In this section, we look at a few of Python's many standard utility modules to solve common problems.
The *os* and *os.path* modules include many functions to interact with the file system. The *shutil* module can copy files.
## Example pulls filenames from a dir, prints their relative and absolute pathsdef printdir(dir):filenames = os.listdir(dir)for filename in filenames:print filename ## foo.txtprint os.path.join(dir, filename) ## dir/foo.txt (relative to current dir)print os.path.abspath(os.path.join(dir, filename)) ## /home/nick/dir/foo.txt
Exploring a module works well with the built-in python help() and dir() functions. In the interpreter, do an "import os", and then use these commands look at what's available in the module: dir(os), help(os.listdir), dir(os.path), help(os.path.dirname).
The *commands* module is a simple way to run an external command and capture its output.
Note: The commands module and the popen2 module are deprecated as of Python 2.6 and removed in 3.x. The subprocess module replaces these modules. In particular, the subprocess module discusses replacements for these modules in the subprocess-replacements section.
## Given a dir path, run an external 'ls -l' on it --## shows how to call an external programdef listdir(dir):cmd = 'ls -l ' + dirprint "Command to run:", cmd ## good to debug cmd before actually running it(status, output) = commands.getstatusoutput(cmd)if status: ## Error case, print the command's output to stderr and exitsys.stderr.write(output)sys.exit(status)print output ## Otherwise do something with the command's output
An exception represents a run-time error that halts the normal execution at a particular line and transfers control to error handling code. This section just introduces the most basic uses of exceptions. For example a run-time error might be that a variable used in the program does not have a value (ValueError .. you've probably seen that one a few times), or a file open operation error because a file does not exist (IOError). Learn more in the exceptions tutorial and see the entire exception list.
Without any error handling code (as we have done thus far), a run-time exception just halts the program with an error message. That's a good default behavior, and you've seen it many times. You can add a "try/except" structure to your code to handle exceptions, like this:
try:## Either of these two lines could throw an IOError, say## if the file does not exist or the read() encounters a low level error.f = open(filename, 'rU')text = f.read()f.close()except IOError:## Control jumps directly to here if any of the above lines throws IOError.sys.stderr.write('problem reading:' + filename)## In any case, the code then continues with the line after the try/except
The try: section includes the code which might throw an exception. The except: section holds the code to run if there is an exception. If there is no exception, the except: section is skipped (that is, that code is for error handling only, not the "normal" case for the code). You can get a pointer to the exception object itself with syntax "except IOError as e: .." (e points to the exception object).
The module *urllib* provides url fetching -- making a url look like a file you can read from. The *urlparse* module can take apart and put together urls.
In Python 3, urllib and urllib2 are merged into urllib.request, and urlparse becomes urllib.parse. All of their exceptions are in urllib.error.
## Given a url, try to retrieve it. If it's text/html,## print its base url and its text.def wget(url):ufile = urllib.urlopen(url) ## get file-like object for urlinfo = ufile.info() ## meta-info about the url contentif info.gettype() == 'text/html':print 'base url:' + ufile.geturl()text = ufile.read() ## read all its textprint text
The above code works fine, but does not include error handling if a url does not work for some reason. Here's a version of the function which adds try/except logic to print an error message if the url operation fails.
## Version that uses try/except to print an error message if the## urlopen() fails.def wget2(url):try:ufile = urllib.urlopen(url)if ufile.info().gettype() == 'text/html':print ufile.read()except IOError:print 'problem reading url:', url
Based on content from https://developers.google.com/edu/python/utilities licensed under the Creative Commons Attribution 3.0 License, and code samples are licensed under the Apache 2.0 License