Lecture 19, Tue 03/12

File IO cont.

Recorded Lecture: 3_12_24

Common Ways to Read Data From Files (cont.)

• read() method – Reads the entire file into one string
• readline() method - Read everything from the current position to the next \n (or to the end of the file, EOF). If nothing left to read, returns an empty string.
• readlines() method - Read all the lines in the file into a list
• for loop - Read a line of text (separated by \n) each iteration
• Examples reading from file (example.txt) located in the same directory as our .py file:

This is Line 1
This is Line 2
This is Line 3

# Reading entire contents of file into program
infile = open('example.txt') 
data = infile.read()
print(data)
infile.close()

# Reading a single line (up to and including \n) in file
infile = open('example.txt')
data = infile.readline()
print(data)
data = infile.readline()
print(data)
data = infile.readline()
print(data)
infile.close()

# Creating a list where elements are a line (includes \n)
infile = open('example.txt')
data = infile.readlines()
print(data)
infile.close()

# Traversing through the file line-by-line in a for loop (includes \n)
infile = open('example.txt')
for line in infile:
	print(line)
infile.close()

Writing to Files

• We can write contents from our program into a file
• When writing to a file, we need to pass 'w' into the open function
  • If the file doesn’t exist, then Python will create a new file
  • If the file does exist, then writing to the file will clear all the existing content and write new content to the file
• Examples:

# Example writing to a new file `example2.txt`
outfile = open('example2.txt', 'w')
outfile.write("This is something new!")
outfile.close()

# Example of overwriting contents of a file
outfile = open('example2.txt', 'w')
outfile.write("Completely new text ... old text is gone!\n")
outfile.write("This is a new new line")
outfile.close()

Appending to Files

• If we don’t want to overwrite the contents of the file, we can choose to add on to the contents by appending more information
  • We need to pass the 'a' into the open function to append contents

# Example of appending contents of a file
outfile = open('example2.txt', 'a')
outfile.write("Something new!\n")
outfile.write("Another line!")
outfile.close()

# Example copying / writing from a file to another file.
infile = open('example.txt', 'r')
outfile = open('copy.txt', 'w')
for line in infile:
	outfile.write(line)
infile.close()
outfile.close()

with Statement

• As we mentioned before, it’s good practice to close a file after we open it
• For File IO, we can use the with statement
  • This allows us to open a file, execute some code within the with block, and automatically closes the file once the statements in the with block are done executing
• Example:

with open('example.txt', 'w') as outfile:
	outfile.write("Writing to the file - line 1\n")
	outfile.write("Exiting with block")

# After with block is finished executing, outfile is automatically closed
print("Done")

The os Module

• Python provides a module that interfaces with the Operating System (OS) running the Python program
• The OS module can be useful for many things. One thing is file paths may look different between Macs and Windows machines and are separated with different characters
  • Mac: /
  • Windows: \
• We can determine the path separator with os.path.sep
• So if we want to create a file path programmatically, we can join the correct separator with the folders / subfolders appropriately
• Example:

print(os.path.join("Users", "richert", "Desktop", "UCSB", "CS8W", "lecture.py")) # Users/richert/Desktop/UCSB/CS8W/lecture.py
# Note that the file separator would be different if I ran this on Windows

The CSV Reader

• A common file format for spreadsheets is comma separated values (csv) and are usually stored with a .csv extension
• Python has the csv module to help us read and process csv files
• For example, we can create the following csv file with the following data for student grades (sample.csv):

Student 1,100,98,100
Student 2,100,100,95
Student 3,100,95,90

• Note that if we open this with a spreadsheet editor, the data separated with commas are stored in individual cells
• The csv module has the method reader that takes in the csv opened file and an optional delimiter (, is used by default)
  • It returns an iterable object where each element is a list containing each data piece separated by commas
  • Note that each piece of data is a string, so we may need to convert the data into numerical types to do numerical computations
• Example:

import csv

with open('sample.csv', 'r') as csvFile:
	csvReader = csv.reader(csvFile) # could use a different delimiter param
	for row in csvReader:
		print(row)

• Another example: Calculate the average score for each student

import csv

with open('sample.csv', 'r') as csvFile:
	csvReader = csv.reader(csvFile)

	for row in csvReader:
		total = 0
		for i in range(1, len(row)):
			total += float(row[i])
		print(f"{row[0]}: average = {total / 3:.2f}")

• Another example: Drop the lowest score from the list of grades (there could be any ways to solve this)

import csv

def dropLowest(fileName):
	with open(fileName, 'r') as csvFile:

		with open("updatedSample.csv", 'w') as outFile:
			csvReader = csv.reader(csvFile)
			for row in csvReader:
				maxValue = 1000
				minIndex = -1
				for i in range(1, len(row)):
					if float(row[i]) < maxValue:
						maxValue = float(row[i])
						minIndex = i
				row.pop(minIndex)

				newRow = ""
				for element in row:
					newRow += element + ","
				newRow = newRow[:-1]
				outFile.write(newRow + "\n")

dropLowest("sample.csv")