In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "We can include other contexts through the use of the include directive."

A block of code is set as follows:

import matplotlib.pyplot as plt
from matplotlib.collections import LineCollection
from tutorial import track_loader
tracks = track_loader('polygons.shp')
# Filter out non-tracks (unassociated polygons given trackID of -9)
tracks = {tid: t for tid, t in tracks.items() if tid != -9}

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

    polys = [p for p in cells.polygons] 
    for p in polys: 
        p.set_visible(True) 
        p.set_alpha(0.0) 

    def update(frame, polys): 
        for i, p in enumerate(polys): 
            alpha = 0.0 if i > frame else 1.0 / ((frame - i + 1)**2) 
            p.set_alpha(alpha) 

    ax.set_xlabel("Longitude")
    ax.set_ylabel("Latitude")
    strmanim = FuncAnimation(fig, update, frameCnt, 
                             fargs=(polys,)) 
    plt.show()

Any command-line input or output is written as follows:

$ pip install matplotlib

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Now click on the Selection radio button and you will find that you can select a polygon again."