# 20.5. `urllib` — Open arbitrary resources by URL

注解

The `urllib` module has been split into parts and renamed in Python 3 to `urllib.request`, `urllib.parse`, and `urllib.error`. The 2to3 tool will automatically adapt imports when converting your sources to Python 3. Also note that the `urllib.request.urlopen()` function in Python 3 is equivalent to `urllib2.urlopen()` and that `urllib.urlopen()` has been removed.

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.

参见

The Requests package is recommended for a higher-level HTTP client interface.

在 2.7.9 版更改: For HTTPS URIs, `urllib` performs all the neccessary certificate and hostname checks by default.

警告

For Python versions earlier than 2.7.9, urllib does not attempt to validate the server certificates of HTTPS URIs. Use at your own risk!

## 20.5.1. High-level interface

`urllib.``urlopen`(url[, data[, proxies[, context]]])

  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()`, `getcode()` 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()`, `getcode()` and `geturl()` methods, these methods have the same interface as for file objects — see section File Objects 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.

  The `getcode()` method returns the HTTP status code that was sent with the response, or `None` if the URL is no HTTP 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`, or `ftp_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
      ...

  The `no_proxy` environment variable can be used to specify hosts which shouldn’t be reached via proxy; if set, it should be a comma-separated list of hostname suffixes, optionally with `:port` appended, for example `cern.ch,ncsa.uiuc.edu,some.host:8080`.

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

  In a Mac OS X environment, `urlopen()` will retrieve proxy information from the OS X System Configuration Framework, which can be managed with Network System Preferences panel.

  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)

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

  The context parameter may be set to a `ssl.SSLContext` instance to configure the SSL settings that are used if `urlopen()` makes a HTTPS connection.

  在 2.3 版更改: Added the proxies support.

  在 2.6 版更改: Added `getcode()` to returned object and support for the `no_proxy` environment variable.

  在 2.7.9 版更改: The context parameter was added. All the neccessary certificate and hostname checks are done by default.

  2.6 版后已移除: The `urlopen()` function has been removed in Python 3 in favor of `urllib2.urlopen()`.

`urllib.``urlretrieve`(url[, filename[, reporthook[, data[, context]]]])

  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 callable that will be called once on establishment of the network connection and once after each block read thereafter. The callable 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.

  The context parameter may be set to a `ssl.SSLContext` instance to configure the SSL settings that are used if `urlretrieve()` makes a HTTPS connection.

  在 2.5 版更改: `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.

  在 2.7.9 版更改: The context parameter was added. All the neccessary certificate and hostname checks are done by default.

`urllib.``_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()

`urllib.``urlcleanup`()

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

## 20.5.2. Utility functions

`urllib.``quote`(string[, safe])

  Replace special characters in string using the `%xx` escape. Letters, digits, and the characters `'_.-'` are never quoted. By default, this function is intended for quoting the path section of the URL. The optional safe parameter specifies additional characters that should not be quoted — its default value is `'/'`.

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

`urllib.``quote_plus`(string[, safe])

  Like `quote()`, but also replaces spaces by plus signs, as required for quoting HTML form values when building up a query string to go into a URL. Plus signs in the original string are escaped unless they are included in safe. It also does not have safe default to `'/'`.

`urllib.``unquote`(string)

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

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

`urllib.``unquote_plus`(string)

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

`urllib.``urlencode`(query[, doseq])

  Convert a mapping object or a sequence of two-element tuples to a “percent-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. 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 value element in itself can be a sequence and in that case, if the optional parameter doseq is evaluates to `True`, individual `key=value` pairs separated by `'&'` are generated for each element of the value sequence for the key. The order of parameters in the encoded string will match the order of parameter tuples in the sequence. The `urlparse` module provides the functions `parse_qs()` and `parse_qsl()` which are used to parse query strings into Python data structures.

`urllib.``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.

`urllib.``url2pathname`(path)

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

`urllib.``getproxies`()

  This helper function returns a dictionary of scheme to proxy server URL mappings. It scans the environment for variables named `<scheme>_proxy`, in case insensitive way, for all operating systems first, and when it cannot find it, looks for proxy information from Mac OSX System Configuration for Mac OS X and Windows Systems Registry for Windows. If both lowercase and uppercase environment variables exist (and disagree), lowercase is preferred.

  注解

  If the environment variable `REQUEST_METHOD` is set, which usually indicates your script is running in a CGI environment, the environment variable `HTTP_PROXY` (uppercase `_PROXY`) will be ignored. This is because that variable can be injected by a client using the “Proxy:” HTTP header. If you need to use an HTTP proxy in a CGI environment, either use `ProxyHandler` explicitly, or make sure the variable name is in lowercase (or at least the `_proxy` suffix).

注解

urllib also exposes certain utility functions like splittype, splithost and others parsing URL into various components. But it is recommended to use `urlparse` for parsing URLs rather than using these functions directly. Python 3 does not expose these helper functions from `urllib.parse` module.

## 20.5.3. URL Opener objects

class `urllib.``URLopener`([proxies[, context[, **x509]]])

  Base class for opening and reading URLs. Unless you need to support opening objects using schemes other than `http:`, `ftp:`, 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.

  The context parameter may be a `ssl.SSLContext` instance. If given, it defines the SSL settings the opener uses to make HTTPS connections.

  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.

  在 2.7.9 版更改: The context parameter was added. All the neccessary certificate and hostname checks are done by default.

  `open`(fullurl[, data])

    Open fullurl using the appropriate protocol. This method sets up cache and proxy information, then calls the appropriate open method with its input arguments. If the scheme is not recognized, `open_unknown()` is called. The data argument has the same meaning as the data argument of `urlopen()`.

  `open_unknown`(fullurl[, data])

    Overridable interface to open unknown URL types.

  `retrieve`(url[, filename[, reporthook[, data]]])

    Retrieves the contents of url and places it in filename. The return value is a tuple consisting of a local filename and either a `mimetools.Message` object containing the response headers (for remote URLs) or `None` (for local URLs). The caller must then open and read the contents of filename. If filename is not given and the URL refers to a local file, the input filename is returned. If the URL is non-local and filename is not given, the filename is the output of `tempfile.mktemp()` with a suffix that matches the suffix of the last path component of the input URL. If reporthook is given, it must be a function accepting three numeric parameters. It will be called after each chunk of data is read from the network. reporthook is ignored for local URLs.

    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.

  `version`

    Variable that specifies the user agent of the opener object. To get `urllib` to tell servers that it is a particular user agent, set this in a subclass as a class variable or in the constructor before calling the base constructor.

class `urllib.``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.

  注解

  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`.

  注解

  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.

  The `FancyURLopener` class offers one additional method that should be overloaded to provide the appropriate behavior:

  `prompt_user_passwd`(host, realm)

    Return information needed to authenticate the user at the given host in the specified security realm. The return value should be a tuple, `(user, password)`, which can be used for basic authentication.

    The implementation prompts for this information on the terminal; an application should override this method to use an appropriate interaction model in the local environment.

exception `urllib.``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.

  2.5 新版功能.

## 20.5.4. `urllib` Restrictions

- Currently, only the following protocols are supported: HTTP, (versions 0.9 and 1.0), 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. 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`.

## 20.5.5. Examples

Here is an example session that uses the `GET` method to retrieve a URL containing parameters:

    >>> import urllib
    >>> params = urllib.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
    >>> f = urllib.urlopen("http://www.musi-cal.com/cgi-bin/query?%s" % params)
    >>> print f.read()

The following example uses the `POST` method instead:

    >>> import urllib
    >>> params = urllib.urlencode({'spam': 1, 'eggs': 2, 'bacon': 0})
    >>> f = urllib.urlopen("http://www.musi-cal.com/cgi-bin/query", params)
    >>> print f.read()

The following example uses an explicitly specified HTTP proxy, overriding environment settings:

    >>> import urllib
    >>> proxies = {'http': 'http://proxy.example.com:8080/'}
    >>> opener = urllib.FancyURLopener(proxies)
    >>> f = opener.open("http://www.python.org")
    >>> f.read()

The following example uses no proxies at all, overriding environment settings:

    >>> import urllib
    >>> opener = urllib.FancyURLopener({})
    >>> f = opener.open("http://www.python.org/")
    >>> f.read()