that isn’t a simple image file. That’s fine; you’ll just skip those. If your selector doesn’t find any elements, then soup.select('#comic img') will return a blank list. When that happens, the program can just print an error message and move on without downloading the image.

Otherwise, the selector will return a list containing one <img> element. You can get the src attribute from this <img> element and pass it to requests.get() to download the comic’s image file.

Make your code look like the following:

#! python3
# downloadXkcd.py - Downloads every single XKCD comic.

import requests, os, bs4

--snip--

        # Save the image to ./xkcd.
        imageFile = open(os.path.join('xkcd', os.path.basename(comicUrl)), 'wb')
        for chunk in res.iter_content(100000):
            imageFile.write(chunk)
        imageFile.close()

    # Get the Prev button's url.
    prevLink = soup.select('a[rel="prev"]')[0]
    url = 'http://xkcd.com' + prevLink.get('href')

print('Done.')

At this point, the image file of the comic is stored in the res variable. You need to write this image data to a file on the hard drive.

You’ll need a filename for the local image file to pass to open(). The comicUrl will have a value like 'http://imgs.xkcd.com/comics/heartbleed_explanation.png'—which you might have noticed looks a lot like a file path. And in fact, you can call os.path.basename() with comicUrl, and it will return just the last part of the URL, 'heartbleed_explanation.png'. You can use this as the filename when saving the image to your hard drive. You join this name with the name of your xkcd folder using os.path.join() so that your program uses backslashes (\) on Windows and forward slashes (/) on OS X and Linux. Now that you finally have the filename, you can call open() to open a new file in 'wb' “write binary” mode.

Remember from earlier in this chapter that to save files you’ve downloaded using Requests, you need to loop over the return value of the iter_content() method. The code in the for loop writes out chunks of the image data (at most 100,000 bytes each) to the file and then you close the file. The image is now saved to your hard drive.

Afterward, the selector 'a[rel="prev"]' identifies the <a> element with the rel attribute set to prev, and you can use this <a> element’s href attribute to get the previous comic’s URL, which gets stored in url. Then the while loop begins the entire download process again for this comic.

The output of this program will look like this:

Downloading page http://xkcd.com...
Downloading image http://imgs.xkcd.com/comics/phone_alarm.png...
Downloading page http://xkcd.com/1358/...
Downloading image http://imgs.xkcd.com/comics/nro.png...
Downloading page http://xkcd.com/1357/...
Downloading image http://imgs.xkcd.com/comics/free_speech.png...
Downloading page http://xkcd.com/1356/...
Downloading image http://imgs.xkcd.com/comics/orbital_mechanics.png...
Downloading page http://xkcd.com/1355/...
Downloading image http://imgs.xkcd.com/comics/airplane_message.png...
Downloading page http://xkcd.com/1354/...
Downloading image http://imgs.xkcd.com/comics/heartbleed_explanation.png...
--snip--

This project is a good example of a program that can automatically follow links in order to scrape large amounts of data from the Web. You can learn about Beautiful Soup’s other features from its documentation at http://www.crummy.com/software/BeautifulSoup/bs4/doc/.

Downloading pages and following links are the basis of many web crawling programs. Similar programs could also do the following:

The requests and BeautifulSoup modules are great as long as you can figure out the URL you need to pass to requests.get(). However, sometimes this isn’t so easy to find. Or perhaps the website you want your program to navigate requires you to log in first. The selenium module will give your programs the power to perform such sophisticated tasks.

For these examples, you’ll need the Firefox web browser. This will be the browser that you control. If you don’t already have Firefox, you can download it for free from http://getfirefox.com/.

Importing the modules for Selenium is slightly tricky. Instead of import selenium, you need to run from selenium import webdriver. (The exact reason why the selenium module is set up this way is beyond the scope of this book.) After that, you can launch the Firefox browser with Selenium. Enter the following into the interactive shell:

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
>>> type(browser)
<class 'selenium.webdriver.firefox.webdriver.WebDriver'>
>>> browser.get('http://inventwithpython.com')

You’ll notice when webdriver.Firefox() is called, the Firefox web browser starts up. Calling type() on the value webdriver.Firefox() reveals it’s of the WebDriver data type. And calling browser.get('http://inventwithpython.com') directs the browser to http://inventwithpython.com/. Your browser should look something like Figure 11-7.

Figure 11-7. After calling webdriver.Firefox() and get() in IDLE, the Firefox browser appears.

WebDriver objects have quite a few methods for finding elements on a page. They are divided into the find_element_* and find_elements_* methods. The find_element_* methods return a single WebElement object, representing the first element on the page that matches your query. The find_elements_* methods return a list of WebElement_* objects for every matching element on the page.

Table 11-3 shows several examples of find_element_* and find_elements_* methods being called on a WebDriver object that’s stored in the variable browser.

browser.find_element_by_class_name(name)
browser.find_elements_by_class_name(name)

browser.find_element_by_css_selector(selector)
browser.find_elements_by_css_selector(selector)

browser.find_element_by_id(id)
browser.find_elements_by_id(id)

browser.find_element_by_link_text(text)
browser.find_elements_by_link_text(text)

browser.find_element_by_partial_link_text(text)
browser.find_elements_by_partial_link_text(text)

browser.find_element_by_name(name)
browser.find_elements_by_name(name)

browser.find_element_by_tag_name(name)
browser.find_elements_by_tag_name(name)

Except for the *_by_tag_name() methods, the arguments to all the methods are case sensitive. If no elements exist on the page that match what the method is looking for, the selenium module raises a NoSuchElement exception. If you do not want this exception to crash your program, add try and except statements to your code.

Once you have the WebElement object, you can find out more about it by reading the attributes or calling the methods in Table 11-4.

For example, open a new file editor and enter the following program:

from selenium import webdriver
browser = webdriver.Firefox()
browser.get('http://inventwithpython.com')
try:
    elem = browser.find_element_by_class_name('bookcover')
    print('Found <%s> element with that class name!' % (elem.tag_name))
except:
    print('Was not able to find an element with that name.')

Here we open Firefox and direct it to a URL. On this page, we try to find elements with the class name 'bookcover', and if such an element is found, we print its tag name using the tag_name attribute. If no such element was found, we print a different message.

This program will output the following:

Found <img> element with that class name!

We found an element with the class name 'bookcover' and the tag name 'img'.

WebElement objects returned from the find_element_* and find_elements_* methods have a click() method that simulates a mouse click on that element. This method can be used to follow a link, make a selection on a radio button, click a Submit button, or trigger whatever else might happen when the element is clicked by the mouse. For example, enter the following into the interactive shell:

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
>>> browser.get('http://inventwithpython.com')
>>> linkElem = browser.find_element_by_link_text('Read It Online')
>>> type(linkElem)
<class 'selenium.webdriver.remote.webelement.WebElement'>
>>> linkElem.click() # follows the "Read It Online" link

This opens Firefox to http://inventwithpython.com/, gets the WebElement object for the <a> element with the text Read It Online, and then simulates clicking that <a> element. It’s just like if you clicked the link yourself; the browser then follows that link.

Sending keystrokes to text fields on a web page is a matter of finding the <input> or <textarea> element for that text field and then calling the send_keys() method. For example, enter the following into the interactive shell:

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
>>> browser.get('https://mail.yahoo.com')
>>> emailElem = browser.find_element_by_id('login-username')
>>> emailElem.send_keys('not_my_real_email')
>>> passwordElem = browser.find_element_by_id('login-passwd')
>>> passwordElem.send_keys('12345')
>>> passwordElem.submit()

As long as Gmail hasn’t changed the id of the Username and Password text fields since this book was published, the previous code will fill in those text fields with the provided text. (You can always use the browser’s inspector to verify the id.) Calling the submit() method on any element will have the same result as clicking the Submit button for the form that element is in. (You could have just as easily called emailElem.submit(), and the code would have done the same thing.)

Selenium has a module for keyboard keys that are impossible to type into a string value, which function much like escape characters. These values are stored in attributes in the selenium.webdriver.common.keys module. Since that is such a long module name, it’s much easier to run from selenium.webdriver.common.keys import Keys at the top of your program; if you do, then you can simply write Keys anywhere you’d normally have to write selenium.webdriver.common.keys. Table 11-5 lists the commonly used Keys variables.

For example, if the cursor is not currently in a text field, pressing the HOME and END keys will scroll the browser to the top and bottom of the page, respectively. Enter the following into the interactive shell, and notice how the send_keys() calls scroll the page:

>>> from selenium import webdriver
>>> from selenium.webdriver.common.keys import Keys
>>> browser = webdriver.Firefox()
>>> browser.get('http://nostarch.com')
>>> htmlElem = browser.find_element_by_tag_name('html')
>>> htmlElem.send_keys(Keys.END)     # scrolls to bottom
>>> htmlElem.send_keys(Keys.HOME)    # scrolls to top

The <html> tag is the base tag in HTML files: The full content of the HTML file is enclosed within the <html> and </html> tags. Calling browser.find_element_by_tag_name('html') is a good place to send keys to the general web page. This would be useful if, for example, new content is loaded once you’ve scrolled to the bottom of the page.

Selenium can simulate clicks on various browser buttons as well through the following methods:

Selenium can do much more beyond the functions described here. It can modify your browser’s cookies, take screenshots of web pages, and run custom JavaScript. To learn more about these features, you can visit the Selenium documentation at http://selenium-python.readthedocs.org/.

Write a program that takes an email address and string of text on the command line and then, using Selenium, logs into your email account and sends an email of the string to the provided address. (You might want to set up a separate email account for this program.)

This would be a nice way to add a notification feature to your programs. You could also write a similar program to send messages from a Facebook or Twitter account.

Write a program that goes to a photo-sharing site like Flickr or Imgur, searches for a category of photos, and then downloads all the resulting images. You could write a program that works with any photo site that has a search feature.

2048 is a simple game where you combine tiles by sliding them up, down, left, or right with the arrow keys. You can actually get a fairly high score by repeatedly sliding in an up, right, down, and left pattern over and over again. Write a program that will open the game at https://gabrielecirulli.github.io/2048/ and keep sending up, right, down, and left keystrokes to automatically play the game.

Write a program that, given the URL of a web page, will attempt to download every linked page on the page. The program should flag any pages that have a 404 “Not Found” status code and print them out as broken links.