Introduction to Python

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File and Directory Listings

writing to files using the file object

To open a file for writing, use the 2nd argument 'w'.

fh = open('new_file.txt', 'w')
fh.write("here's a line of text\n")
fh.write('I add the newlines explicitly if I want to write to the file\n')
fh.close()

• Files can be opened for writing or appending, but not usually for both.
• Note that we are explicitly adding newlines to the end of each line. The write() method doesn't do this for us.
• Special note: opening an existing file for writing truncates (erases) the file, so use with care!

appending to files using the file object

To open a file for appending, use the 2nd argument 'a'.

fh = open('new_file.txt', 'a')
fh.write("20250505 1203   something happened\n")
fh.close()

• Appending is usually used for log files, that are often updated by adding new lines to the file.
• Again, note that we are explicitly adding newlines to the end of each line.

use os.getcwd() to show the present/current working directory.

Below program assumes were are starting in our home directory:

import os                # os ('operating system') module talks
                         # to the os (for file access & more)

cwd = os.getcwd()        # str, '/Users/david'

print(cwd)

• The 'os' module gives us access to the operating system and its work and environment (files, folders, programs, etc.).
• Again, the pwd is the location where we 'are' currently.
• An important note: VS Code uses the workspace folder as the pwd when you run programs through the IDE, whereas PyCharm uses the directory the script is in.

a sample file tree

We'll use this tree to explore relative filepaths.

dir1
├── file1.txt
├── test1.py
│
├── dir2a
│   ├── file2a.txt
│   ├── test2a.py
│   │
│   ├── dir3a
│   │   ├── file3a.txt
│   │   ├── test3a.py
│   │   │
│   │   └── dir4
│   │       ├── file4.txt
│   │       └── test4.py
└── dir2b
    ├── file2b.txt
    ├── test2b.py
    │
    └── dir3b
       ├── file3b.txt
       └── test3b.py

• this tree can be found among your course files

relative filepaths

These paths locate files relative to the present working directory.

If the file you want to open is in the same directory as the script you're executing, use the filename alone:

fh = open('filename.txt')

relative filepaths: parent directory

To reach the parent directory, prepend the filename with ../

fh = open('../filename.txt')

relative filepaths: child directory

To reach the child directory, prepend the filename with the name of the child directory.

fh = open('childdir/filename.txt')

relative filepaths: sibling directory

To reach a sibling directory, prepend the filename with ../ and the name of the child directory.

fh = open('../childdir/filename.txt')

To reach a sibling directory, we must go "up, then down" by using ../ to go to the parent, then the sibling directory name to go down to the child.

absolute filepaths

These paths locate files from the root of the filesystem.

In Windows, absolute paths begin with a drive letter, usually C:\:

""" test3a.py:  open and read a file """

filepath = r'C:\Users\david\Desktop\python_data\dir1\file1.txt'
fh = open(filepath)

print(fh.read())

(Note that r'' should be used when expressing in our Python program any Windows paths that contain backslashes.)

On the Mac, absolute paths begin with a forward slash:

""" test3a.py:  open and read a file """

filepath = '/Users/david/Desktop/python_data/dir1/file1.txt'
fh = open(filepath)

print(fh.read())

(The above paths assume that the python_data folder is in the Desktop directory; your may have placed yours elsewhere on your system. Of course, the above paths also assume that my home directory is called david/; yours is likely different.)

os.path.join()

This function joins together directory and file strings with slashes appropriate to the current operating system.

dirname = '/Users/david'
filename = 'journal.txt'

filepath = os.path.join(dirname, filename)             # '/Users/david/journal.txt'

filepath2 = os.path.join(dirname, 'backup', filename)  # '/Users/david/backup/journal.txt'

• this function inserts slashes (when needed) in between directory name and filename items, joining them together into one string
• the slash inserted will be forward slash on Mac/Linux, and backslash on Windows machines
• Keep in mind these are only strings, and this is a string join method. Its work is not affecting or reading any existing files or directories - it is only manipulating these strings.

os.listdir(): list a directory

os.listdir() can read the contents of any directory.

import os

mydirectory = '/Users/david'

items = os.listdir(mydirectory)

for item in items:                                # 'photos'

    item_path = os.path.join(mydirectory, item)

    print(item_path)  # /Users/david/photos/
                      # /Users/david/backups/
                      # /Users/david/college_letter.docx
                      # /Users/david/notes.txt
                      # /Users/david/finances.xlsx

Note the os.path.join() call. This is a standard algorithm for looping through a directory -- each item must be joined to the directory to ensure that the filepath is correct.

exceptions for missing or incorrect files or directories

Several exceptions can indicate a file or directory misfire.

• depending on your OS type and Python version, you may see any of these exceptions
• exceptions like FileNotFoundError are very specific, but more general exceptions like WindowsError or OSError may indicate one of the more specific conditions (missing file, missing directory, attempting to open a directory, etc.)

traversing a directory tree with os.walk()

os.walk() visits every directory in a directory tree so we can list files and folders.

import os
root_dir = '/Users/david'
for root, dirs, files in os.walk(root_dir):

    for tdir in dirs:                    # loop through dirs in this directory
        print(os.path.join(root, tdir))  # print full path to tdir

    for tfile in files:                  # loop through files in this dir
        print(os.path.join(root, tfile)) # print full path to file

• os.walk() traverses an entire directory tree
• at each iteration of the 'for' loop, it visits each node (i.e., directory), one at a time
• starting with the supplied directory (above, root_dir), it visits each subdirectory, traveling to every node beneath the root (again, node means particular directory)

At each iteration, these three variables are assigned these values:

• root: string, the "node" or directory currently being read
• dirs: list, names of directories found in the current directory
• files: list, names of files found in the current directory