# 11.2 `urllib` -- Open an arbitrary object given by URL.

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()`, `info()` and `geturl()`.

  Except for the `info()` and `geturl()` methods, 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 meta-information associated with the URL. When the method is HTTP, these headers are those returned by the server at the head of the retrieved HTML page (including Content-Length and Content-Type). When the method is FTP, a Content-Length header will be present if (as is now usual) the server passed back a file length in response to the FTP retrieval request. When the method is local-file, returned headers will include a Date representing the file's last-modified time, a Content-Length giving file size, and a Content-Type containing a guess at the file's type. See also the description of the `mimetools` module.

  The `geturl()` method returns the real URL of the page. In some cases, the HTTP server redirects a client to another URL. The `urlopen()` function handles this transparently, but in some cases the caller needs to know which URL the client was redirected to. The `geturl()` method can be used to get at this redirected URL.

  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[, filename[, hook]])
  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()`.

  The second argument, if present, specifies the file location to copy to (if absent, the location will be a tempfile with a generated name). The third argument, if present, is a hook function that will be called once on establishment of the network connection and once after each block read thereafter. The hook will be passed three arguments; a count of blocks transferred so far, a block size in bytes, and the total size of the file. The third argument may be `-1` on older FTP servers which do not return a file size in response to a retrieval request.

`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`.