495 lines
21 KiB
TeX
495 lines
21 KiB
TeX
\section{\module{urllib} ---
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Open arbitrary resources by URL}
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\declaremodule{standard}{urllib}
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\modulesynopsis{Open an arbitrary network resource by URL (requires sockets).}
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\index{WWW}
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\index{World Wide Web}
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\index{URL}
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This module provides a high-level interface for fetching data across
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the World Wide Web. In particular, the \function{urlopen()} function
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is similar to the built-in function \function{open()}, but accepts
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Universal Resource Locators (URLs) instead of filenames. Some
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restrictions apply --- it can only open URLs for reading, and no seek
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operations are available.
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It defines the following public functions:
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\begin{funcdesc}{urlopen}{url\optional{, data\optional{, proxies}}}
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Open a network object denoted by a URL for reading. If the URL does
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not have a scheme identifier, or if it has \file{file:} as its scheme
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identifier, this opens a local file (without universal newlines);
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otherwise it opens a socket to a server somewhere on the network. If
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the connection cannot be made
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the \exception{IOError} exception is raised. If all went well, a
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file-like object is returned. This supports the following methods:
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\method{read()}, \method{readline()}, \method{readlines()}, \method{fileno()},
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\method{close()}, \method{info()} and \method{geturl()}. It also has
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proper support for the iterator protocol.
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One caveat: the \method{read()} method, if the size argument is
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omitted or negative, may not read until the end of the data stream;
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there is no good way to determine that the entire stream from a socket
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has been read in the general case.
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Except for the \method{info()} and \method{geturl()} methods,
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these methods have the same interface as for
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file objects --- see section \ref{bltin-file-objects} in this
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manual. (It is not a built-in file object, however, so it can't be
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used at those few places where a true built-in file object is
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required.)
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The \method{info()} method returns an instance of the class
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\class{mimetools.Message} containing meta-information associated
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with the URL. When the method is HTTP, these headers are those
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returned by the server at the head of the retrieved HTML page
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(including Content-Length and Content-Type). When the method is FTP,
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a Content-Length header will be present if (as is now usual) the
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server passed back a file length in response to the FTP retrieval
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request. A Content-Type header will be present if the MIME type can
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be guessed. When the method is local-file, returned headers will include
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a Date representing the file's last-modified time, a Content-Length
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giving file size, and a Content-Type containing a guess at the file's
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type. See also the description of the
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\refmodule{mimetools}\refstmodindex{mimetools} module.
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The \method{geturl()} method returns the real URL of the page. In
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some cases, the HTTP server redirects a client to another URL. The
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\function{urlopen()} function handles this transparently, but in some
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cases the caller needs to know which URL the client was redirected
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to. The \method{geturl()} method can be used to get at this
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redirected URL.
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If the \var{url} uses the \file{http:} scheme identifier, the optional
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\var{data} argument may be given to specify a \code{POST} request
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(normally the request type is \code{GET}). The \var{data} argument
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must be in standard \mimetype{application/x-www-form-urlencoded} format;
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see the \function{urlencode()} function below.
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The \function{urlopen()} function works transparently with proxies
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which do not require authentication. In a \UNIX{} or Windows
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environment, set the \envvar{http_proxy}, or \envvar{ftp_proxy}
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environment variables to a URL that identifies
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the proxy server before starting the Python interpreter. For example
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(the \character{\%} is the command prompt):
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\begin{verbatim}
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% http_proxy="http://www.someproxy.com:3128"
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% export http_proxy
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% python
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...
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\end{verbatim}
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In a Windows environment, if no proxy environment variables are set,
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proxy settings are obtained from the registry's Internet Settings
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section.
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In a Macintosh environment, \function{urlopen()} will retrieve proxy
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information from Internet\index{Internet Config} Config.
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Alternatively, the optional \var{proxies} argument may be used to
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explicitly specify proxies. It must be a dictionary mapping scheme
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names to proxy URLs, where an empty dictionary causes no proxies to be
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used, and \code{None} (the default value) causes environmental proxy
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settings to be used as discussed above. For example:
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\begin{verbatim}
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# Use http://www.someproxy.com:3128 for http proxying
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proxies = {'http': 'http://www.someproxy.com:3128'}
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filehandle = urllib.urlopen(some_url, proxies=proxies)
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# Don't use any proxies
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filehandle = urllib.urlopen(some_url, proxies={})
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# Use proxies from environment - both versions are equivalent
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filehandle = urllib.urlopen(some_url, proxies=None)
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filehandle = urllib.urlopen(some_url)
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\end{verbatim}
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The \function{urlopen()} function does not support explicit proxy
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specification. If you need to override environmental proxy settings,
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use \class{URLopener}, or a subclass such as \class{FancyURLopener}.
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Proxies which require authentication for use are not currently
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supported; this is considered an implementation limitation.
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\versionchanged[Added the \var{proxies} support]{2.3}
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\end{funcdesc}
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\begin{funcdesc}{urlretrieve}{url\optional{, filename\optional{,
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reporthook\optional{, data}}}}
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Copy a network object denoted by a URL to a local file, if necessary.
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If the URL points to a local file, or a valid cached copy of the
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object exists, the object is not copied. Return a tuple
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\code{(\var{filename}, \var{headers})} where \var{filename} is the
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local file name under which the object can be found, and \var{headers}
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is whatever the \method{info()} method of the object returned by
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\function{urlopen()} returned (for a remote object, possibly cached).
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Exceptions are the same as for \function{urlopen()}.
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The second argument, if present, specifies the file location to copy
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to (if absent, the location will be a tempfile with a generated name).
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The third argument, if present, is a hook function that will be called
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once on establishment of the network connection and once after each
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block read thereafter. The hook will be passed three arguments; a
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count of blocks transferred so far, a block size in bytes, and the
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total size of the file. The third argument may be \code{-1} on older
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FTP servers which do not return a file size in response to a retrieval
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request.
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If the \var{url} uses the \file{http:} scheme identifier, the optional
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\var{data} argument may be given to specify a \code{POST} request
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(normally the request type is \code{GET}). The \var{data} argument
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must in standard \mimetype{application/x-www-form-urlencoded} format;
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see the \function{urlencode()} function below.
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\versionchanged[
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\function{urlretrieve()} will raise \exception{ContentTooShortError}
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when it detects that the amount of data available
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was less than the expected amount (which is the size reported by a
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\var{Content-Length} header). This can occur, for example, when the
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download is interrupted.
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The \var{Content-Length} is treated as a lower bound: if there's more data
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to read, urlretrieve reads more data, but if less data is available,
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it raises the exception.
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You can still retrieve the downloaded data in this case, it is stored
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in the \member{content} attribute of the exception instance.
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If no \var{Content-Length} header was supplied, urlretrieve can
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not check the size of the data it has downloaded, and just returns it.
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In this case you just have to assume that the download was successful]{2.5}
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\end{funcdesc}
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\begin{datadesc}{_urlopener}
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The public functions \function{urlopen()} and
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\function{urlretrieve()} create an instance of the
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\class{FancyURLopener} class and use it to perform their requested
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actions. To override this functionality, programmers can create a
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subclass of \class{URLopener} or \class{FancyURLopener}, then assign
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an instance of that class to the
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\code{urllib._urlopener} variable before calling the desired function.
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For example, applications may want to specify a different
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\mailheader{User-Agent} header than \class{URLopener} defines. This
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can be accomplished with the following code:
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\begin{verbatim}
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import urllib
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class AppURLopener(urllib.FancyURLopener):
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version = "App/1.7"
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urllib._urlopener = AppURLopener()
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\end{verbatim}
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\end{datadesc}
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\begin{funcdesc}{urlcleanup}{}
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Clear the cache that may have been built up by previous calls to
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\function{urlretrieve()}.
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\end{funcdesc}
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\begin{funcdesc}{quote}{string\optional{, safe}}
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Replace special characters in \var{string} using the \samp{\%xx} escape.
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Letters, digits, and the characters \character{_.-} are never quoted.
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The optional \var{safe} parameter specifies additional characters
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that should not be quoted --- its default value is \code{'/'}.
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Example: \code{quote('/\~{}connolly/')} yields \code{'/\%7econnolly/'}.
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\end{funcdesc}
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\begin{funcdesc}{quote_plus}{string\optional{, safe}}
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Like \function{quote()}, but also replaces spaces by plus signs, as
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required for quoting HTML form values. Plus signs in the original
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string are escaped unless they are included in \var{safe}. It also
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does not have \var{safe} default to \code{'/'}.
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\end{funcdesc}
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\begin{funcdesc}{unquote}{string}
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Replace \samp{\%xx} escapes by their single-character equivalent.
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Example: \code{unquote('/\%7Econnolly/')} yields \code{'/\~{}connolly/'}.
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\end{funcdesc}
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\begin{funcdesc}{unquote_plus}{string}
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Like \function{unquote()}, but also replaces plus signs by spaces, as
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required for unquoting HTML form values.
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\end{funcdesc}
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\begin{funcdesc}{urlencode}{query\optional{, doseq}}
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Convert a mapping object or a sequence of two-element tuples to a
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``url-encoded'' string, suitable to pass to
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\function{urlopen()} above as the optional \var{data} argument. This
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is useful to pass a dictionary of form fields to a \code{POST}
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request. The resulting string is a series of
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\code{\var{key}=\var{value}} pairs separated by \character{\&}
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characters, where both \var{key} and \var{value} are quoted using
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\function{quote_plus()} above. If the optional parameter \var{doseq} is
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present and evaluates to true, individual \code{\var{key}=\var{value}} pairs
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are generated for each element of the sequence.
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When a sequence of two-element tuples is used as the \var{query} argument,
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the first element of each tuple is a key and the second is a value. The
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order of parameters in the encoded string will match the order of parameter
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tuples in the sequence.
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The \refmodule{cgi} module provides the functions
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\function{parse_qs()} and \function{parse_qsl()} which are used to
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parse query strings into Python data structures.
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\end{funcdesc}
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\begin{funcdesc}{pathname2url}{path}
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Convert the pathname \var{path} from the local syntax for a path to
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the form used in the path component of a URL. This does not produce a
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complete URL. The return value will already be quoted using the
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\function{quote()} function.
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\end{funcdesc}
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\begin{funcdesc}{url2pathname}{path}
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Convert the path component \var{path} from an encoded URL to the local
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syntax for a path. This does not accept a complete URL. This
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function uses \function{unquote()} to decode \var{path}.
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\end{funcdesc}
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\begin{classdesc}{URLopener}{\optional{proxies\optional{, **x509}}}
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Base class for opening and reading URLs. Unless you need to support
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opening objects using schemes other than \file{http:}, \file{ftp:},
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or \file{file:}, you probably want to use
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\class{FancyURLopener}.
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By default, the \class{URLopener} class sends a
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\mailheader{User-Agent} header of \samp{urllib/\var{VVV}}, where
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\var{VVV} is the \module{urllib} version number. Applications can
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define their own \mailheader{User-Agent} header by subclassing
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\class{URLopener} or \class{FancyURLopener} and setting the class
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attribute \member{version} to an appropriate string value in the
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subclass definition.
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The optional \var{proxies} parameter should be a dictionary mapping
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scheme names to proxy URLs, where an empty dictionary turns proxies
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off completely. Its default value is \code{None}, in which case
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environmental proxy settings will be used if present, as discussed in
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the definition of \function{urlopen()}, above.
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Additional keyword parameters, collected in \var{x509}, may be used for
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authentication of the client when using the \file{https:} scheme. The keywords
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\var{key_file} and \var{cert_file} are supported to provide an
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SSL key and certificate; both are needed to support client authentication.
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\class{URLopener} objects will raise an \exception{IOError} exception
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if the server returns an error code.
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\end{classdesc}
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\begin{classdesc}{FancyURLopener}{...}
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\class{FancyURLopener} subclasses \class{URLopener} providing default
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handling for the following HTTP response codes: 301, 302, 303, 307 and
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401. For the 30x response codes listed above, the
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\mailheader{Location} header is used to fetch the actual URL. For 401
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response codes (authentication required), basic HTTP authentication is
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performed. For the 30x response codes, recursion is bounded by the
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value of the \var{maxtries} attribute, which defaults to 10.
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For all other response codes, the method \method{http_error_default()}
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is called which you can override in subclasses to handle the error
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appropriately.
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\note{According to the letter of \rfc{2616}, 301 and 302 responses to
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POST requests must not be automatically redirected without
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confirmation by the user. In reality, browsers do allow automatic
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redirection of these responses, changing the POST to a GET, and
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\module{urllib} reproduces this behaviour.}
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The parameters to the constructor are the same as those for
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\class{URLopener}.
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\note{When performing basic authentication, a
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\class{FancyURLopener} instance calls its
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\method{prompt_user_passwd()} method. The default implementation asks
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the users for the required information on the controlling terminal. A
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subclass may override this method to support more appropriate behavior
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if needed.}
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\end{classdesc}
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\begin{excclassdesc}{ContentTooShortError}{msg\optional{, content}}
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This exception is raised when the \function{urlretrieve()} function
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detects that the amount of the downloaded data is less than the
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expected amount (given by the \var{Content-Length} header). The
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\member{content} attribute stores the downloaded (and supposedly
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truncated) data.
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\versionadded{2.5}
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\end{excclassdesc}
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Restrictions:
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\begin{itemize}
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\item
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Currently, only the following protocols are supported: HTTP, (versions
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0.9 and 1.0), FTP, and local files.
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\indexii{HTTP}{protocol}
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\indexii{FTP}{protocol}
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\item
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The caching feature of \function{urlretrieve()} has been disabled
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until I find the time to hack proper processing of Expiration time
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headers.
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\item
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There should be a function to query whether a particular URL is in
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the cache.
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\item
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For backward compatibility, if a URL appears to point to a local file
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but the file can't be opened, the URL is re-interpreted using the FTP
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protocol. This can sometimes cause confusing error messages.
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\item
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The \function{urlopen()} and \function{urlretrieve()} functions can
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cause arbitrarily long delays while waiting for a network connection
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to be set up. This means that it is difficult to build an interactive
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Web client using these functions without using threads.
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\item
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The data returned by \function{urlopen()} or \function{urlretrieve()}
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is the raw data returned by the server. This may be binary data
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(such as an image), plain text or (for example) HTML\index{HTML}. The
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HTTP\indexii{HTTP}{protocol} protocol provides type information in the
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reply header, which can be inspected by looking at the
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\mailheader{Content-Type} header. If the
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returned data is HTML, you can use the module
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\refmodule{htmllib}\refstmodindex{htmllib} to parse it.
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\item
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The code handling the FTP\index{FTP} protocol cannot differentiate
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between a file and a directory. This can lead to unexpected behavior
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when attempting to read a URL that points to a file that is not
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accessible. If the URL ends in a \code{/}, it is assumed to refer to
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a directory and will be handled accordingly. But if an attempt to
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read a file leads to a 550 error (meaning the URL cannot be found or
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is not accessible, often for permission reasons), then the path is
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treated as a directory in order to handle the case when a directory is
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specified by a URL but the trailing \code{/} has been left off. This can
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cause misleading results when you try to fetch a file whose read
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permissions make it inaccessible; the FTP code will try to read it,
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fail with a 550 error, and then perform a directory listing for the
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unreadable file. If fine-grained control is needed, consider using the
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\module{ftplib} module, subclassing \class{FancyURLOpener}, or changing
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\var{_urlopener} to meet your needs.
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\item
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This module does not support the use of proxies which require
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authentication. This may be implemented in the future.
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\item
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Although the \module{urllib} module contains (undocumented) routines
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to parse and unparse URL strings, the recommended interface for URL
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manipulation is in module \refmodule{urlparse}\refstmodindex{urlparse}.
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\end{itemize}
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\subsection{URLopener Objects \label{urlopener-objs}}
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\sectionauthor{Skip Montanaro}{skip@mojam.com}
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\class{URLopener} and \class{FancyURLopener} objects have the
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following attributes.
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\begin{methoddesc}[URLopener]{open}{fullurl\optional{, data}}
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Open \var{fullurl} using the appropriate protocol. This method sets
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up cache and proxy information, then calls the appropriate open method with
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its input arguments. If the scheme is not recognized,
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\method{open_unknown()} is called. The \var{data} argument
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has the same meaning as the \var{data} argument of \function{urlopen()}.
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\end{methoddesc}
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\begin{methoddesc}[URLopener]{open_unknown}{fullurl\optional{, data}}
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Overridable interface to open unknown URL types.
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\end{methoddesc}
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\begin{methoddesc}[URLopener]{retrieve}{url\optional{,
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filename\optional{,
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reporthook\optional{, data}}}}
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Retrieves the contents of \var{url} and places it in \var{filename}. The
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return value is a tuple consisting of a local filename and either a
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\class{mimetools.Message} object containing the response headers (for remote
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URLs) or \code{None} (for local URLs). The caller must then open and read the
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contents of \var{filename}. If \var{filename} is not given and the URL
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refers to a local file, the input filename is returned. If the URL is
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non-local and \var{filename} is not given, the filename is the output of
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\function{tempfile.mktemp()} with a suffix that matches the suffix of the last
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path component of the input URL. If \var{reporthook} is given, it must be
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a function accepting three numeric parameters. It will be called after each
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chunk of data is read from the network. \var{reporthook} is ignored for
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local URLs.
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If the \var{url} uses the \file{http:} scheme identifier, the optional
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\var{data} argument may be given to specify a \code{POST} request
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(normally the request type is \code{GET}). The \var{data} argument
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must in standard \mimetype{application/x-www-form-urlencoded} format;
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see the \function{urlencode()} function below.
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\end{methoddesc}
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\begin{memberdesc}[URLopener]{version}
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Variable that specifies the user agent of the opener object. To get
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\refmodule{urllib} to tell servers that it is a particular user agent,
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set this in a subclass as a class variable or in the constructor
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before calling the base constructor.
|
|
\end{memberdesc}
|
|
|
|
The \class{FancyURLopener} class offers one additional method that
|
|
should be overloaded to provide the appropriate behavior:
|
|
|
|
\begin{methoddesc}[FancyURLopener]{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,
|
|
\code{(\var{user}, \var{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.
|
|
\end{methoddesc}
|
|
|
|
|
|
\subsection{Examples}
|
|
\nodename{Urllib Examples}
|
|
|
|
Here is an example session that uses the \samp{GET} method to retrieve
|
|
a URL containing parameters:
|
|
|
|
\begin{verbatim}
|
|
>>> 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()
|
|
\end{verbatim}
|
|
|
|
The following example uses the \samp{POST} method instead:
|
|
|
|
\begin{verbatim}
|
|
>>> 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()
|
|
\end{verbatim}
|
|
|
|
The following example uses an explicitly specified HTTP proxy,
|
|
overriding environment settings:
|
|
|
|
\begin{verbatim}
|
|
>>> import urllib
|
|
>>> proxies = {'http': 'http://proxy.example.com:8080/'}
|
|
>>> opener = urllib.FancyURLopener(proxies)
|
|
>>> f = opener.open("http://www.python.org")
|
|
>>> f.read()
|
|
\end{verbatim}
|
|
|
|
The following example uses no proxies at all, overriding environment
|
|
settings:
|
|
|
|
\begin{verbatim}
|
|
>>> import urllib
|
|
>>> opener = urllib.FancyURLopener({})
|
|
>>> f = opener.open("http://www.python.org/")
|
|
>>> f.read()
|
|
\end{verbatim}
|