5.8. Standard input, output, and error

UNIX users are already familiar with the concept of standard input, standard output, and standard error. This section is for the rest of you.

Standard output and standard error (commonly abbreviated stdout and stderr) are pipes that are built into every UNIX system. When you print something, it goes to the stdout pipe; when your program crashes and prints out debugging information (like a traceback in Python), it goes to the stderr pipe. Both of these pipes are ordinarily just connected to the terminal window where you are working, so when a program prints, you see the output, and when a program crashes, you see the debugging information. (If you’re working on a system with a window-based Python IDE, stdout and stderr default to your “Interactive Window”.)

Example 5.32. Introducing stdout and stderr

>>> for i in range(3):
...     print 'Dive in'             1
Dive in
Dive in
Dive in
>>> import sys
>>> for i in range(3):
...     sys.stdout.write('Dive in') 2
Dive inDive inDive in
>>> for i in range(3):
...     sys.stderr.write('Dive in') 3
Dive inDive inDive in
1 As we saw in Example 3.28, we can use Python’s built-in range function to build simple counter loops that repeat something a set number of times.
2 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.
3 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.

stdout and stderr are both file-like objects, like the ones we discussed in Abstracting input sources, but they are both write-only. They have no read method, only write. Still, they are file-like objects, and you can assign any other file- or file-like object to them to redirect their output.

Example 5.33. Redirecting output

[f8dy@oliver kgp]$ python stdout.py
Dive in
[f8dy@oliver kgp]$ cat out.log
This message will be logged instead of displayed

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

import sys

print 'Dive in'                                          1
saveout = sys.stdout                                     2
fsock = open('out.log', 'w')                             3
sys.stdout = fsock                                       4
print 'This message will be logged instead of displayed' 5
sys.stdout = saveout                                     6
fsock.close()                                            7
1 This will print to the IDE “Interactive Window” (or the terminal, if running the script from the command line).
2 Always save stdout before redirecting it, so you can set it back to normal later.
3 Open a new file for writing.
4 Redirect all further output to the new file we just opened.
5 This will be “printed” to the log file only; it will not be visible in the IDE window or on the screen.
6 Set stdout back to the way it was before we mucked with it.
7 Close the log file.

Redirecting stderr works exactly the same way, using sys.stderr instead of sys.stdout.

Example 5.34. Redirecting error information

[f8dy@oliver kgp]$ python stderr.py
[f8dy@oliver 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.

import sys

fsock = open('error.log', 'w')               1
sys.stderr = fsock                           2
raise Exception, 'this error will be logged' 3 4
1 Open the log file where we want to store debugging information.
2 Redirect standard error by assigning the file object of our newly-opened log file to stderr.
3 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.
4 Also note that we’re not explicitly closing our log file, nor are we setting stderr back to its original value. This is fine, since once the program crashes (due to our 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.

Standard input, on the other hand, is a read-only file object, and it represents the data flowing into the program from some previous program. This will likely not make much sense to classic Mac OS users, or even Windows users unless you were ever fluent on the MS-DOS command line. The way it works is that you can construct a chain of commands in a single line, so that one program’s output becomes the input for the next program in the chain. The first program simply outputs to standard output (without doing any special redirecting itself, just doing normal print statements or whatever), and the next program reads from standard input, and the operating system takes care of connecting one program’s output to the next program’s input.

Example 5.35. Chaining commands

[f8dy@oliver kgp]$ python kgp.py -g binary.xml         1
[f8dy@oliver kgp]$ cat binary.xml                      2
<?xml version="1.0"?>
<!DOCTYPE grammar PUBLIC "-//diveintopython.org//DTD Kant Generator Pro v1.0//EN" "kgp.dtd">
<ref id="bit">
<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>
[f8dy@oliver kgp]$ cat binary.xml | python kgp.py -g - 3 4
1 As we saw in Diving in, this will print a string of eight random bits, 0 or 1.
2 This simply prints out the entire contents of binary.xml. (Windows users should use type instead of cat.)
3 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 our Python script.
4 Instead of specifying a module (like binary.xml), we specify “-”, which causes our 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 we specified the grammar filename directly, but think of the expansion possibilities here. Instead of simply doing cat binary.xml, we could run a script that dynamically generates the grammar, then we can pipe it into our script. It could come from anywhere: a database, or some grammar-generating meta-script, or whatever. The point is that we don’t have to change our kgp.py script at all to incorporate any of this functionality. All we have to do is be able to take grammar files from standard input, and we can separate all the other logic into another program.

So how does our script “know” to read from standard input when the grammar file is “-”? It’s not magic; it’s just code.

Example 5.36. Reading from standard input in kgp.py

def openAnything(source):
    if source == "-":    1
        import sys
        return sys.stdin

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

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