Working with Files & Directories in Python

All examples assume Python 3, unless otherwise stated

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 file into string

Heads-up: This will break if the file doesn't fit in memory

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

# use variable `contents`

Buffered reading

Process a large text file, but without without loading it all to memory at the same time (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 (dangerous 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.

Process file line by line

See above: Buffered Reading

Check 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

List files in directory

To show all files that exist 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

I.e. for file in directory, do something.

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

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 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(<source-path>, <target-path>)

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"