The os module in the Python standard library (from the docs) "provides a portable way of using operating system dependent functionality". The most commonly used features are demonstrated below.

In each of the code snippets that follow, the following imports may be needed:

from datetime import datetime
import os
from pathlib import Path
import stat

Use the dir function to get a list of all the attributes and methods in a module. For example:

print(dir(os))
print(dir(os.path))
print(dir(os.path.sys))

Get and change current working directory

# Print current working directory.
print(os.getcwd())

# Change current working directory.
os.chdir('/Users/Mark') # absolute path
os.chdir('..') # relative path to parent directory

Get files and directories

# List files and directories in current directory.
print(os.listdir())

# Get list of filenames in current directory with a .csv file extension.
csv_files = filter(lambda name: name.endswith('.csv'), os.listdir())
print(list(csv_files))

Create/remove/rename files and directories

# Make (create) a new directory.
# This raises FileExistsError if it already exists.
os.mkdir('foo')

# Make a directory tree where any of them may already exist.
# This raises FileExistsError if the last
# directory name in the path already exists.
os.makedirs('foo/bar/baz')

# Remove a single directory.
# This raises OSError if the directory is not empty.
# Intermediate directories are not removed.
os.rmdir('foo/bar/baz')

# Remove a directory tree working from bottom to top.
# Intermediate directories are removed if they are empty.
os.removedirs('foo/bar/baz')

# Touch to create an empty file.
Path('test.csv').touch()

# Remove a file.
# This raises FileNotFoundError if the file does not exist.
os.remove('text.csv')

# Rename a file or directory.
# This raises FileNotFoundError if the file does not exist.
os.rename('test.csv', 'renamed.csv')

Recursively walk a directory

The os.walk method returns a generator that recursively walks a given directory. Each yielded value is a tuple that contains:

• dirpath: directory path
• dirnames: list of all directories found at dirpath
• filenames: list of all files found at dirpath

By default this does not follow symbolic links, but it will if followlinks=True is passed.

# Walk a directory tree to find all the .py files.
cwd = os.getcwd()
for dirpath, dirnames, filenames in os.walk(cwd):
    #print('dirnames =', dirnames)
    #print('filenames =', filenames)
    py_files = list(filter(lambda name: name.endswith('.py'), filenames))
    if py_files:
        print(dirpath, 'contains', py_files)

When only files whose names match some pattern will be processed, another approach is to use the standard library glob module.

The glob method returns a list of matching relative file paths. The iglob method returns an iterator so all the matching relative file paths don't need to be stored before iteration begins.

In patterns passed to glob methods:

• ? matches any single character.
• * matches any run of characters.
• ** matches any subdirectory tree, including the current directory.

For example:

import glob

# Match all Python source files in and below the current directory.
PATTERN = '**/*.py'

files = glob.glob(PATTERN)
print(files)

for file in glob.iglob(PATTERN):
    print(file)

Get environment variables

# Get all environment variable names and values.
print(os.environ) # prints a dict

# Get the value of a specific environment variable.
dir_path = os.environ.get('PYTHON_DIR')

Write and read files

# Build a file path using an environment variable.
dir_path = os.environ.get('PYTHON_DIR')
FILE_NAME = 'test.csv'
# Using os.path.join instead of string concatenation
# ensures correct handling of path separator characters.
file_path = os.path.join(dir_path, FILE_NAME)

# Create a CSV file.
with open(file_path, 'w') as f:
    f.write('name,breed\n')
    f.write('Comet,Whippet\n')

# Read the file all at once.
with open(file_path, 'r') as f:
    contents = f.read()
    lines = contents.split('\n')
    print('lines =', lines)

# Read the file one line at a time.
with open(file_path, 'r') as f:
    while True:
        line = f.readline()
        if not line:
            break
        print('line =', line, end='') # omits newline at end

Get information about files

The os.stat method returns an object with many fields.

• st_atime is the timestamp at which the file was last accessed.
  On macOS, this is not updated simply by cat'ing a file.
• st_ctime is the timestamp at which the file was created.
• st_mtime is the timestamp at which the file was last modified.
• st_dev is the unique identifier for the device on which the file resides.
• st_gid is the group id of the file owner.
• st_ino is the OS-specific unique identifier for the file.
• st_mode encodes the file type and mode (permissions)
• st_nlink is the number of hard links to the file (often 1).
• st_size is the size in bytes.
• st_uid is the user id of the owner.

# Get statistics for a file or directory.
stats = os.stat('os/test.txt')
print(stats)

print('modified at', datetime.fromtimestamp(stats.st_mtime))
mode = stats.st_mode
print('is directory?', stat.S_ISDIR(mode))
print('is regular file?', stat.S_ISREG(mode))

def have_permission(stats, permission):
    return stat.S_IMODE(stats.st_mode) & permission == permission
can_read = lambda stats: have_permission(stats, stat.S_IREAD)
can_write = lambda stats: have_permission(stats, stat.S_IWRITE)
can_execute = lambda stats: have_permission(stats, stat.S_IEXEC)

print('can read =', can_read(stats))
print('can write =', can_write(stats))
print('can execute =', can_execute(stats))

# Using os.path ...

print('basename =', os.path.basename(file_path)) # test.csv
print('dirname =', os.path.dirname(file_path)) # PYTHON_DIR value
print('split =', os.path.split(file_path)) # tuple of PYTHON_DIR value and test.csv
print('splitext =', os.path.splitext(file_path))
# tuple of PYTHON_DIR value + file name
# and file extension including leading dot

print('exists =', os.path.exists(file_path)) # True
print('isdir =', os.path.isdir(file_path)) # False
print('isfile =', os.path.isfile(file_path)) # True
print('islink =', os.path.islink(file_path)) # False

print('size =', os.path.getsize(file_path), 'bytes')

atime = os.path.getatime(file_path)
ctime = os.path.getctime(file_path)
mtime = os.path.getmtime(file_path)
print('atime =', atime, datetime.fromtimestamp(atime))
print('ctime =', ctime, datetime.fromtimestamp(ctime))
print('mtime =', mtime, datetime.fromtimestamp(mtime))

Get OS information

print('pathsep =', os.path.pathsep) # : on macOS
print('sep =', os.path.sep) # / on macOS
print('platform =', os.path.sys.platform) # darwin on macOS
print('version =', os.path.sys.version) # 3.8.5 - version of Python
print('version_info =', os.path.sys.version_info)
# tuple containing major, minor, micro,
# and releaselevel of Python executable