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3.3 Print

The print statement simply prints objects. Technically, it writes the textual representation of objects to the standard output stream. The standard output stream happens to be the same as the C stdout stream and usually maps to the window where you started your Python program (unless you've redirected it to a file in your system's shell).

In Chapter 2, we also saw file methods that write text. The print statement is similar, but more focused: print writes objects to the stdout stream (with some default formatting), but file write methods write strings to files. Since the standard output stream is available in Python as the stdout object in the built-in sys module (aka sys.stdout), it's possible to emulate print with file writes (see below), but print is easier to use.

Table 3.4 lists the print statement's forms.

Table?.4. Print Statement Forms



print spam, ham

Print objects to sys.stdout, add a space between

print spam, ham,

Same, but don't add newline at end

By default, print adds a space between items separated by commas and adds a linefeed at the end of the current output line. To suppress the linefeed (so you can add more text on the same line later), end your print statement with a comma, as shown in the second line of the table. To suppress the space between items, you can instead build up an output string using the string concatenation and formatting tools in Chapter 2:

>>> print "a", "b"
a b
>>> print "a" + "b"
>>> print "%s...%s" % ("a", "b")

3.3.1 The Python "Hello World" Program

And now, without further delay, here's the script you've all been waiting for (drum roll please)梩he hello world program in Python. Alas, it's more than a little anticlimactic. To print a hello world message in Python, you simply print it:

>>> print 'hello world'                 # print a string object
hello world

>>> 'hello world'                       # interactive prints
'hello world'

>>> import sys                          # printing the hard way
>>> sys.stdout.write('hello world\n')
hello world

Printing is as simple as it should be in Python; although you can achieve the same effect by calling the write method of the sys.stdout file object, the print statement is provided as a simpler tool for simple printing jobs. Since expression results are echoed in the interactive command line, you often don't even need to use a print statement there; simply type expressions you'd like to have printed.

Why You Will Care: print and stdout

The equivalence between the print statement and writing to sys.stdout is important to notice. It's possible to reassign sys.stdout to a user-defined object that provides the same methods as files (e.g., write). Since the print statement just sends text to the sys.stdout.write method, you can capture printed text in your programs by assigning sys.stdout to an object whose write method saves the text. For instance, you can send printed text to a GUI window by defining an object with a write method that does the routing. We'll see an example of this trick later in the book, but abstractly, it looks like this:

class FileFaker:
    def write(self, string):
        # do something with the string

import sys
sys.stdout = FileFaker()
print someObjects            # sends to the write method of the class

Python's built-in raw_input() function reads from the sys.stdin file, so you can intercept read requests in a similar way (using classes that implement file-like read methods). Notice that since print text goes to the stdout stream, it's the way to print HTML in CGI scripts (see Chapter 9). It also means you can redirect Python script input and output at the operating system's command line, as usual:

python script.py < inputfile > outputfile
python script.py | filter

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