11.9. "linecache" --- Random access to text lines
*************************************************

**Source code:** Lib/linecache.py

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

The "linecache" module allows one to get any line from a Python source
file, while attempting to optimize internally, using a cache, the
common case where many lines are read from a single file.  This is
used by the "traceback" module to retrieve source lines for inclusion
in  the formatted traceback.

The "tokenize.open()" function is used to open files. This function
uses "tokenize.detect_encoding()" to get the encoding of the file; in
the absence of an encoding token, the file encoding defaults to UTF-8.

The "linecache" module defines the following functions:

linecache.getline(filename, lineno, module_globals=None)

   Get line *lineno* from file named *filename*. This function will
   never raise an exception --- it will return "''" on errors (the
   terminating newline character will be included for lines that are
   found).

   If a file named *filename* is not found, the function will look for
   it in the module search path, "sys.path", after first checking for
   a **PEP 302** "__loader__" in *module_globals*, in case the module
   was imported from a zipfile or other non-filesystem import source.

linecache.clearcache()

   Clear the cache.  Use this function if you no longer need lines
   from files previously read using "getline()".

linecache.checkcache(filename=None)

   Check the cache for validity.  Use this function if files in the
   cache  may have changed on disk, and you require the updated
   version.  If *filename* is omitted, it will check all the entries
   in the cache.

linecache.lazycache(filename, module_globals)

   Capture enough detail about a non-file-based module to permit
   getting its lines later via "getline()" even if *module_globals* is
   "None" in the later call. This avoids doing I/O until a line is
   actually needed, without having to carry the module globals around
   indefinitely.

   3.5 新版功能.

示例:

   >>> import linecache
   >>> linecache.getline(linecache.__file__, 8)
   'import sys\n'
