Python 3

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Introduction to Python

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File and Directory Listings

writing to files using the file object

Opening an existing file for writing truncates the file.

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.

appending to files

Appending is usually used for log files.

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()

Again, note that we are explicitly adding newlines to the end of each line.

show the present/current working directory.

The pwd is the location from which we run our programs.

import os

cwd = os.getcwd()        # str (your current directory)

print(cwd)

a sample file tree

this tree can be found among your course files.

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

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: parent 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 with 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 with 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 elements, 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 directly related to any files or directories.

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.

exception type	example trigger
FileNotFoundError	attempt to open a file not in this location
FileExistsError	attempt to create a directory (or in some cases a file) that already exists
IsADirectoryError	attempt to open() a file that is already a directory
NotADirectoryError	attempt to os.listdir() a directory that is not a directory
PermissionError	attempt to read or write a file or directory to which you haven't the permissions
WindowsError, OSError	these exception types are sometimes raised in place of one or more of the above when on a Windows computer

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

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

root: str, 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

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