11.2 Standard Module urllib

This module provides a high-level interface for fetching data across the World-Wide Web. In particular, the urlopen() function is similar to the built-in function open(), but accepts Universal Resource Locators (URLs) instead of filenames. Some restrictions apply -- it can only open URLs for reading, and no seek operations are available.

It defines the following public functions:

urlopen (url[, data])

Open a network object denoted by a URL for reading. If the URL does not have a scheme identifier, or if it has "file:" as its scheme identifier, this opens a local file; otherwise it opens a socket to a server somewhere on the network. If the connection cannot be made, or if the server returns an error code, the IOError exception is raised. If all went well, a file-like object is returned. This supports the following methods: read(), readline(), readlines(), fileno(), close() and info().

Except for the info() method, these methods have the same interface as for file objects -- see section 2.1 in this manual. (It is not a built-in file object, however, so it can't be used at those few places where a true built-in file object is required.)

The info() method returns an instance of the class mimetools.Message containing the headers received from the server, if the protocol uses such headers (currently the only supported protocol that uses this is HTTP). See the description of the mimetools module.

If the url uses the "http:" scheme identifier, the optional data argument may be given to specify a POST request (normally the request type is GET). The data argument must in standard "application/x-www-form-urlencoded" format; see the urlencode() function below.

urlretrieve (url)

Copy a network object denoted by a URL to a local file, if necessary. If the URL points to a local file, or a valid cached copy of the object exists, the object is not copied. Return a tuple (filename, headers) where filename is the local file name under which the object can be found, and headers is either None (for a local object) or whatever the info() method of the object returned by urlopen() returned (for a remote object, possibly cached). Exceptions are the same as for urlopen().

urlcleanup ()

Clear the cache that may have been built up by previous calls to urlretrieve().

quote (string[, safe])

Replace special characters in string using the "%xx" escape. Letters, digits, and the characters "_,.-" are never quoted. The optional safe parameter specifies additional characters that should not be quoted -- its default value is '/'.

Example: quote('/connolly/') yields '/%7econnolly/'.

quote_plus (string[, safe])

Like quote(), but also replaces spaces by plus signs, as required for quoting HTML form values. Plus signs in the original string are escaped unless they are included in safe.

unquote (string)

Replace "%xx" escapes by their single-character equivalent.

Example: unquote('/%7Econnolly/') yields '/connolly/'.

unquote_plus (string)

Like unquote(), but also replaces plus signs by spaces, as required for unquoting HTML form values.

urlencode (dict)

Convert a dictionary to a ``url-encoded'' string, suitable to pass to urlopen() above as the optional data argument. This is useful to pass a dictionary of form fields to a POST request. The resulting string is a series of key=value pairs separated by & characters, where both key and value are quoted using quote_plus() above.

Restrictions:

• Currently, only the following protocols are supported: HTTP, (versions 0.9 and 1.0), Gopher (but not Gopher-+), FTP, and local files.

• The caching feature of urlretrieve() has been disabled until I find the time to hack proper processing of Expiration time headers.

• There should be a function to query whether a particular URL is in the cache.

• For backward compatibility, if a URL appears to point to a local file but the file can't be opened, the URL is re-interpreted using the FTP protocol. This can sometimes cause confusing error messages.

• The urlopen() and urlretrieve() functions can cause arbitrarily long delays while waiting for a network connection to be set up. This means that it is difficult to build an interactive web client using these functions without using threads.

• The data returned by urlopen() or urlretrieve() is the raw data returned by the server. This may be binary data (e.g. an image), plain text or (for example) HTML. The HTTP protocol provides type information in the reply header, which can be inspected by looking at the content-type header. For the Gopher protocol, type information is encoded in the URL; there is currently no easy way to extract it. If the returned data is HTML, you can use the module htmllib to parse it.

• Although the urllib module contains (undocumented) routines to parse and unparse URL strings, the recommended interface for URL manipulation is in module urlparse.