Triple Quotes Code Multiline Block Strings

So far, you’ve seen single quotes, double quotes, escapes, and raw strings in action. Python also has a triple-quoted string literal format, sometimes called a block string, that is a syntactic convenience for coding multiline text data. This form begins with three quotes (of either the single or double variety), is followed by any number of lines of text, and is closed with the same triple-quote sequence that opened it. Single and double quotes embedded in the string’s text may be, but do not have to be, escaped—the string does not end until Python sees three unescaped quotes of the same kind used to start the literal. For example:

>>> mantra = """Always look
...  on the bright
... side of life."""
>>>
>>> mantra
'Always look\n on the bright\nside of life.'

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This string spans three lines (in some interfaces, the interactive prompt changes to ... on continuation lines; IDLE simply drops down one line). Python collects all the triple-quoted text into a single multiline string, with embedded newline characters (\n) at the places where your code has line breaks. Notice that, as in the literal, the second line in the result has a leading space, but the third does not—what you type is truly what you get. To see the string with the newlines interpreted, print it instead of echoing:

>>> print(mantra)
Always look
 on the bright
side of life.

Triple-quoted strings are useful any time you need multiline text in your program; for example, to embed multiline error messages or HTML or XML code in your source code files. You can embed such blocks directly in your scripts without resorting to external text files or explicit concatenation and newline characters.

Triple-quoted strings are also commonly used for documentation strings, which are string literals that are taken as comments when they appear at specific points in your file (more on these later in the book). These don’t have to be triple-quoted blocks, but they usually are to allow for multiline comments.

Finally, triple-quoted strings are also sometimes used as a “horribly hackish” way to temporarily disable lines of code during development (OK, it’s not really too horrible, and it’s actually a fairly common practice). If you wish to turn off a few lines of code and run your script again, simply put three quotes above and below them, like this:

X = 1
"""
import os                            # Disable this code temporarily
print(os.getcwd())
"""
Y = 2

I said this was hackish because Python really does make a string out of the lines of code disabled this way, but this is probably not significant in terms of performance. For large sections of code, it’s also easier than manually adding hash marks before each line and later removing them. This is especially true if you are using a text editor that does not have support for editing Python code specifically. In Python, practicality often beats aesthetics.

 

[17] If you need to care about binary data files, the chief distinction is that you open them in binary mode (using open mode flags with a b, such as 'rb', 'wb', and so on). In Python 3.0, binary file content is a bytes string, with an interface similar to that of normal strings; in 2.6, such content is a normal str string. See also the standard struct module introduced in Chapter 9, which can parse binary data loaded from a file, and the extended coverage of binary files and byte strings in Chapter 36.

[18] In classes, I’ve met people who have indeed committed most or all of this table to memory; I’d probably think that was really sick, but for the fact that I’m a member of the set, too.