Reading & Writing Files

Happy Friday!

Quick Announcements

• Problem set 5 is due next week Monday.
• The deadline for courses withdrawal is today Friday October 23 at 5pm.

Recall the Moon Thresholding Problem

• On wednesday, we got the files from the Discord and completed the first task
• Initial tasks:
  • Add a dark blue background rectangle to the window
  • Add a GImage of the moon.png image centered in the window

Problem 2b: Odd Pixels

• Extract the pixel array from the moon image
• This moon image is a grayscale image, so the Red, Green, and Blue components are always equal
• Looping over all the pixels, how many pixels are darker than a value of 30?

Problem 2c: Thresholding

• Modify your last code to actually update the pixels in the array
  • If they are darker than 30, set them to a perfectly black pixel
  • Otherwise, set them to a perfectly white pixel
• Pass the updated pixel array into GImage(pix_array) and add the result to the window
• Play around with the thresholds to see different effects
• Since you are adding it on top of the original, you can also play around with giving the pixels some transparency (or making them different colors!)

Reading & Writing Files

• Programs often need to work with collections of data that are too large to reasonably exist typed all out in the code
  • Easier to read in the values of a list from some external data file
• A file is the generic name for any named collection of data maintained on some permanent storage media attached to a computer
• Files can contain information encoded in many different ways
  • Most common is the text file
  • Contains character data like you’d find in a string

Strings vs Text Files

• While strings and text files both store characters, there are some important differences:
  • The longevity of the data stored
    • The value of a string variable lasts only as long as the string exists, is not overridden, or is not thrown out when a function completes
    • Information in a text file exists until the file is deleted
  • How data is read in
    • You have access to all the characters in a string variable pretty much immediately
    • Data from text files is generally read in sequentially, starting from the beginning and proceeding until the end of the file is reached

Reading Text Files

• The general approach for reading a text file is to first open the file and associate that file with a variable, commonly called its file handle

• We will also use the with keyword to ensure that Python cleans up after itself (closes the file) when we are done with it.

  with open(filename) as file_handle:
    # Code to read the file using the file_handle

• Python gives you several ways to actually read in the data

  • read reads the entire file in as a string
  • readline or readlines reads a single line or lines from the file
  • read alongside splitlines gets you a list of line strings
  • Can use the file handle as an iterator to loop over

Entire file ⟶ String

• The read method reads the entire file into a string, with includes newline characters (\n) to mark the end of lines

• Simple, but can be cumbersome to work with the newline characters, and, for large files, it can take a large amount of memory

• As an example, the file:

  One fish
  two fish
  red fish
  blue fish

  would get read as "One fish\ntwo fish\nred fish\nblue fish"

Line by Line

• Of the ways to read the file in a string at a time, using the file handler as an iterator and looping is probably best and certainly most flexible

• Leads to code that looks like:

  with open(filename) as f:
      for line in f:
          # Do something with the line

• Note that most strategies preserve the newline character, which you very likely do not want, so be ready to strip them out before doing more processing

String Methods That Involve Lists

• str.split() splits str into a list of substrings at each whitespace character
• str.split(separator) splits str into a list of substrings at each occurrence of separator
• str.splitlines() splits str into a list of substrings at each line break
• sep.join(list) joins the elements of list into a string, using sep to separate them

Powers Combined

• So long as your files are not gigantic, using read and then the splitlines method can be a good option

• This does remove the newline characters, since it splits the string at them

  with open(filename) as f:
      lines = f.read().splitlines()
  # Then you can do whatever you want with the list of lines

Writing Text Files

• You can write text files using almost the same syntax as reading:

  with open(filename, mode) as file_handle:
      # Code to write the file using file_handle

• Note the mode parameter to open here! Mode is a string which is either

  • "w" to write a new file (or overwrite an existing file)
  • "a" to append new contents to the end of an existing file

• The file handler supports the methods:

  • .write(some_string) to write a string to the file
  • .writelines(iterable_of_strings) to write each iterable element to the file

Aren’t you Exceptional

Ignore Yoda, there is a try

Example: Requesting Existing File

Today’s Group Problems LO’s

• Understanding how to open a file
• Reading contents from a file
• Writing text back to a file

Problem 1: Inconceivable Code

My name is Inigo Montoya.
You killed my father.
Prepare to die.

with open('PBride.txt') as f:
    for line in f:
        w = line.split()
        if w[0] == "You":
            print(w[-1])

c = read('PBride.txt')
print(c.find("father"))

with open('PBride.txt') as f:
    c = f.read().splitlines()
    print(c[1][4])

with open('PBride.txt') as f:
    c = f.read()
    i = c.find("f")
    print(c[i:i+6])

Problem 2a: The Great White Whale

• If you have not done so, get the starter files from the Discord
• This is a coding problem. Work in pairs on a single computer
  • Discuss your algorithm BEFORE YOU WRITE ANY CODE. Make sure everyone understands.
• You are working in Prob2.py
• Initial tasks:
  • Read in the contents of moby_dick.txt, which contains the first paragraph from the classic text.
  • Print the text to the screen without any line breaks. (It will still wrap at the end of your terminal, that is ok.)

Problem 2b: The Great White Swine?

• Swap who is typing!
• Now you want to “Pig-Latinify” the text
• Will require extracting individual words from each string and passing into the imported word2piglatin function
  • This function has been written to be able to work with punctuation. So you don’t need to strip out punctuation. You can just pass in any string of characters that appears between two space.
• Print out the “Pig-Latinified” text to the terminal!

Problem 2c: The Great White Publisher

• Swap who is typing!
• This new version of Moby Dick is too good to let vanish!
• Write it to a new file, called moby_bacon.txt
• Format it across the same number of lines in this file as in the original