File Input and Output

Overview

Variables do not stay after your program is done. Files can help save data after your program is finished. A file is just a large string.

New Concepts

File Paths and File Extensions

A file can be referred to in two ways, its filename and its path. The path is the location of the file while the filename is the name of the file. For example, if I was the US President and I had the nuclear launch codes, the path would probably be something like;

C:\Users\Trump\Documents\nuclear_launch_codes.txt

In this case, the filename of the launch codes is "nuclear_launch_codes.txt" and the "Users", "Trump", and "Documents" are known as folders or directories. the C:\ in Windows is called the root folder. Root means the base or the lowest you can go, like a plant. The root folder contains everything. In Linux or macOS, the root folder is /. Windows and Unix-based operating systems deal with files different. Feel free to ask questions.

As a note, USB Drives are represented differently in Windows and Unix-based operating systems. In Windows, USB Drives are shown with an entirely different root folder like D:\ or E:. In Linux and macOS, the drives are attached to the root folder, sometimes under mnt.

(Show Tree structure of a folder hierarchy)

Backslash on Windows and Forward Slash on macOS and Linux

Another difference is that Windows paths are written with backslashes (). As macOS and Linux uses forward slashes (/). To make python work on all OS-es, you will need to handle both cases, which will be shown further down.

A helpful function is os.path.join(), it will return a string with a file path using the correct path seperators

  >> import os
  >> path = os.path.join('usr', 'bin', 'spam')
  'usr\\bin\\spam' # on windoge
  'usr/bin/spam    # on best OS

You need double back slashes when using Windows because you need to use an escape character smh

Current Working Directory (directory is another name for folder)

Follow along with the Python interactive shell. You may or may not get the same output. Do not worry, we are more concerned with you trying out the functions so you can see what they do.

os.getcwd() - gets current working directory os.chdir(path) - changes the directory

  >> import os
  >> os.getcwd()
  'C:\\Python34'
  >> os.chdir('C:\\Windows\\System32')
  >> os.getcwd()
  'C:\\Windows\\System32'
  >> os.chdir('C:\\RandomAFLocation')

  FileNotFoundError

Absolute vs. Relative Paths

An absolute path is a path that starts from root. A relative starts from working directory.

For example, C:\Users\Trump\Documents\nuclear_launch_codes.txt is the absolute path. While if I am in the Trump folder, .\Documents\nuclear_launch_codes.txt is the relative path. We can try this out in the windows command line.

Show graphic/example to compare and contrast

. (dot) for this directory .. (dot-dot) for parent folder

Creating New Folders

Use os.makedirs(path) to create a folder

The os.path Module

Whenever we are working with paths, we will be using the os.path module. To import this, we only need to type in "import os". This module gives us a few additional functions that can help us. Try to follow along with the Python interactive shell

Handling Abssolute and Relative path

os.path.abspath(path) - returns a string with the absolute path of the argument

  >> os.path.abspath('.')

os.path.isabs(path) - returns True if argument is an absolute path

  >> os.path.isabs('.') # Why is this sometimes false?
  False
  >> os.path.isabs(os.path.abspath('.')) # Why is this true?
  True

*os.path.relpath(path, start) - returns a string of a relative path from the start to path. If a start is not given, the current working directory is used

  >> os.path.relpath('C:\\Windows', 'C:\\')
  'Windows'

os.path.dirname(path) will return a string of everything that comes before the last slash os.path.basename(path) will return a string of everything that comes after the last slash

  >> path = 'C:\\Windows\\System32\\calc.exe'
  >> os.path.basename(path)
  'calc.exe'
  >> os.path.dirname(path)
  'C:\\Windows\\System32'

os.path.split(path) takes a path and returns a tuple of the base + dirname

  >> path = 'C:\\Windows\\System32\\calc.exe'
  >> os.path.split(path)
  ('C:\\Windows\\System32', 'calc.exe')

Finding File Sizes and Folder contents

os.path.getsize(path) returns the size in bytes of the file os.listdir(path) returns a list of strings for each file in the path

>> totalSize = 0
>> # What does this for-loop do?
>> for filename in os.listdir('.')
     totalSize = totalSize + os.path.getsize(os.path.join('.', filename))
>> print(totalSize) 

os.path.abspath() os.path.basename() os.path.exists() file.startswith() file.endswith() open() write() os.path.join()

Need list() dict.keys() dict.values() random.shuffle()