Doc/library/linecache.rst

:mod:`!linecache` --- Random access to text lines
=================================================

.. module:: linecache
   :synopsis: Provides random access to individual lines from text files.

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

--------------

The :mod:`!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 :mod:`traceback` module
to retrieve source lines for inclusion in  the formatted traceback.

The :func:`tokenize.open` function is used to open files. This
function uses :func:`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 :mod:`!linecache` module defines the following functions:


.. function:: 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).

   .. index:: triple: module; search; path

   If *filename* indicates a frozen module (starting with ``'<frozen '``), the function
   will attempt to get the real file name from ``module_globals['__file__']`` if
   *module_globals* is not ``None``.

   If a file named *filename* is not found, the function first checks
   for a :pep:`302` ``__loader__`` in *module_globals*.
   If there is such a loader and it defines a ``get_source`` method,
   then that determines the source lines
   (if ``get_source()`` returns ``None``, then ``''`` is returned).
   Finally, if *filename* is a relative filename,
   it is looked up relative to the entries in the module search path, ``sys.path``.

   .. versionchanged:: 3.14

      Support *filename* of frozen modules.


.. function:: clearcache()

   Clear the cache.  Use this function if you no longer need lines from files
   previously read using :func:`getline`.


.. function:: 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.

.. function:: lazycache(filename, module_globals)

   Capture enough detail about a non-file-based module to permit getting its
   lines later via :func:`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.

   .. versionadded:: 3.5

Example::

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