EC
ENOTDIR
Linux / POSIXERRORCommonFile SystemHIGH confidence

Not a Directory

Production Risk

Common when path construction logic has a bug — a config value intended to be a directory path points to a file.

What this means

ENOTDIR (errno 20) is returned when a path component that is expected to be a directory is in fact a regular file or other non-directory. This commonly surfaces when a parent directory in a path is actually a file.

Why it happens
  1. 1A component of a path is a regular file, not a directory (e.g., /etc/hosts/foo where /etc/hosts is a file)
  2. 2Calling opendir() on a regular file
  3. 3Calling chdir() to a path that is not a directory
How to reproduce

Using a file as a directory component in a path.

trigger — this will error
trigger — this will error
import os
os.listdir('/etc/hosts/subdir')
# /etc/hosts is a file, not a directory

expected output

NotADirectoryError: [Errno 20] Not a directory: '/etc/hosts/subdir'

Fix

Verify each path component is actually a directory

WHEN When building paths dynamically from user input or config

Verify each path component is actually a directory
import os
path = '/etc/hosts'
if os.path.isdir(path):
    os.listdir(path)
else:
    raise ValueError(f"{path} is not a directory")

Why this works

Use os.path.isdir() before treating a path as a directory. In C, use stat() and check S_ISDIR(st.st_mode).

Sources
Official documentation ↗

Linux Programmer Manual errno(3)

stat(2) ↗

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