statcache.py

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"""Maintain a cache of stat() information on files.

There are functions to reset the cache or to selectively remove items.
"""

import os as _os
from stat import *

__all__ = ["stat","reset","forget","forget_prefix","forget_dir",
           "forget_except_prefix","isdir"]

# The cache.  Keys are pathnames, values are os.stat outcomes.
# Remember that multiple threads may be calling this!  So, e.g., that
# cache.has_key(path) returns 1 doesn't mean the cache will still contain
# path on the next line.  Code defensively.

cache = {}

def stat(path):
    """Stat a file, possibly out of the cache."""
    ret = cache.get(path, None)
    if ret is None:
        cache[path] = ret = _os.stat(path)
    return ret

def reset():
    """Clear the cache."""
    cache.clear()

# For thread saftey, always use forget() internally too.
def forget(path):
    """Remove a given item from the cache, if it exists."""
    try:
        del cache[path]
    except KeyError:
        pass

def forget_prefix(prefix):
    """Remove all pathnames with a given prefix."""
    for path in cache.keys():
        if path.startswith(prefix):
            forget(path)

def forget_dir(prefix):
    """Forget a directory and all entries except for entries in subdirs."""

    # Remove trailing separator, if any.  This is tricky to do in a
    # x-platform way.  For example, Windows accepts both / and \ as
    # separators, and if there's nothing *but* a separator we want to
    # preserve that this is the root.  Only os.path has the platform
    # knowledge we need.
    from os.path import split, join
    prefix = split(join(prefix, "xxx"))[0]
    forget(prefix)
    for path in cache.keys():
        # First check that the path at least starts with the prefix, so
        # that when it doesn't we can avoid paying for split().
        if path.startswith(prefix) and split(path)[0] == prefix:
            forget(path)

def forget_except_prefix(prefix):
    """Remove all pathnames except with a given prefix.

    Normally used with prefix = '/' after a chdir().
    """

    for path in cache.keys():
        if not path.startswith(prefix):
            forget(path)

def isdir(path):
    """Return 1 if directory, else 0."""
    try:
        st = stat(path)
    except _os.error:
        return 0
    return S_ISDIR(st[ST_MODE])