# 18.5 `urllib` -- Open arbitrary resources 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[, proxies]])

  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 (without universal newlines); otherwise it opens a socket to a server somewhere on the network. If the connection cannot be made 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()`. It also has proper support for the iterator protocol. One caveat: the `read()` method, if the size argument is omitted or negative, may not read until the end of the data stream; there is no good way to determine that the entire stream from a socket has been read in the general case.

  Except for the `info()` and `geturl()` methods, these methods have the same interface as for file objects -- see section 3.9 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. A Content-Type header will be present if the MIME type can be guessed. 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 be in standard application/x-www-form-urlencoded format; see the `urlencode()` function below.

  The `urlopen()` function works transparently with proxies which do not require authentication. In a Unix or Windows environment, set the http_proxy, ftp_proxy or gopher_proxy environment variables to a URL that identifies the proxy server before starting the Python interpreter. For example (the "`%`" is the command prompt):

      % http_proxy="http://www.someproxy.com:3128"
      % export http_proxy
      % python
      ...

  In a Windows environment, if no proxy environment variables are set, proxy settings are obtained from the registry's Internet Settings section.

  In a Macintosh environment, `urlopen()` will retrieve proxy information from Internet Config.

  Alternatively, the optional proxies argument may be used to explicitly specify proxies. It must be a dictionary mapping scheme names to proxy URLs, where an empty dictionary causes no proxies to be used, and `None` (the default value) causes environmental proxy settings to be used as discussed above. For example:

      # Use http://www.someproxy.com:3128 for http proxying
      proxies = {'http': 'http://www.someproxy.com:3128'}
      filehandle = urllib.urlopen(some_url, proxies=proxies)
      # Don't use any proxies
      filehandle = urllib.urlopen(some_url, proxies={})
      # Use proxies from environment - both versions are equivalent
      filehandle = urllib.urlopen(some_url, proxies=None)
      filehandle = urllib.urlopen(some_url)

  The `urlopen()` function does not support explicit proxy specification. If you need to override environmental proxy settings, use `URLopener`, or a subclass such as `FancyURLopener`.

  Proxies which require authentication for use are not currently supported; this is considered an implementation limitation.

  Changed in version 2.3: Added the proxies support.

`urlretrieve`( | url[, filename[, reporthook[, data]]])

  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 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.

  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.

  Changed in version 2.5: `urlretrieve()` will raise `ContentTooShortError` when it detects that the amount of data available was less than the expected amount (which is the size reported by a Content-Length header). This can occur, for example, when the download is interrupted.

  The Content-Length is treated as a lower bound: if there's more data to read, urlretrieve reads more data, but if less data is available, it raises the exception.

  You can still retrieve the downloaded data in this case, it is stored in the `content` attribute of the exception instance.

  If no Content-Length header was supplied, urlretrieve can not check the size of the data it has downloaded, and just returns it. In this case you just have to assume that the download was successful.

`_urlopener`
  The public functions `urlopen()` and `urlretrieve()` create an instance of the `FancyURLopener` class and use it to perform their requested actions. To override this functionality, programmers can create a subclass of `URLopener` or `FancyURLopener`, then assign an instance of that class to the `urllib._urlopener` variable before calling the desired function. For example, applications may want to specify a different User-Agent: header than `URLopener` defines. This can be accomplished with the following code:

      import urllib

      class AppURLopener(urllib.FancyURLopener):
          version = "App/1.7"

      urllib._urlopener = AppURLopener()

`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. It also does not have safe default to `'/'`.

`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`( | query[, doseq])

  Convert a mapping object or a sequence of two-element tuples 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. If the optional parameter doseq is present and evaluates to true, individual `key=value` pairs are generated for each element of the sequence. When a sequence of two-element tuples is used as the query argument, the first element of each tuple is a key and the second is a value. The order of parameters in the encoded string will match the order of parameter tuples in the sequence. The `cgi` module provides the functions `parse_qs()` and `parse_qsl()` which are used to parse query strings into Python data structures.

`pathname2url`( | path)

  Convert the pathname path from the local syntax for a path to the form used in the path component of a URL. This does not produce a complete URL. The return value will already be quoted using the `quote()` function.

`url2pathname`( | path)

  Convert the path component path from an encoded URL to the local syntax for a path. This does not accept a complete URL. This function uses `unquote()` to decode path.

class `URLopener`( | [proxies[, **x509]])

  Base class for opening and reading URLs. Unless you need to support opening objects using schemes other than http:, ftp:, gopher: or file:, you probably want to use `FancyURLopener`.

  By default, the `URLopener` class sends a User-Agent: header of "`urllib/VVV`", where VVV is the `urllib` version number. Applications can define their own User-Agent: header by subclassing `URLopener` or `FancyURLopener` and setting the class attribute `version` to an appropriate string value in the subclass definition.

  The optional proxies parameter should be a dictionary mapping scheme names to proxy URLs, where an empty dictionary turns proxies off completely. Its default value is `None`, in which case environmental proxy settings will be used if present, as discussed in the definition of `urlopen()`, above.

  Additional keyword parameters, collected in x509, may be used for authentication of the client when using the https: scheme. The keywords key_file and cert_file are supported to provide an SSL key and certificate; both are needed to support client authentication.

  `URLopener` objects will raise an `IOError` exception if the server returns an error code.

class `FancyURLopener`( | ...)

  `FancyURLopener` subclasses `URLopener` providing default handling for the following HTTP response codes: 301, 302, 303, 307 and 401. For the 30x response codes listed above, the Location: header is used to fetch the actual URL. For 401 response codes (authentication required), basic HTTP authentication is performed. For the 30x response codes, recursion is bounded by the value of the maxtries attribute, which defaults to 10.

  For all other response codes, the method `http_error_default()` is called which you can override in subclasses to handle the error appropriately.

  Note: According to the letter of RFC 2616, 301 and 302 responses to POST requests must not be automatically redirected without confirmation by the user. In reality, browsers do allow automatic redirection of these responses, changing the POST to a GET, and `urllib` reproduces this behaviour.

  The parameters to the constructor are the same as those for `URLopener`.

  Note: When performing basic authentication, a `FancyURLopener` instance calls its `prompt_user_passwd()` method. The default implementation asks the users for the required information on the controlling terminal. A subclass may override this method to support more appropriate behavior if needed.

exception `ContentTooShortError`( | msg[, content])

  This exception is raised when the `urlretrieve()` function detects that the amount of the downloaded data is less than the expected amount (given by the Content-Length header). The `content` attribute stores the downloaded (and supposedly truncated) data. New in version 2.5.

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 (such as 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.

- The code handling the FTP protocol cannot differentiate between a file and a directory. This can lead to unexpected behavior when attempting to read a URL that points to a file that is not accessible. If the URL ends in a `/`, it is assumed to refer to a directory and will be handled accordingly. But if an attempt to read a file leads to a 550 error (meaning the URL cannot be found or is not accessible, often for permission reasons), then the path is treated as a directory in order to handle the case when a directory is specified by a URL but the trailing `/` has been left off. This can cause misleading results when you try to fetch a file whose read permissions make it inaccessible; the FTP code will try to read it, fail with a 550 error, and then perform a directory listing for the unreadable file. If fine-grained control is needed, consider using the `ftplib` module, subclassing `FancyURLOpener`, or changing _urlopener to meet your needs.

- This module does not support the use of proxies which require authentication. This may be implemented in the future.

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