Introduction to Python

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File Tests and Manipulations

os.path.isfile() and os.path.isdir()

With these functions we can see whether a file is a plain file, or a directory.

import os                         # os ('operating system') module talks
                                  # to the os (for file access & more)
mydirectory = '/Users/david'

items = os.listdir(mydirectory)   # list of strings, files found in this directory

for item in items:                # str, first file or dir found in directory

    item_path = os.path.join(mydirectory, item)  # join directory name and file or dir

    if os.path.isdir(item_path):
        print(f"{item}:  directory")
    elif os.path.isfile(item_path):
        print(f"{item}:  file")
                                     # photos:  directory
                                     # backups:  directory
                                     # college_letter.docx:  file
                                     # notes.txt:  file
                                     # finances.xlsx:  file

• again, os.path.join() is here connecting the directory being listed to a file or directory found in it, to construct a path to the file or directory
• .isdir() returns True if the listing is a directory
• .isfile() returns True if the listing is a file

os.path.exists()

This function tests to see if a file exists on the filesystem.

import os

fn = input('please enter a file or directory name:  ')
if not os.path.exists(fn):
    print('item does not exist')

elif os.path.isfile(fn):
    print('item is a file')

elif os.path.isdir(fn):
    print('item is a directory')

• Keep in mind that if an item doesn't even exist, .isdir() and .isfile() will return False
• This could lead you to believe that a non-existent entity is a dir based on getting False from .isfile(), or a file based on getting False from .isdir()
• Therefore if we're getting False from .isdir() or .isfile(), we should check to see if the item even exists.
• .isfile() and .isdir() will also return False if the item is another kind of entity -- depending on your operating system, an entry representing a link, a block device, or a socket may be encountered.

read file size with os.path.getsize()

os.path.getsize() takes a filename and returns the size of the file in bytes

import os

mydirectory = '/Users/david'

items = os.listdir(mydirectory)

for item in items:
    item_path = os.path.join(mydirectory, item)
    item_size = os.path.getsize(item_path)
    print(f"{item_path}:  {item_size} bytes")

• Remember, as before, that when looping through a directory, Python won't be able to find a file unless its path is prepended. This is why os.path.join() is so important.

moving or renaming a file

moving and renaming a file are essentailly the same thing

import os

filename = 'file1.txt'
new_filename = 'newname.txt'

os.rename(filename, new_filename)

import os

filename = 'file1.txt'      # or could be a filepath incluing directory
move_to_dir = 'old/'

# renaming file1.txt to old/file1.txt
os.rename(filename, os.path.join(move_to_dir, filename))

• in the first example, we simply rename a file
• in the second example, we retain the old filename, but give it a new path. This is the same as moving the file to a new location - in a sense, we're renaming the path.

copying or backing up a file

import shutil                      # the 'shell utilities' module

filename = 'file1.txt'
backup_filename = 'file1.txt_bk'   # must be a filepath, including filename

shutil.copyfile(filename, backup_filename)

import shutil

filename = 'file1.txt'
target_dir = 'backup'              # can be a filepath or just a directory name

shutil.copy(filename, target_dir)  # dst can be a folder; use shutil.copy2()

• shutil ("shell utilities") is a module for doing all kinds of operations on files, directories, and the like
• shutil.copy() can copy a file to a new name or pathname, or to a new directory
• it's important to note that the file "metadata" (creation date, modify date, etc.) will not be copied with the file

creating a directory: os.mkdir()

This function is named after the unix utility mkdir.

import os

os.mkdir('newdir')

A new directory will be created if one does not already exist.

removing a directory or file tree: os.remove() and shutil.rmtree()

If your directory is not empty, shutil.rmtree must be used.

import os
import shutil

os.mkdir('newdir')

wfh = open('newdir/newfile.txt', 'w')  # creating a file in the dir
wfh.write('some data')
wfh.close()

os.rmdir('newdir')        # OSError: [Errno 66] Directory not empty: 'newdir'

shutil.rmtree('newdir')   # success

• if a directory is empty, os.rmdir() can remove it
• if the directory is not empty, this function will fail
• in that case, shutil.rmtree() will succeed
• .rmtree() can also remove entire trees of directories and files
• obviously, care must be taken before removing an entire file tree!

copying a file tree

Again, take care when working with entire trees!

import shutil

shutil.copytree('olddir', 'newdir')

Regardless of what files and folders are in the directory to be copied, all files and folders (and indeed all child folders and files within those) will be copied to the new name or location.