Working with Files & Directories in Python

All examples assume Python 3, unless otherwise stated

Write string to file

write is a 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 file into string

Heads-up: Will crash if the file doesn't fit in memory. See Buffered Reading

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

# use variable `contents`

Buffered reading

Process text files line by line without loading it all to memory at once:

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

You can also use f.readline() to read a whole line¹ or f.read(size), which takes an argument indicating the maximum number of bytes to be read into memory.

Check file exists

Returns True if the given path exists and it's a file.

To test if a path exists (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

Check directory exists

import os

# path is either absolute or relative to working dir
os.path.isdir(path_to_directory)

List files in directory

To show all files in a directory, use os.listdir(path_to_directory):

import os

os.listdir(path_to_directory)
>>> ['file1.txt', 'file2.json', 'myfile.png']

Loop over files in directory

for file in directory

import os

root_path = "/path/to/directory"

for file_name in os.listdir(root):

    # /path/to/directory/myfile.txt
    full_path_to_file = os.path.join(root, file_name)

Loop over files in directory, recursively

Loop over all files recursively under a path (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 something with directory_path

    for filename in filenames:

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

Delete file

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

import os

# raises FileNotFoundError if file doesn't exist
os.remove("/path/to/file")

Move/rename file

Use: os.replace(from, to)

import os

# raises FileNotFoundError if source file doesn't exist
os.replace('/path/to/source/file','/path/to/new/location/file')

Copy file

Use: shutil.copy(<from>,<to>) and shutil.copy2(<from>,<to>). Both methods are used the same way. See differences below.

Shutil is part of Python's standard library; there's no need to install anything.

• Example 1: Target path is the full path to the new location

  import shutil
  
  source_path = '/path/to/source/source-file'
  target_path = '/path/to/target/target-file'
  
  # raises FileNotFoundError if source file doesn't exist
  # target file is overwritten if it already exists!
  shutil.copy(source_path,target_path)
  # >>> '/path/to/target/target-file'
  

• Example 2: Target path is the path to the new directory

  import shutil
  
  source_path = '/path/to/my-file'
  target_path = '/path/to/another/directory/'
  
  # raises FileNotFoundError if source file doesn't exist
  # target file is overwritten if it already exists!
  shutil.copy(source_path,target_path)
  # >>> '/path/to/another/directory/my-file'
  

Add directory to system PATH

That is, the PATH environment variable.

import os

os.environ["PATH"] += os.pathsep + "/path/to/directory"

1: This equates to reading the whole file into memory if the file has no line breaks