"ftplib" --- FTP 协议客户端
***************************

**源代码：** Lib/ftplib.py

======================================================================

This module defines the class "FTP" and a few related items. The "FTP"
class implements the client side of the FTP protocol.  You can use
this to write Python programs that perform a variety of automated FTP
jobs, such as mirroring other FTP servers.  It is also used by the
module "urllib.request" to handle URLs that use FTP.  For more
information on FTP (File Transfer Protocol), see Internet **RFC 959**.

The default encoding is UTF-8, following **RFC 2640**.

Here's a sample session using the "ftplib" module:

   >>> from ftplib import FTP
   >>> ftp = FTP('ftp.us.debian.org')  # connect to host, default port
   >>> ftp.login()                     # user anonymous, passwd anonymous@
   '230 Login successful.'
   >>> ftp.cwd('debian')               # change into "debian" directory
   >>> ftp.retrlines('LIST')           # list directory contents
   -rw-rw-r--    1 1176     1176         1063 Jun 15 10:18 README
   ...
   drwxr-sr-x    5 1176     1176         4096 Dec 19  2000 pool
   drwxr-sr-x    4 1176     1176         4096 Nov 17  2008 project
   drwxr-xr-x    3 1176     1176         4096 Oct 10  2012 tools
   '226 Directory send OK.'
   >>> with open('README', 'wb') as fp:
   >>>     ftp.retrbinary('RETR README', fp.write)
   '226 Transfer complete.'
   >>> ftp.quit()

这个模块定义了以下内容：

class ftplib.FTP(host='', user='', passwd='', acct='', timeout=None, source_address=None, *, encoding='utf-8')

   Return a new instance of the "FTP" class.  When *host* is given,
   the method call "connect(host)" is made.  When *user* is given,
   additionally the method call "login(user, passwd, acct)" is made
   (where *passwd* and *acct* default to the empty string when not
   given).  The optional *timeout* parameter specifies a timeout in
   seconds for blocking operations like the connection attempt (if is
   not specified, the global default timeout setting will be used).
   *source_address* is a 2-tuple "(host, port)" for the socket to bind
   to as its source address before connecting. The *encoding*
   parameter specifies the encoding for directories and filenames.

   The "FTP" class supports the "with" statement, e.g.:

   >>> from ftplib import FTP
   >>> with FTP("ftp1.at.proftpd.org") as ftp:
   ...     ftp.login()
   ...     ftp.dir()
   ... 
   '230 Anonymous login ok, restrictions apply.'
   dr-xr-xr-x   9 ftp      ftp           154 May  6 10:43 .
   dr-xr-xr-x   9 ftp      ftp           154 May  6 10:43 ..
   dr-xr-xr-x   5 ftp      ftp          4096 May  6 10:43 CentOS
   dr-xr-xr-x   3 ftp      ftp            18 Jul 10  2008 Fedora
   >>>

   3.2 版更變: 支持了 "with" 语句。

   3.3 版更變: *source_address* parameter was added.

   3.9 版更變: If the *timeout* parameter is set to be zero, it will
   raise a "ValueError" to prevent the creation of a non-blocking
   socket. The *encoding* parameter was added, and the default was
   changed from Latin-1 to UTF-8 to follow **RFC 2640**.

class ftplib.FTP_TLS(host='', user='', passwd='', acct='', keyfile=None, certfile=None, context=None, timeout=None, source_address=None, *, encoding='utf-8')

   A "FTP" subclass which adds TLS support to FTP as described in
   **RFC 4217**. Connect as usual to port 21 implicitly securing the
   FTP control connection before authenticating. Securing the data
   connection requires the user to explicitly ask for it by calling
   the "prot_p()" method.  *context* is a "ssl.SSLContext" object
   which allows bundling SSL configuration options, certificates and
   private keys into a single (potentially long-lived) structure.
   Please read Security considerations for best practices.

   *keyfile* and *certfile* are a legacy alternative to *context* --
   they can point to PEM-formatted private key and certificate chain
   files (respectively) for the SSL connection.

   3.2 版新加入.

   3.3 版更變: *source_address* parameter was added.

   3.4 版更變: The class now supports hostname check with
   "ssl.SSLContext.check_hostname" and *Server Name Indication* (see
   "ssl.HAS_SNI").

   3.6 版後已棄用: *keyfile* and *certfile* are deprecated in favor of
   *context*. Please use "ssl.SSLContext.load_cert_chain()" instead,
   or let "ssl.create_default_context()" select the system's trusted
   CA certificates for you.

   3.9 版更變: If the *timeout* parameter is set to be zero, it will
   raise a "ValueError" to prevent the creation of a non-blocking
   socket. The *encoding* parameter was added, and the default was
   changed from Latin-1 to UTF-8 to follow **RFC 2640**.

   Here's a sample session using the "FTP_TLS" class:

      >>> ftps = FTP_TLS('ftp.pureftpd.org')
      >>> ftps.login()
      '230 Anonymous user logged in'
      >>> ftps.prot_p()
      '200 Data protection level set to "private"'
      >>> ftps.nlst()
      ['6jack', 'OpenBSD', 'antilink', 'blogbench', 'bsdcam', 'clockspeed', 'djbdns-jedi', 'docs', 'eaccelerator-jedi', 'favicon.ico', 'francotone', 'fugu', 'ignore', 'libpuzzle', 'metalog', 'minidentd', 'misc', 'mysql-udf-global-user-variables', 'php-jenkins-hash', 'php-skein-hash', 'php-webdav', 'phpaudit', 'phpbench', 'pincaster', 'ping', 'posto', 'pub', 'public', 'public_keys', 'pure-ftpd', 'qscan', 'qtc', 'sharedance', 'skycache', 'sound', 'tmp', 'ucarp']

exception ftplib.error_reply

   Exception raised when an unexpected reply is received from the
   server.

exception ftplib.error_temp

   Exception raised when an error code signifying a temporary error
   (response codes in the range 400--499) is received.

exception ftplib.error_perm

   Exception raised when an error code signifying a permanent error
   (response codes in the range 500--599) is received.

exception ftplib.error_proto

   Exception raised when a reply is received from the server that does
   not fit the response specifications of the File Transfer Protocol,
   i.e. begin with a digit in the range 1--5.

ftplib.all_errors

   The set of all exceptions (as a tuple) that methods of "FTP"
   instances may raise as a result of problems with the FTP connection
   (as opposed to programming errors made by the caller).  This set
   includes the four exceptions listed above as well as "OSError" and
   "EOFError".

也參考:

  Module "netrc"
     Parser for the ".netrc" file format.  The file ".netrc" is
     typically used by FTP clients to load user authentication
     information before prompting the user.


FTP Objects
===========

Several methods are available in two flavors: one for handling text
files and another for binary files.  These are named for the command
which is used followed by "lines" for the text version or "binary" for
the binary version.

"FTP" instances have the following methods:

FTP.set_debuglevel(level)

   Set the instance's debugging level.  This controls the amount of
   debugging output printed.  The default, "0", produces no debugging
   output.  A value of "1" produces a moderate amount of debugging
   output, generally a single line per request.  A value of "2" or
   higher produces the maximum amount of debugging output, logging
   each line sent and received on the control connection.

FTP.connect(host='', port=0, timeout=None, source_address=None)

   Connect to the given host and port.  The default port number is
   "21", as specified by the FTP protocol specification.  It is rarely
   needed to specify a different port number.  This function should be
   called only once for each instance; it should not be called at all
   if a host was given when the instance was created.  All other
   methods can only be used after a connection has been made. The
   optional *timeout* parameter specifies a timeout in seconds for the
   connection attempt. If no *timeout* is passed, the global default
   timeout setting will be used. *source_address* is a 2-tuple "(host,
   port)" for the socket to bind to as its source address before
   connecting.

   Raises an auditing event "ftplib.connect" with arguments "self",
   "host", "port".

   3.3 版更變: *source_address* parameter was added.

FTP.getwelcome()

   Return the welcome message sent by the server in reply to the
   initial connection.  (This message sometimes contains disclaimers
   or help information that may be relevant to the user.)

FTP.login(user='anonymous', passwd='', acct='')

   Log in as the given *user*.  The *passwd* and *acct* parameters are
   optional and default to the empty string.  If no *user* is
   specified, it defaults to "'anonymous'".  If *user* is
   "'anonymous'", the default *passwd* is "'anonymous@'".  This
   function should be called only once for each instance, after a
   connection has been established; it should not be called at all if
   a host and user were given when the instance was created.  Most FTP
   commands are only allowed after the client has logged in.  The
   *acct* parameter supplies "accounting information"; few systems
   implement this.

FTP.abort()

   Abort a file transfer that is in progress.  Using this does not
   always work, but it's worth a try.

FTP.sendcmd(cmd)

   Send a simple command string to the server and return the response
   string.

   Raises an auditing event "ftplib.sendcmd" with arguments "self",
   "cmd".

FTP.voidcmd(cmd)

   Send a simple command string to the server and handle the response.
   Return nothing if a response code corresponding to success (codes
   in the range 200--299) is received.  Raise "error_reply" otherwise.

   Raises an auditing event "ftplib.sendcmd" with arguments "self",
   "cmd".

FTP.retrbinary(cmd, callback, blocksize=8192, rest=None)

   Retrieve a file in binary transfer mode.  *cmd* should be an
   appropriate "RETR" command: "'RETR filename'". The *callback*
   function is called for each block of data received, with a single
   bytes argument giving the data block. The optional *blocksize*
   argument specifies the maximum chunk size to read on the low-level
   socket object created to do the actual transfer (which will also be
   the largest size of the data blocks passed to *callback*).  A
   reasonable default is chosen. *rest* means the same thing as in the
   "transfercmd()" method.

FTP.retrlines(cmd, callback=None)

   Retrieve a file or directory listing in the encoding specified by
   the *encoding* parameter at initialization. *cmd* should be an
   appropriate "RETR" command (see "retrbinary()") or a command such
   as "LIST" or "NLST" (usually just the string "'LIST'"). "LIST"
   retrieves a list of files and information about those files. "NLST"
   retrieves a list of file names. The *callback* function is called
   for each line with a string argument containing the line with the
   trailing CRLF stripped.  The default *callback* prints the line to
   "sys.stdout".

FTP.set_pasv(val)

   Enable "passive" mode if *val* is true, otherwise disable passive
   mode. Passive mode is on by default.

FTP.storbinary(cmd, fp, blocksize=8192, callback=None, rest=None)

   Store a file in binary transfer mode.  *cmd* should be an
   appropriate "STOR" command: ""STOR filename"". *fp* is a *file
   object* (opened in binary mode) which is read until EOF using its
   "read()" method in blocks of size *blocksize* to provide the data
   to be stored. The *blocksize* argument defaults to 8192.
   *callback* is an optional single parameter callable that is called
   on each block of data after it is sent. *rest* means the same thing
   as in the "transfercmd()" method.

   3.2 版更變: *rest* parameter added.

FTP.storlines(cmd, fp, callback=None)

   Store a file in line mode.  *cmd* should be an appropriate "STOR"
   command (see "storbinary()").  Lines are read until EOF from the
   *file object* *fp* (opened in binary mode) using its "readline()"
   method to provide the data to be stored.  *callback* is an optional
   single parameter callable that is called on each line after it is
   sent.

FTP.transfercmd(cmd, rest=None)

   Initiate a transfer over the data connection.  If the transfer is
   active, send an "EPRT" or  "PORT" command and the transfer command
   specified by *cmd*, and accept the connection.  If the server is
   passive, send an "EPSV" or "PASV" command, connect to it, and start
   the transfer command.  Either way, return the socket for the
   connection.

   If optional *rest* is given, a "REST" command is sent to the
   server, passing *rest* as an argument.  *rest* is usually a byte
   offset into the requested file, telling the server to restart
   sending the file's bytes at the requested offset, skipping over the
   initial bytes.  Note however that the "transfercmd()" method
   converts *rest* to a string with the *encoding* parameter specified
   at initialization, but no check is performed on the string's
   contents.  If the server does not recognize the "REST" command, an
   "error_reply" exception will be raised.  If this happens, simply
   call "transfercmd()" without a *rest* argument.

FTP.ntransfercmd(cmd, rest=None)

   Like "transfercmd()", but returns a tuple of the data connection
   and the expected size of the data.  If the expected size could not
   be computed, "None" will be returned as the expected size.  *cmd*
   and *rest* means the same thing as in "transfercmd()".

FTP.mlsd(path="", facts=[])

   List a directory in a standardized format by using "MLSD" command
   (**RFC 3659**).  If *path* is omitted the current directory is
   assumed. *facts* is a list of strings representing the type of
   information desired (e.g. "["type", "size", "perm"]").  Return a
   generator object yielding a tuple of two elements for every file
   found in path.  First element is the file name, the second one is a
   dictionary containing facts about the file name.  Content of this
   dictionary might be limited by the *facts* argument but server is
   not guaranteed to return all requested facts.

   3.3 版新加入.

FTP.nlst(argument[, ...])

   Return a list of file names as returned by the "NLST" command.  The
   optional *argument* is a directory to list (default is the current
   server directory).  Multiple arguments can be used to pass non-
   standard options to the "NLST" command.

   備註:

     If your server supports the command, "mlsd()" offers a better
     API.

FTP.dir(argument[, ...])

   Produce a directory listing as returned by the "LIST" command,
   printing it to standard output.  The optional *argument* is a
   directory to list (default is the current server directory).
   Multiple arguments can be used to pass non-standard options to the
   "LIST" command.  If the last argument is a function, it is used as
   a *callback* function as for "retrlines()"; the default prints to
   "sys.stdout".  This method returns "None".

   備註:

     If your server supports the command, "mlsd()" offers a better
     API.

FTP.rename(fromname, toname)

   Rename file *fromname* on the server to *toname*.

FTP.delete(filename)

   Remove the file named *filename* from the server.  If successful,
   returns the text of the response, otherwise raises "error_perm" on
   permission errors or "error_reply" on other errors.

FTP.cwd(pathname)

   Set the current directory on the server.

FTP.mkd(pathname)

   Create a new directory on the server.

FTP.pwd()

   Return the pathname of the current directory on the server.

FTP.rmd(dirname)

   Remove the directory named *dirname* on the server.

FTP.size(filename)

   Request the size of the file named *filename* on the server.  On
   success, the size of the file is returned as an integer, otherwise
   "None" is returned. Note that the "SIZE" command is not
   standardized, but is supported by many common server
   implementations.

FTP.quit()

   Send a "QUIT" command to the server and close the connection. This
   is the "polite" way to close a connection, but it may raise an
   exception if the server responds with an error to the "QUIT"
   command.  This implies a call to the "close()" method which renders
   the "FTP" instance useless for subsequent calls (see below).

FTP.close()

   Close the connection unilaterally.  This should not be applied to
   an already closed connection such as after a successful call to
   "quit()". After this call the "FTP" instance should not be used any
   more (after a call to "close()" or "quit()" you cannot reopen the
   connection by issuing another "login()" method).


FTP_TLS Objects
===============

"FTP_TLS" class inherits from "FTP", defining these additional
objects:

FTP_TLS.ssl_version

   The SSL version to use (defaults to "ssl.PROTOCOL_SSLv23").

FTP_TLS.auth()

   Set up a secure control connection by using TLS or SSL, depending
   on what is specified in the "ssl_version" attribute.

   3.4 版更變: The method now supports hostname check with
   "ssl.SSLContext.check_hostname" and *Server Name Indication* (see
   "ssl.HAS_SNI").

FTP_TLS.ccc()

   Revert control channel back to plaintext.  This can be useful to
   take advantage of firewalls that know how to handle NAT with non-
   secure FTP without opening fixed ports.

   3.3 版新加入.

FTP_TLS.prot_p()

   Set up secure data connection.

FTP_TLS.prot_c()

   Set up clear text data connection.
