Working with Files & the Filesystem in Python: Reference and Examples

WIP Alert This is a work in progress. Current information is correct but more content may be added in the future.

All examples assume Python 3, unless otherwise stated

The full list of methods you can call on a file object can be found here: Python 3: io.IOBASE

Manipulating filesystem paths is a very common need in most if not all applications; most problems can be solved using either os.path or pathlib modules.

Pathlib is used in some examples here; it comes by default with Python 3 but you can also use it in Python 2 via Pip: pip install pathlib

Write string to file

write is just one method of class File. See all here

# will be overwritten if it already exists!
with open("filename.txt","w") as f:
    f.write("foo bar")

Read the whole file contents to a string

with open("filename.txt","r") as f:
    contents = f.read()

Read a file line-by-line, without loading it all to memory (buffered reading)

This is easy with python:

with open("path/to/file") as f:
    for line in f:
        # process the line

You can also use f.readline() to read a whole line (be careful if the file has no line breaks) or f.read(size), which takes an argument indicating the maximum number of bytes to be read into memory.

Test if a file exists

This method returns True if the given path exists and it's a file.

If you just want to test if a path exists (be it a file, directory or even link), use os.path.exists() instead.

import os.path

path_to_file = "/path/to/file"

if os.path.isfile(path_to_file):
    # it exists, and it is a regular
    # (i.e. not a directory) file

Relative to Absolute Path

Use os.path.abspath. But remember that the relative path is calculated with respect to your current working directory.

Note that this will not warn you if you supply a path that resolves to a file/dir that doesn't exist

If you want the relative path to be calculated with respect to the current file location, don't forget to prepend __file__+"/" to the relative path!

import os

# path is relative to the current WORKING directory!
abs_path = os.path.abspath("../../other-dir")

# path is relative to the directory where THIS file
# is located!
abs_path = os.path.abspath(__file__+"/../../other-dir")

Relative to absolute path in Jupyter notebooks

"NameError: name 'file' is not defined"

Use "__file__" (in quotes) instead:

import os

# on a jupyter notebook
abs_path = os.path.abspath("__file__"+"/../../other-dir")

Get file name from file path (basename)

Use os.path.basename()

import os

print(os.path.basename("/path/to/some/file.txt"))
# "file.txt"

Get path to file without the basename

This is the inverse of the above. Note the trailing slash is not included.

Just use

import os

print(os.path.dirname("/path/to/some/file.txt"))
# "/path/to/some"

Get current working directory

The current working directory (or cwd) is the directory the user that first called the script was located.

import os

os.getcwd()

Get the directory where this (current) file is located

Note that this is not at all the same thing as the current working directory (seen above)

import os

# __file__ points to the where the current file is located
os.path.dirname(os.path.realpath(__file__))

List files in directory

import os

os.listdir(path_to_directory)

Loop over files in directory, recursively

To loop over all files recursively under some path (i.e. including subdirectories):

import os

# you may omit trailing slashes
# i.e. '/tmp' works too
path = '/tmp/'

for root, directories, filenames in os.walk(path):
    for directory in directories:

        directory_path = os.path.join(root, directory)
        # do somthing with directory_path

    for filename in filenames:

        file_path = os.path.join(root,filename)
        # do something with directory_path

Delete a file

Use os.remove(path-to-file).

Add directory to PATH

Suppose you have a directory structure that looks like this:

myproject/
├── helpers
│   └── mymodule.py
├── utils
│   └── myothermodule.py
└── stuff
    └── file1.py

# stuff/file1.py
import os
import sys

# find out the absolute path to the directory
# you want to add to your path
other_dir = os.path.abspath(__file__+"/../")

sys.path.insert(0,other_dir)

# now you can import stuff like this
from helpers import mymodule
from utils import myothermodule

See also

• Python 3 Tutorial - Input and Output

• Python 3 Tutorial - Input and Output - Methods of File Objects

• Python 3 docs - File and Directory Access

• Python 3 - os.path

• Python 3 - pathlib