Dive Into Python

Example 10.8. Introducing stdout and stderr

>>> for i in range(3):
...     print 'Dive in'             
Dive in
Dive in
Dive in
>>> import sys
>>> for i in range(3):
...     sys.stdout.write('Dive in') 
Dive inDive inDive in
>>> for i in range(3):
...     sys.stderr.write('Dive in') 
Dive inDive inDive in

	As you saw in Example 6.9, “Simple Counters”, you can use Python's built-in range function to build simple counter loops that repeat something a set number of times.
	stdout is a file-like object; calling its write function will print out whatever string you give it. In fact, this is what the print function really does; it adds a carriage return to the end of the string you're printing, and calls sys.stdout.write.
	In the simplest case, stdout and stderr send their output to the same place: the Python IDE (if you're in one), or the terminal (if you're running Python from the command line). Like stdout, stderr does not add carriage returns for you; if you want them, add them yourself.

Example 10.9. Redirecting output

[you@localhost kgp]$ python stdout.py
Dive in
[you@localhost kgp]$ cat out.log
This message will be logged instead of displayed

(On Windows, you can use type instead of cat to display the contents of a file.)

If you have not already done so, you can download this and other examples used in this book.

#stdout.py
import sys

print 'Dive in'                                          
saveout = sys.stdout                                     
fsock = open('out.log', 'w')                             
sys.stdout = fsock                                       
print 'This message will be logged instead of displayed' 
sys.stdout = saveout                                     
fsock.close()                                            

	This will print to the IDE “Interactive Window” (or the terminal, if running the script from the command line).
	Always save stdout before redirecting it, so you can set it back to normal later.
	Open a file for writing. If the file doesn't exist, it will be created. If the file does exist, it will be overwritten.
	Redirect all further output to the new file you just opened.
	This will be “printed” to the log file only; it will not be visible in the IDE window or on the screen.
	Set stdout back to the way it was before you mucked with it.
	Close the log file.

Example 10.10. Redirecting error information

[you@localhost kgp]$ python stderr.py
[you@localhost kgp]$ cat error.log
Traceback (most recent line last):
  File "stderr.py", line 5, in ?
    raise Exception, 'this error will be logged'
Exception: this error will be logged

If you have not already done so, you can download this and other examples used in this book.

#stderr.py
import sys

fsock = open('error.log', 'w')               
sys.stderr = fsock                           
raise Exception, 'this error will be logged'  

	Open the log file where you want to store debugging information.
	Redirect standard error by assigning the file object of the newly-opened log file to stderr.
	Raise an exception. Note from the screen output that this does not print anything on screen. All the normal traceback information has been written to error.log.
	Also note that you're not explicitly closing your log file, nor are you setting stderr back to its original value. This is fine, since once the program crashes (because of the exception), Python will clean up and close the file for us, and it doesn't make any difference that stderr is never restored, since, as I mentioned, the program crashes and Python ends. Restoring the original is more important for stdout, if you expect to go do other stuff within the same script afterwards.

Example 10.11. Printing to stderr

>>> print 'entering function'
entering function
>>> import sys
>>> print >> sys.stderr, 'entering function' 
entering function

This shorthand syntax of the print statement can be used to write to any open file, or file-like object. In this case, you can redirect a single print statement to stderr without affecting subsequent print statements.

Example 10.12. Chaining commands

[you@localhost kgp]$ python kgp.py -g binary.xml         
01100111
[you@localhost kgp]$ cat binary.xml                      
<?xml version="1.0"?>
<!DOCTYPE grammar PUBLIC "-//book.diveintopython.org//DTD Kant Generator Pro v1.0//EN" "kgp.dtd">
<grammar>
<ref id="bit">
  <p>0</p>
  <p>1</p>
</ref>
<ref id="byte">
  <p><xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/>\
<xref id="bit"/><xref id="bit"/><xref id="bit"/><xref id="bit"/></p>
</ref>
</grammar>
[you@localhost kgp]$ cat binary.xml | python kgp.py -g -  
10110001

	As you saw in Section 9.1, “Diving in”, this will print a string of eight random bits, 0 or 1.
	This simply prints out the entire contents of binary.xml. (Windows users should use type instead of cat.)
	This prints the contents of binary.xml, but the “|” character, called the “pipe” character, means that the contents will not be printed to the screen. Instead, they will become the standard input of the next command, which in this case calls your Python script.
	Instead of specifying a module (like binary.xml), you specify “-”, which causes your script to load the grammar from standard input instead of from a specific file on disk. (More on how this happens in the next example.) So the effect is the same as the first syntax, where you specified the grammar filename directly, but think of the expansion possibilities here. Instead of simply doing cat binary.xml, you could run a script that dynamically generates the grammar, then you can pipe it into your script. It could come from anywhere: a database, or some grammar-generating meta-script, or whatever. The point is that you don't need to change your kgp.py script at all to incorporate any of this functionality. All you need to do is be able to take grammar files from standard input, and you can separate all the other logic into another program.

Example 10.13. Reading from standard input in kgp.py

def openAnything(source):
    if source == "-":    
        import sys
        return sys.stdin

    # try to open with urllib (if source is http, ftp, or file URL)
    import urllib
    try:

[... snip ...]

This is the openAnything function from toolbox.py, which you previously examined in Section 10.1, “Abstracting input sources”. All you've done is add three lines of code at the beginning of the function to check if the source is “-”; if so, you return sys.stdin. Really, that's it! Remember, stdin is a file-like object with a read method, so the rest of the code (in kgp.py, where you call openAnything) doesn't change a bit.