Dive Into Python Python for experienced programmers |

## 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')
    >>> xmldoc = minidom.parse(fsock)
    >>> fsock.close()
    >>> print xmldoc
    <?xml version="1.0" ?>
    <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>

| First, we open the file on disk. This gives us a file object.
| 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.
| 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')
    >>> xmldoc = minidom.parse(usock)
    >>> usock.close()
    >>> print xmldoc.toxml()
    <?xml version="1.0" ?>
    <rdf:RDF xmlns="http://my.netscape.com/rdf/simple/0.9/"
     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">

    <channel>
    <title>Slashdot</title>
    <link>http://slashdot.org/</link>
    <description>News for nerds, stuff that matters</description>
    </channel>

    <image>
    <title>Slashdot</title>
    <url>http://images.slashdot.org/topics/topicslashdot.gif</url>
    <link>http://slashdot.org/</link>
    </image>

    <item>
    <title>To HDTV or Not to HDTV?</title>
    <link>http://slashdot.org/article.pl?sid=01/12/28/0421241</link>
    </item>

    [...snip...]

| 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.
| 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.
| As soon as you're done with it, be sure to close the file-like object that `urlopen` gives you.
| 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)
    >>> print xmldoc.toxml()
    <?xml version="1.0" ?>
    <grammar><ref id="bit"><p>0</p><p>1</p></ref></grammar>

| `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)
    >>> ssock.read()
    "<grammar><ref id='bit'><p>0</p><p>1</p></ref></grammar>"
    >>> ssock.read()
    ''
    >>> ssock.seek(0)
    >>> ssock.read(15)
    '<grammar><ref i'
    >>> ssock.read(15)
    "d='bit'><p>0</p"
    >>> ssock.read()
    '><p>1</p></ref></grammar>'
    >>> ssock.close()

| 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.
| 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.
| 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.
| 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.
| You can also read the string in chunks, by passing a `size` parameter to the `read` method.
| 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)
    >>> print xmldoc.toxml()
    <?xml version="1.0" ?>
    <grammar><ref id="bit"><p>0</p><p>1</p></ref></grammar>

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

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

        # treat source as string
        import StringIO
        return StringIO.StringIO(str(source))

| 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.
| 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.
| 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.
| 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
            sock.close()
            return xmldoc