✏ EDIT: linecache.py

"""Cache lines from files.

This is intended to read lines from modules imported -- hence if a filename
is not found, it will look down the module search path for a file by
that name.
"""

import sys
import os

__all__ = ["getline", "clearcache", "checkcache"]

def getline(filename, lineno, module_globals=None):
    lines = getlines(filename, module_globals)
    if 1 <= lineno <= len(lines):
        return lines[lineno-1]
    else:
        return ''

# The cache

cache = {} # The cache

def clearcache():
    """Clear the cache entirely."""

global cache
    cache = {}

def getlines(filename, module_globals=None):
    """Get the lines for a file from the cache.
    Update the cache if it doesn't contain an entry for this file already."""

if filename in cache:
        return cache[filename][2]

try:
        return updatecache(filename, module_globals)
    except MemoryError:
        clearcache()
        return []

def checkcache(filename=None):
    """Discard cache entries that are out of date.
    (This is not checked upon each call!)"""

if filename is None:
        filenames = cache.keys()
    else:
        if filename in cache:
            filenames = [filename]
        else:
            return

for filename in filenames:
        size, mtime, lines, fullname = cache[filename]
        if mtime is None:
            continue   # no-op for files loaded via a __loader__
        try:
            stat = os.stat(fullname)
        except os.error:
            del cache[filename]
            continue
        if size != stat.st_size or mtime != stat.st_mtime:
            del cache[filename]

def updatecache(filename, module_globals=None):
    """Update a cache entry and return its list of lines.
    If something's wrong, print a message, discard the cache entry,
    and return an empty list."""

if filename in cache:
        del cache[filename]
    if not filename or (filename.startswith('<') and filename.endswith('>')):
        return []

fullname = filename
    try:
        stat = os.stat(fullname)
    except OSError:
        basename = filename

# Try for a __loader__, if available
        if module_globals and '__loader__' in module_globals:
            name = module_globals.get('__name__')
            loader = module_globals['__loader__']
            get_source = getattr(loader, 'get_source', None)

if name and get_source:
                try:
                    data = get_source(name)
                except (ImportError, IOError):
                    pass
                else:
                    if data is None:
                        # No luck, the PEP302 loader cannot find the source
                        # for this module.
                        return []
                    cache[filename] = (
                        len(data), None,
                        [line+'\n' for line in data.splitlines()], fullname
                    )
                    return cache[filename][2]

# Try looking through the module search path, which is only useful
        # when handling a relative filename.
        if os.path.isabs(filename):
            return []

for dirname in sys.path:
            # When using imputil, sys.path may contain things other than
            # strings; ignore them when it happens.
            try:
                fullname = os.path.join(dirname, basename)
            except (TypeError, AttributeError):
                # Not sufficiently string-like to do anything useful with.
                continue
            try:
                stat = os.stat(fullname)
                break
            except os.error:
                pass
        else:
            return []
    try:
        with open(fullname, 'rU') as fp:
            lines = fp.readlines()
    except IOError:
        return []
    if lines and not lines[-1].endswith('\n'):
        lines[-1] += '\n'
    size, mtime = stat.st_size, stat.st_mtime
    cache[filename] = size, mtime, lines, fullname
    return lines

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