Pathlib & File System

Path Objects and Composition

pathlib.Path models paths as objects that join with / and decompose cleanly:

from pathlib import Path

p = Path("data") / "raw" / "sales.csv"
p.parts        # ('data', 'raw', 'sales.csv')
p.name         # 'sales.csv'
p.stem         # 'sales'
p.suffix       # '.csv'
p.parent       # data/raw
list(p.parents)          # ancestors, nearest first

Cross-platform joining is automatic: the same code yields data/raw/sales.csv on POSIX and backslashes on Windows. Conversions are explicit — str(path) for APIs, os.fspath(path) where a protocol is expected. Note p.absolute() prepends the cwd without touching the filesystem, while p.resolve() also normalizes .. and follows symlinks (non-strict by default since 3.6).

Reading, Writing, and Walking

from pathlib import Path

cfg = Path("config.json")
cfg.write_text('{"mode": "prod"}', encoding="utf-8")   # always pass encoding
data = cfg.read_text(encoding="utf-8")

Path("out/logs").mkdir(parents=True, exist_ok=True)    # idempotent mkdir -p

for csv in Path("data").glob("**/*.csv"):              # rglob("**/*.csv") is identical
    print(csv, csv.stat().st_size)

if cfg.exists() and cfg.is_file():
    cfg.replace("config.bak")     # atomic overwrite/rename via os.replace

Directory walking replaces os.walk for most cases: sorted(d.iterdir()), filtering with is_dir()/suffix checks. Path.open() returns a normal file object, so context managers (with p.open() as f:) still manage closure. Never rely on default encodings — locale differences corrupt UTF-8 data on Windows.

Pitfalls and Interview Angles

Relative paths are promises, not facts. Path("in.csv") re-resolves against the current working directory every time it is used — a script run from another directory breaks. Anchor with Path(__file__).resolve().parent for file-relative resources.

pathlib rejects bytes paths. Byte-oriented code needs os functions; Path(os.fsdecode(raw)) bridges when necessary.

Glob surprises: patterns match names, not content; glob does not follow .. out of the tree in newer Pythons, and hidden files require glob(".*") explicitly.

Interview angle: contrast os.path.join (string-based, null-usable) with pathlib (object-oriented, richer API), and explain why resolve() is the safe way to canonicalize before comparing two paths.