# 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 URLs (Universal Resource Locators) 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)
  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 last one, these methods have the same interface as for file objects -- see the section on File Objects earlier in this manual. (It's 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.

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

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

`quote_plus` (string[, addsafe])
  Like `quote()`, but also replaces spaces by plus signs, as required for quoting HTML form values.

`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.

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`.