19.1.14. "email.utils": Miscellaneous utilities
***********************************************

**Source code:** Lib/email/utils.py

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

There are a couple of useful utilities provided in the "email.utils"
module:

email.utils.localtime(dt=None)

   Return local time as an aware datetime object.  If called without
   arguments, return current time.  Otherwise *dt* argument should be
   a "datetime" instance, and it is converted to the local time zone
   according to the system time zone database.  If *dt* is naive (that
   is, "dt.tzinfo" is "None"), it is assumed to be in local time.  In
   this case, a positive or zero value for *isdst* causes "localtime"
   to presume initially that summer time (for example, Daylight Saving
   Time) is or is not (respectively) in effect for the specified time.
   A negative value for *isdst* causes the "localtime" to attempt to
   divine whether summer time is in effect for the specified time.

   3.3 新版功能.

email.utils.make_msgid(idstring=None, domain=None)

   Returns a string suitable for an **RFC 2822**-compliant *Message-
   ID* header.  Optional *idstring* if given, is a string used to
   strengthen the uniqueness of the message id.  Optional *domain* if
   given provides the portion of the msgid after the '@'.  The default
   is the local hostname.  It is not normally necessary to override
   this default, but may be useful certain cases, such as a
   constructing distributed system that uses a consistent domain name
   across multiple hosts.

   在 3.2 版更改: Added the *domain* keyword.

The remaining functions are part of the legacy ("Compat32") email API.
There is no need to directly use these with the new API, since the
parsing and formatting they provide is done automatically by the
header parsing machinery of the new API.

email.utils.quote(str)

   Return a new string with backslashes in *str* replaced by two
   backslashes, and double quotes replaced by backslash-double quote.

email.utils.unquote(str)

   Return a new string which is an *unquoted* version of *str*. If
   *str* ends and begins with double quotes, they are stripped off.
   Likewise if *str* ends and begins with angle brackets, they are
   stripped off.

email.utils.parseaddr(address)

   Parse address -- which should be the value of some address-
   containing field such as *To* or *Cc* -- into its constituent
   *realname* and *email address* parts.  Returns a tuple of that
   information, unless the parse fails, in which case a 2-tuple of
   "('', '')" is returned.

email.utils.formataddr(pair, charset='utf-8')

   The inverse of "parseaddr()", this takes a 2-tuple of the form
   "(realname, email_address)" and returns the string value suitable
   for a *To* or *Cc* header.  If the first element of *pair* is
   false, then the second element is returned unmodified.

   Optional *charset* is the character set that will be used in the
   **RFC 2047** encoding of the "realname" if the "realname" contains
   non-ASCII characters.  Can be an instance of "str" or a "Charset".
   Defaults to "utf-8".

   在 3.3 版更改: Added the *charset* option.

email.utils.getaddresses(fieldvalues)

   This method returns a list of 2-tuples of the form returned by
   "parseaddr()". *fieldvalues* is a sequence of header field values
   as might be returned by "Message.get_all".  Here's a simple example
   that gets all the recipients of a message:

      from email.utils import getaddresses

      tos = msg.get_all('to', [])
      ccs = msg.get_all('cc', [])
      resent_tos = msg.get_all('resent-to', [])
      resent_ccs = msg.get_all('resent-cc', [])
      all_recipients = getaddresses(tos + ccs + resent_tos + resent_ccs)

email.utils.parsedate(date)

   Attempts to parse a date according to the rules in **RFC 2822**.
   however, some mailers don't follow that format as specified, so
   "parsedate()" tries to guess correctly in such cases.  *date* is a
   string containing an **RFC 2822** date, such as  ""Mon, 20 Nov 1995
   19:12:08 -0500"".  If it succeeds in parsing the date,
   "parsedate()" returns a 9-tuple that can be passed directly to
   "time.mktime()"; otherwise "None" will be returned.  Note that
   indexes 6, 7, and 8 of the result tuple are not usable.

email.utils.parsedate_tz(date)

   Performs the same function as "parsedate()", but returns either
   "None" or a 10-tuple; the first 9 elements make up a tuple that can
   be passed directly to "time.mktime()", and the tenth is the offset
   of the date's timezone from UTC (which is the official term for
   Greenwich Mean Time) [1].  If the input string has no timezone, the
   last element of the tuple returned is "None".  Note that indexes 6,
   7, and 8 of the result tuple are not usable.

email.utils.parsedate_to_datetime(date)

   The inverse of "format_datetime()".  Performs the same function as
   "parsedate()", but on success returns a "datetime".  If the input
   date has a timezone of "-0000", the "datetime" will be a naive
   "datetime", and if the date is conforming to the RFCs it will
   represent a time in UTC but with no indication of the actual source
   timezone of the message the date comes from.  If the input date has
   any other valid timezone offset, the "datetime" will be an aware
   "datetime" with the corresponding a "timezone" "tzinfo".

   3.3 新版功能.

email.utils.mktime_tz(tuple)

   Turn a 10-tuple as returned by "parsedate_tz()" into a UTC
   timestamp (seconds since the Epoch).  If the timezone item in the
   tuple is "None", assume local time.

email.utils.formatdate(timeval=None, localtime=False, usegmt=False)

   Returns a date string as per **RFC 2822**, e.g.:

      Fri, 09 Nov 2001 01:08:47 -0000

   Optional *timeval* if given is a floating point time value as
   accepted by "time.gmtime()" and "time.localtime()", otherwise the
   current time is used.

   Optional *localtime* is a flag that when "True", interprets
   *timeval*, and returns a date relative to the local timezone
   instead of UTC, properly taking daylight savings time into account.
   The default is "False" meaning UTC is used.

   Optional *usegmt* is a flag that when "True", outputs a  date
   string with the timezone as an ascii string "GMT", rather than a
   numeric "-0000". This is needed for some protocols (such as HTTP).
   This only applies when *localtime* is "False".  The default is
   "False".

email.utils.format_datetime(dt, usegmt=False)

   Like "formatdate", but the input is a "datetime" instance.  If it
   is a naive datetime, it is assumed to be "UTC with no information
   about the source timezone", and the conventional "-0000" is used
   for the timezone. If it is an aware "datetime", then the numeric
   timezone offset is used. If it is an aware timezone with offset
   zero, then *usegmt* may be set to "True", in which case the string
   "GMT" is used instead of the numeric timezone offset.  This
   provides a way to generate standards conformant HTTP date headers.

   3.3 新版功能.

email.utils.decode_rfc2231(s)

   Decode the string *s* according to **RFC 2231**.

email.utils.encode_rfc2231(s, charset=None, language=None)

   Encode the string *s* according to **RFC 2231**.  Optional
   *charset* and *language*, if given is the character set name and
   language name to use.  If neither is given, *s* is returned as-is.
   If *charset* is given but *language* is not, the string is encoded
   using the empty string for *language*.

email.utils.collapse_rfc2231_value(value, errors='replace', fallback_charset='us-ascii')

   When a header parameter is encoded in **RFC 2231** format,
   "Message.get_param" may return a 3-tuple containing the character
   set, language, and value.  "collapse_rfc2231_value()" turns this
   into a unicode string.  Optional *errors* is passed to the *errors*
   argument of "str"'s "encode()" method; it defaults to "'replace'".
   Optional *fallback_charset* specifies the character set to use if
   the one in the **RFC 2231** header is not known by Python; it
   defaults to "'us-ascii'".

   For convenience, if the *value* passed to
   "collapse_rfc2231_value()" is not a tuple, it should be a string
   and it is returned unquoted.

email.utils.decode_params(params)

   Decode parameters list according to **RFC 2231**.  *params* is a
   sequence of 2-tuples containing elements of the form "(content-
   type, string-value)".

-[ 脚注 ]-

[1] Note that the sign of the timezone offset is the opposite of
    the sign of the "time.timezone" variable for the same timezone;
    the latter variable follows the POSIX standard while this module
    follows **RFC 2822**.
