Doc/liburllib.tex

   1 \section{Standard Module \module{urllib}}
   2 \label{module-urllib}
   3 \stmodindex{urllib}
   4 \index{WWW}
   5 \index{World-Wide Web}
   6 \index{URL}
   7
   8
   9 This module provides a high-level interface for fetching data across
  10 the World-Wide Web.  In particular, the \function{urlopen()} function
  11 is similar to the built-in function \function{open()}, but accepts
  12 Universal Resource Locators (URLs) instead of filenames.  Some
  13 restrictions apply --- it can only open URLs for reading, and no seek
  14 operations are available.
  15
  16 It defines the following public functions:
  17
  18 \begin{funcdesc}{urlopen}{url}
  19 Open a network object denoted by a URL for reading.  If the URL does
  20 not have a scheme identifier, or if it has \file{file:} as its scheme
  21 identifier, this opens a local file; otherwise it opens a socket to a
  22 server somewhere on the network.  If the connection cannot be made, or
  23 if the server returns an error code, the \exception{IOError} exception
  24 is raised.  If all went well, a file-like object is returned.  This
  25 supports the following methods: \method{read()}, \method{readline()},
  26 \method{readlines()}, \method{fileno()}, \method{close()} and
  27 \method{info()}.
  28 Except for the last one, these methods have the same interface as for
  29 file objects --- see section \ref{bltin-file-objects} in this
  30 manual.  (It is not a built-in file object, however, so it can't be
  31 used at those few places where a true built-in file object is
  32 required.)
  33
  34 The \method{info()} method returns an instance of the class
  35 \class{mimetools.Message} containing the headers received from the
  36 server, if the protocol uses such headers (currently the only
  37 supported protocol that uses this is HTTP).  See the description of
  38 the \module{mimetools}\refstmodindex{mimetools} module.
  39 \end{funcdesc}
  40
  41 \begin{funcdesc}{urlretrieve}{url}
  42 Copy a network object denoted by a URL to a local file, if necessary.
  43 If the URL points to a local file, or a valid cached copy of the
  44 object exists, the object is not copied.  Return a tuple
  45 \code{(\var{filename}, \var{headers})} where \var{filename} is the
  46 local file name under which the object can be found, and \var{headers}
  47 is either \code{None} (for a local object) or whatever the
  48 \method{info()} method of the object returned by \function{urlopen()}
  49 returned (for a remote object, possibly cached).  Exceptions are the
  50 same as for \function{urlopen()}.
  51 \end{funcdesc}
  52
  53 \begin{funcdesc}{urlcleanup}{}
  54 Clear the cache that may have been built up by previous calls to
  55 \function{urlretrieve()}.
  56 \end{funcdesc}
  57
  58 \begin{funcdesc}{quote}{string\optional{, addsafe}}
  59 Replace special characters in \var{string} using the \samp{\%xx} escape.
  60 Letters, digits, and the characters \character{_,.-} are never quoted.
  61 The optional \var{addsafe} parameter specifies additional characters
  62 that should not be quoted --- its default value is \code{'/'}.
  63
  64 Example: \code{quote('/\~connolly/')} yields \code{'/\%7econnolly/'}.
  65 \end{funcdesc}
  66
  67 \begin{funcdesc}{quote_plus}{string\optional{, addsafe}}
  68 Like \function{quote()}, but also replaces spaces by plus signs, as
  69 required for quoting HTML form values.
  70 \end{funcdesc}
  71
  72 \begin{funcdesc}{unquote}{string}
  73 Replace \samp{\%xx} escapes by their single-character equivalent.
  74
  75 Example: \code{unquote('/\%7Econnolly/')} yields \code{'/\~connolly/'}.
  76 \end{funcdesc}
  77
  78 \begin{funcdesc}{unquote_plus}{string}
  79 Like \function{unquote()}, but also replaces plus signs by spaces, as
  80 required for unquoting HTML form values.
  81 \end{funcdesc}
  82
  83 Restrictions:
  84
  85 \begin{itemize}
  86
  87 \item
  88 Currently, only the following protocols are supported: HTTP, (versions
  89 0.9 and 1.0), Gopher (but not Gopher-+), FTP, and local files.
  90 \indexii{HTTP}{protocol}
  91 \indexii{Gopher}{protocol}
  92 \indexii{FTP}{protocol}
  93
  94 \item
  95 The caching feature of \function{urlretrieve()} has been disabled
  96 until I find the time to hack proper processing of Expiration time
  97 headers.
  98
  99 \item
 100 There should be a function to query whether a particular URL is in
 101 the cache.
 102
 103 \item
 104 For backward compatibility, if a URL appears to point to a local file
 105 but the file can't be opened, the URL is re-interpreted using the FTP
 106 protocol.  This can sometimes cause confusing error messages.
 107
 108 \item
 109 The \function{urlopen()} and \function{urlretrieve()} functions can
 110 cause arbitrarily long delays while waiting for a network connection
 111 to be set up.  This means that it is difficult to build an interactive
 112 web client using these functions without using threads.
 113
 114 \item
 115 The data returned by \function{urlopen()} or \function{urlretrieve()}
 116 is the raw data returned by the server.  This may be binary data
 117 (e.g. an image), plain text or (for example) HTML.  The HTTP protocol
 118 provides type information in the reply header, which can be inspected
 119 by looking at the \code{content-type} header.  For the Gopher protocol,
 120 type information is encoded in the URL; there is currently no easy way
 121 to extract it.  If the returned data is HTML, you can use the module
 122 \module{htmllib}\refstmodindex{htmllib} to parse it.
 123 \index{HTML}
 124 \indexii{HTTP}{protocol}
 125 \indexii{Gopher}{protocol}
 126
 127 \item
 128 Although the \module{urllib} module contains (undocumented) routines
 129 to parse and unparse URL strings, the recommended interface for URL
 130 manipulation is in module \module{urlparse}\refstmodindex{urlparse}.
 131
 132 \end{itemize}