Understanding Python Basics for Raspberry Pi
This chapter is from the book
This chapter is from the book
This chapter is from the book
In this hour, you will get a chance to learn some Python basics, such as using the print function to display output. You will read about using variables and how to assign them values, and you will gain an understanding of their data types. By the end of the hour, you will know how to get data into a script by using the input function, and you will be writing your first Python script!
Producing Python Script Output
Understanding how to produce output from a Python script is a good starting point for those who are new to the Python programming language. You can get instant feedback on your Python statements from the Python interactive interpreter and gently experiment with proper syntax. The print function, which you met in Hour 3, “Setting Up a Programming Environment,” is a good place to focus your attention.
Exploring the print Function
A function is a group of python statements that are put together as a unit to perform a specific task. You can simply enter a single Python statement to perform a task for you.
The print function’s task is to output items. The “items” to output are correctly called an argument. The basic syntax of the print function is as follows:
print (argument)
The argument portion of the print function can be characters, such as ABC or 123. It can also be values stored in variables. You will learn about variables later in this hour.
Using Characters as print Function Arguments
To display characters (also called string literals) using the print function, you need to enclose the characters in either a set of single quotes or double quotes. Listing 4.1 shows using a pair of single quotes to enclose characters (a sentence) so it can be used as a print function argument.
LISTING 4.1 Using a Pair of Single Quotes to Enclose Characters
>>> print ('This is an example of using single quotes.')
This is an example of using single quotes.
>>>
Listing 4.2 shows the use of double quotes with the print function. You can see that the output that results from both Listing 4.1 and Listing 4.2 does not contain the quotation marks, only the characters.
LISTING 4.2 Using a Pair of Double Quotes to Enclose Characters
>>> print ("This is an example of using double quotes.")
This is an example of using double quotes.
>>>
Sometimes you need to output a string of characters that contain a single quote to show possession or a contraction. In such a case, you use double quotes around the print function argument, as shown in Listing 4.3.
LISTING 4.3 Protecting a Single Quote with Double Quotes
>>> print ("This example protects the output's single quote.")
This example protects the output's single quote.
>>>
At other times, you need to output a string of characters that contain double quotes, such as for a quotation. Listing 4.4 shows an example of protecting a quote, using single quotes in the argument.
LISTING 4.4 Protecting a Double Quote with Single Quotes
>>> print ('I said, "I need to protect my quotation!" and did so.')
I said, "I need to protect my quotation!" and did so.
>>>
Formatting Output with the print Function
You can perform various output formatting features by using the print function. For example, you can insert a single blank line by using the print function with no arguments, like this:
print ()
The screen in Figure 4.1 shows a short Python script that inserts a blank line between two other lines of output.
)
FIGURE 4.1 Adding a blank line in script output.
Another way to format output using the print function is via triple quotes. Triple quotes are simply three sets of double quotes.
Listing 4.5 shows how you can use triple quotes to embed a linefeed character by pressing the Enter key. When the output is displayed, each embedded linefeed character causes the next sentence to appear on the next line. Thus, linefeed moves your output to the next new line. Notice that you cannot see the linefeed character embedded on each line in the code; you can only see its effect in the output.
LISTING 4.5 Using Triple Quotes
>>> print ("""This is line one.
... This is line two.
... This is line three.""")
This is line one.
This is line two.
This is line three.
>>>
By using triple quotes, you can also protect single and double quotes that you need to be displayed in the output. Listing 4.6 shows the use of triple quotes to protect both single and double quotes in the same character string.
LISTING 4.6 Using Triple Quotes to Protect Single and Double Quotes
>>> print ("""Raz said, "I didn't know about triple quotes!" and laughed.""")
Raz said, "I didn't know about triple quotes!" and laughed.
>>>
Controlling Output with Escape Sequences
An escape sequence is a series of characters that allow a Python statement to “escape” from normal behavior. The new behavior can be the addition of special formatting for the output or the protection of characters typically used in syntax. Escape sequences all begin with the backslash (\) character.
An example of using an escape sequence to add special formatting for output is the \n escape sequence. The \n escape sequence forces any characters listed after it onto the next line of displayed output. This is called a newline, and the formatting character it inserts is a linefeed. Listing 4.7 shows an example of using the \n escape sequence to insert a linefeed. Notice that it causes the output to be formatted exactly as it was Listing 4.5, with triple quotes.
LISTING 4.7 Using an Escape Sequence to Add a Linefeed
>>> print ("This is line one.\nThis is line two.\nAnd this is line three.")
This is line one.
This is line two.
And this is line three.
>>>
Typically, the print function puts a linefeed only at the end of displayed output. However, the print function in Listing 4.7 is forced to “escape” its normal formatting behavior because of the addition of the \n escape sequence.
You can also use escape sequences to protect various characters used in syntax. Listing 4.8 shows the backslash (\) character used to protect a single quote so that it will not be used in the print function’s syntax. Instead, the quote is displayed in the output.
LISTING 4.8 Using an Escape Sequence to Protect Quotes
>>> print ('Use backslash, so the single quote isn\'t noticed.')
Use backslash, so the single quote isn't noticed.
>>>
You can use many different escape sequences in your Python scripts. Table 4.1 shows a few of the available sequences.
TABLE 4.1 A Few Python Escape Sequences
Escape Sequence |
Description |
\' |
Displays a single quote in output. |
\" |
Displays a double quote in output. |
\\ |
Displays a single backslash in output. |
\a |
Produces a “bell” sound with output. |
\f |
Inserts a formfeed into the output. |
\n |
Inserts a linefeed into the output. |
\t |
Inserts a horizontal tab into the output. |
\u#### |
Displays the Unicode character denoted by the character’s four hexadecimal digits (####). |
Notice in Table 4.1 that not only can you insert formatting into your output, you can produce sound as well! Another interesting escape sequence involves displaying Unicode characters in your output.
Now for Something Fun!
Thanks to the Unicode escape sequence, you can print all kinds of characters in your output. You learned a little about Unicode in Hour 3. You can display Unicode characters by using the \u escape sequence. Each Unicode character is represented by a hexadecimal number. You can find these hexadecimal numbers at www.unicode.org/charts. There are lots of Unicode characters!
The hexadecimal number for the pi (∏) symbol is 03c0. To display this symbol using the Unicode escape sequence, you must precede the number with \u in your print function argument. Listing 4.9 displays the pi symbol to output.
LISTING 4.9 Using a Unicode Escape Sequence
>>> print ("I love my Raspberry \u03c0!")
I love my Raspberry π!
>>>