Source code: Lib/linecache.py The linecache module allows one to
get any line from any 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 linecache module defines the following functions: 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.
Clear the cache. Use this function if you no longer need lines
from files previously read using getline(). Check the cache for
validity. Use this function if files in the cache may have
changed on disk,