Doc/lib/liburllib.tex

   1 \section{\module{urllib} ---
   2          Open an arbitrary object given by URL.}
   3 \declaremodule{standard}{urllib}
   4
   5 \modulesynopsis{Open an arbitrary object given by URL (requires sockets).}
   6
   7 \index{WWW}
   8 \index{World-Wide Web}
   9 \index{URL}
  10
  11
  12 This module provides a high-level interface for fetching data across
  13 the World-Wide Web.  In particular, the \function{urlopen()} function
  14 is similar to the built-in function \function{open()}, but accepts
  15 Universal Resource Locators (URLs) instead of filenames.  Some
  16 restrictions apply --- it can only open URLs for reading, and no seek
  17 operations are available.
  18
  19 It defines the following public functions:
  20
  21 \begin{funcdesc}{urlopen}{url\optional{, data}}
  22 Open a network object denoted by a URL for reading.  If the URL does
  23 not have a scheme identifier, or if it has \file{file:} as its scheme
  24 identifier, this opens a local file; otherwise it opens a socket to a
  25 server somewhere on the network.  If the connection cannot be made, or
  26 if the server returns an error code, the \exception{IOError} exception
  27 is raised.  If all went well, a file-like object is returned.  This
  28 supports the following methods: \method{read()}, \method{readline()},
  29 \method{readlines()}, \method{fileno()}, \method{close()},
  30 \method{info()} and \method{geturl()}.
  31
  32 Except for the \method{info()} and \method{geturl()} methods,
  33 these methods have the same interface as for
  34 file objects --- see section \ref{bltin-file-objects} in this
  35 manual.  (It is not a built-in file object, however, so it can't be
  36 used at those few places where a true built-in file object is
  37 required.)
  38
  39 The \method{info()} method returns an instance of the class
  40 \class{mimetools.Message} containing meta-information associated
  41 with the URL.  When the method is HTTP, these headers are those
  42 returned by the server at the head of the retrieved HTML page
  43 (including Content-Length and Content-Type).  When the method is FTP,
  44 a Content-Length header will be present if (as is now usual) the
  45 server passed back a file length in response to the FTP retrieval
  46 request.  When the method is local-file, returned headers will include
  47 a Date representing the file's last-modified time, a Content-Length
  48 giving file size, and a Content-Type containing a guess at the file's
  49 type. See also the description of the
  50 \refmodule{mimetools}\refstmodindex{mimetools} module.
  51
  52 The \method{geturl()} method returns the real URL of the page.  In
  53 some cases, the HTTP server redirects a client to another URL.  The
  54 \function{urlopen()} function handles this transparently, but in some
  55 cases the caller needs to know which URL the client was redirected
  56 to.  The \method{geturl()} method can be used to get at this
  57 redirected URL.
  58
  59 If the \var{url} uses the \file{http:} scheme identifier, the optional
  60 \var{data} argument may be given to specify a \code{POST} request
  61 (normally the request type is \code{GET}).  The \var{data} argument
  62 must in standard \file{application/x-www-form-urlencoded} format;
  63 see the \function{urlencode()} function below.
  64
  65 \end{funcdesc}
  66
  67 \begin{funcdesc}{urlretrieve}{url\optional{, filename\optional{, hook}}}
  68 Copy a network object denoted by a URL to a local file, if necessary.
  69 If the URL points to a local file, or a valid cached copy of the
  70 object exists, the object is not copied.  Return a tuple
  71 \code{(\var{filename}, \var{headers})} where \var{filename} is the
  72 local file name under which the object can be found, and \var{headers}
  73 is either \code{None} (for a local object) or whatever the
  74 \method{info()} method of the object returned by \function{urlopen()}
  75 returned (for a remote object, possibly cached).  Exceptions are the
  76 same as for \function{urlopen()}.
  77
  78 The second argument, if present, specifies the file location to copy
  79 to (if absent, the location will be a tempfile with a generated name).
  80 The third argument, if present, is a hook function that will be called
  81 once on establishment of the network connection and once after each
  82 block read thereafter.  The hook will be passed three arguments; a
  83 count of blocks transferred so far, a block size in bytes, and the
  84 total size of the file.  The third argument may be \code{-1} on older
  85 FTP servers which do not return a file size in response to a retrieval
  86 request.
  87 \end{funcdesc}
  88
  89 \begin{funcdesc}{urlcleanup}{}
  90 Clear the cache that may have been built up by previous calls to
  91 \function{urlretrieve()}.
  92 \end{funcdesc}
  93
  94 \begin{funcdesc}{quote}{string\optional{, safe}}
  95 Replace special characters in \var{string} using the \samp{\%xx} escape.
  96 Letters, digits, and the characters \character{_,.-} are never quoted.
  97 The optional \var{safe} parameter specifies additional characters
  98 that should not be quoted --- its default value is \code{'/'}.
  99
 100 Example: \code{quote('/\~connolly/')} yields \code{'/\%7econnolly/'}.
 101 \end{funcdesc}
 102
 103 \begin{funcdesc}{quote_plus}{string\optional{, safe}}
 104 Like \function{quote()}, but also replaces spaces by plus signs, as
 105 required for quoting HTML form values.  Plus signs in the original
 106 string are escaped unless they are included in \var{safe}.
 107 \end{funcdesc}
 108
 109 \begin{funcdesc}{unquote}{string}
 110 Replace \samp{\%xx} escapes by their single-character equivalent.
 111
 112 Example: \code{unquote('/\%7Econnolly/')} yields \code{'/\~connolly/'}.
 113 \end{funcdesc}
 114
 115 \begin{funcdesc}{unquote_plus}{string}
 116 Like \function{unquote()}, but also replaces plus signs by spaces, as
 117 required for unquoting HTML form values.
 118 \end{funcdesc}
 119
 120 \begin{funcdesc}{urlencode}{dict}
 121 Convert a dictionary to a ``url-encoded'' string, suitable to pass to
 122 \function{urlopen()} above as the optional \var{data} argument.  This
 123 is useful to pass a dictionary of form fields to a \code{POST}
 124 request.  The resulting string is a series of
 125 \code{\var{key}=\var{value}} pairs separated by \character{\&}
 126 characters, where both \var{key} and \var{value} are quoted using
 127 \function{quote_plus()} above.
 128 \end{funcdesc}
 129
 130 Restrictions:
 131
 132 \begin{itemize}
 133
 134 \item
 135 Currently, only the following protocols are supported: HTTP, (versions
 136 0.9 and 1.0), Gopher (but not Gopher-+), FTP, and local files.
 137 \indexii{HTTP}{protocol}
 138 \indexii{Gopher}{protocol}
 139 \indexii{FTP}{protocol}
 140
 141 \item
 142 The caching feature of \function{urlretrieve()} has been disabled
 143 until I find the time to hack proper processing of Expiration time
 144 headers.
 145
 146 \item
 147 There should be a function to query whether a particular URL is in
 148 the cache.
 149
 150 \item
 151 For backward compatibility, if a URL appears to point to a local file
 152 but the file can't be opened, the URL is re-interpreted using the FTP
 153 protocol.  This can sometimes cause confusing error messages.
 154
 155 \item
 156 The \function{urlopen()} and \function{urlretrieve()} functions can
 157 cause arbitrarily long delays while waiting for a network connection
 158 to be set up.  This means that it is difficult to build an interactive
 159 web client using these functions without using threads.
 160
 161 \item
 162 The data returned by \function{urlopen()} or \function{urlretrieve()}
 163 is the raw data returned by the server.  This may be binary data
 164 (e.g. an image), plain text or (for example) HTML\index{HTML}.  The
 165 HTTP\indexii{HTTP}{protocol} protocol provides type information in the
 166 reply header, which can be inspected by looking at the
 167 \code{content-type} header.  For the Gopher\indexii{Gopher}{protocol}
 168 protocol, type information is encoded in the URL; there is currently
 169 no easy way to extract it.  If the returned data is HTML, you can use
 170 the module \refmodule{htmllib}\refstmodindex{htmllib} to parse it.
 171
 172 \item
 173 Although the \module{urllib} module contains (undocumented) routines
 174 to parse and unparse URL strings, the recommended interface for URL
 175 manipulation is in module \refmodule{urlparse}\refstmodindex{urlparse}.
 176
 177 \end{itemize}