5.7. Abstracting input sources

One of Python’s greatest strengths is its dynamic binding, and one powerful use of dynamic binding is the file-like object.

Many functions which require an input source could simply take a filename, go open the file for reading, read it, and close it when they’re done. But they don’t. Instead, they take a file-like object.

In the simplest case, a file-like object is any object with a read method with an optional size parameter, which returns a string. When called with no size parameter, it reads everything there is to read from the input source and returns all the data as a single string. When called with a size parameter, it reads that much from the input source and returns that much data; when called again, it picks up where it left off and returns the next chunk of data.

This is how reading from real files works; the difference is that we’re not limiting ourselves to real files. The input source could be anything: a file on disk, a web page, even a hard-coded string. As long as we pass a file-like object to the function, and the function simply calls the object’s read method, the function can handle any kind of input source without specific code to handle each kind.

In case you were wondering how this relates to XML processing, minidom.parse is one such function which can take a file-like object.

Example 5.25. Parsing XML from a file

>>> from xml.dom import minidom
>>> fsock = open('binary.xml')    1
>>> xmldoc = minidom.parse(fsock) 2
>>> fsock.close()                 3
>>> print xmldoc
<?xml version="1.0" ?>
<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>
1 First, we open the file on disk. This gives us a file object.
2 We pass the file object to minidom.parse, which calls the read method of fsock and reads the XML document from the file on disk.
3 Be sure to call the close method of the file object after we’re done with it. minidom.parse will not do this for you.

Well, that all seems like a colossal waste of time. After all, we’ve already seen that minidom.parse can simply take the filename and do all the opening and closing nonsense automatically. And it’s true that if you know you’re just going to be parsing a local file, you can pass the filename and minidom.parse is smart enough to Do The Right Thing™. But notice how similar -- and easy -- it is to parse an XML document straight from the Internet.

Example 5.26. Parsing XML from a URL

>>> import urllib
>>> usock = urllib.urlopen('http://slashdot.org/slashdot.rdf') 1
>>> xmldoc = minidom.parse(usock)                              2
>>> usock.close()                                              3
>>> print xmldoc.toxml()                                       4
<?xml version="1.0" ?>
<rdf:RDF xmlns="http://my.netscape.com/rdf/simple/0.9/"

<description>News for nerds, stuff that matters</description>


<title>To HDTV or Not to HDTV?</title>

1 As we saw in the previous chapter, urlopen takes a web page URL and returns a file-like object. Most importantly, this object has a read method which returns the HTML source of the web page.
2 Now we pass the file-like object to minidom.parse, which obediently calls the read method of the object and parses the XML data that the read method returns. The fact that this XML data is now coming straight from a web page is completely irrelevant. minidom.parse doesn’t know about web pages, and it doesn’t care about web pages; it just knows about file-like objects.
3 As soon as you’re done with it, be sure to close the file-like object that urlopen gives you.
4 By the way, this URL is real, and it really is XML. It’s an XML representation of the current headlines on Slashdot, a technical news and gossip site.

Example 5.27. Parsing XML from a string (the easy but inflexible way)

>>> contents = "<grammar><ref id='bit'><p>0</p><p>1</p></ref></grammar>"
>>> xmldoc = minidom.parseString(contents) 1
>>> print xmldoc.toxml()
<?xml version="1.0" ?>
<grammar><ref id="bit"><p>0</p><p>1</p></ref></grammar>
1 minidom has a method, parseString, which takes an entire XML document as a string and parses it. You can use this instead of minidom.parse if you know you already have your entire XML document in a string.

OK, so we can use the minidom.parse function for parsing both local files and remote URLs, but for parsing strings, we use... a different function. That means that if we want to be able to take input from a file, a URL, or a string, we’ll need special logic to check whether it’s a string, and call the parseString function instead. How unsatisfying.

If there were a way to turn a string into a file-like object, then we could simply pass this object to minidom.parse. And in fact, there is a module specifically designed for doing just that: StringIO.

Example 5.28. Introducing StringIO

>>> contents = "<grammar><ref id='bit'><p>0</p><p>1</p></ref></grammar>"
>>> import StringIO
>>> ssock = StringIO.StringIO(contents)   1
>>> ssock.read()                          2
"<grammar><ref id='bit'><p>0</p><p>1</p></ref></grammar>"
>>> ssock.read()                          3
>>> ssock.seek(0)                         4
>>> ssock.read(15)                        5
'<grammar><ref i'
>>> ssock.read(15)
>>> ssock.read()
>>> ssock.close()                         6
1 The StringIO module contains a single class, also called StringIO, which allows you to turn a string into a file-like object. The StringIO class takes the string as a parameter when creating an instance.
2 Now we have a file-like object, and we can do all sorts of file-like things with it. Like read, which returns the original string.
3 Calling read again returns an empty string. This is how real file objects work too; once you read the entire file, you can’t read any more without explicitly seeking to the beginning of the file. The StringIO object works the same way.
4 You can explicitly seek to the beginning of the string, just like seeking through a file, by using the seek method of the StringIO object.
5 You can also read the string in chunks, by passing a size parameter to the read method.
6 At any time, read will return the rest of the string that you haven’t read yet. All of this is exactly how file objects work; hence the term file-like object.

Example 5.29. Parsing XML from a string (the file-like object way)

>>> contents = "<grammar><ref id='bit'><p>0</p><p>1</p></ref></grammar>"
>>> ssock = StringIO.StringIO(contents)
>>> xmldoc = minidom.parse(ssock) 1
>>> print xmldoc.toxml()
<?xml version="1.0" ?>
<grammar><ref id="bit"><p>0</p><p>1</p></ref></grammar>
1 Now we can pass the file-like object (really a StringIO) to minidom.parse, which will call the object’s read method and happily parse away, never knowing that its input came from a hard-coded string.

So now we know how to use a single function, minidom.parse, to parse an XML document stored on a web page, in a local file, or in a hard-coded string. For a web page, we use urlopen to get a file-like object; for a local file, we use open; and for a string, we use StringIO. Now let’s take it one step further and generalize these differences as well.

Example 5.30. openAnything

def openAnything(source):                  1
    # try to open with urllib (if source is http, ftp, or file URL)
    import urllib                         
        return urllib.urlopen(source)      2
    except (IOError, OSError):            

    # try to open with native open function (if source is pathname)
        return open(source)                3
    except (IOError, OSError):            

    # treat source as string
    import StringIO                       
    return StringIO.StringIO(str(source))  4
1 The openAnything function takes a single parameter, source, and returns a file-like object. source is a string of some sort; it can either be a URL (like 'http://slashdot.org/slashdot.rdf'), a full or partial pathname to a local file (like 'binary.xml'), or a string that contains actual XML data to be parsed.
2 First, we see if source is a URL. We do this through brute force: we try to open it as a URL and silently ignore errors caused by trying to open something which is not a URL. This is actually elegant in the sense that, if urllib ever supports new types of URLs in the future, we will also support them without recoding.
3 If urllib yelled at us and told us that source wasn’t a valid URL, we assume it’s a path to a file on disk and try to open it. Again, we don’t do anything fancy to check whether source is a valid filename or not (the rules for valid filenames vary wildly between different platforms anyway, so we’d probably get them wrong anyway). Instead, we just blindly open the file, and silently trap any errors.
4 By this point, we have to assume that source is a string that has hard-coded data in it (since nothing else worked), so we use StringIO to create a file-like object out of it and return that. (In fact, since we’re using the str function, source doesn’t even need to be a string; it could be any object, and we’ll use its string representation, as defined by its __str__ special method.)

Now we can use this openAnything function in conjunction with minidom.parse to make a function that takes a source that refers to an XML document somehow (either as a URL, or a local filename, or a hard-coded XML document in a string) and parses it.

Example 5.31. Using openAnything in kgp.py

class KantGenerator:
    def _load(self, source):
        sock = toolbox.openAnything(source)
        xmldoc = minidom.parse(sock).documentElement
        return xmldoc