Lib/posixpath.py

Source:

cpython 3.14 @ ab2d84fe1023/Lib/posixpath.py ↗

posixpath implements os.path on POSIX systems. On Windows, ntpath provides the same interface. The module operates on byte strings and Unicode strings interchangeably.

Map

Lines	Symbol	Role
1-80	join, split, splitext	Path decomposition
81-160	abspath, realpath	Absolute path resolution
161-260	exists, lexists, isfile, isdir, islink	Stat-based checks
261-380	expanduser, expandvars	Shell-style expansion
381-550	commonpath, commonprefix, relpath	Path relationships

Reading

join

# CPython: Lib/posixpath.py:76 join
def join(a, *p):
    sep = _get_sep(a)
    path = a
    for b in p:
        if b.startswith(sep):
            path = b    # absolute component resets the path
        elif not path or path.endswith(sep):
            path += b
        else:
            path += sep + b
    return path

An absolute component in any position resets the accumulated path, matching POSIX shell behavior.

realpath

# CPython: Lib/posixpath.py:196 realpath
def realpath(filename, *, strict=False):
    filename = os.fspath(filename)
    path, ok = _joinrealpath(filename[:0], filename, strict, {})
    return abspath(path)

def _joinrealpath(path, rest, strict, seen):
    # Resolve symlinks iteratively, tracking visited paths to detect cycles
    while rest:
        name, _, rest = rest.partition(sep)
        newpath = join(path, name)
        try:
            target = os.readlink(newpath)
        except OSError:
            path = newpath
            continue
        if target in seen:
            if seen[target] is None:   # cycle
                if strict: raise OSError(...)
                path = newpath
                continue
            path = seen[target]
            continue
        seen[target] = None
        path, ok = _joinrealpath(path, target, strict, seen)
        seen[target] = path
    return path, True

The seen dict prevents infinite loops from symlink cycles.

expanduser

# CPython: Lib/posixpath.py:295 expanduser
def expanduser(path):
    if not path.startswith('~'):
        return path
    i = path.find('/', 1)
    if i < 0:
        i = len(path)
    if i == 1:
        # '~' alone: use HOME env or pwd entry
        userhome = os.environ.get('HOME') or pwd.getpwuid(os.getuid()).pw_dir
    else:
        # '~username': look up in pwd
        pwent = pwd.getpwnam(path[1:i])
        userhome = pwent.pw_dir
    return userhome + path[i:]

commonpath

# CPython: Lib/posixpath.py:490 commonpath
def commonpath(paths):
    split_paths = [path.split(sep) for path in paths]
    try:
        common = split_paths[0]
        for s in split_paths[1:]:
            common = [c for c, x in zip(common, s) if c == x]
    except (TypeError, AttributeError):
        ...
    return sep.join(common)

Differs from commonprefix (which works character-by-character): commonpath works component-by-component.

gopy notes

os.path is used everywhere. posixpath.join, split, splitext, abspath, and exists/isfile/isdir are the critical functions. In gopy these delegate to Go's path/filepath package. expanduser requires os.getuid() and pwd module access. realpath requires os.readlink.