CSE 160

File IO

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Hannah Cheung

Announcements

• HW1 grades released, resubmissions due Sunday, October 22
• Programming Activity + Written Activity 3 released, due Friday, October 20
• HW3 released, due Friday, October 27

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Files

• Files store information when a program is not running
• Important operations:
• open a file
• close a file
• read data
• write data

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File objects and filenames

• A file object represents data on your disk drive
• Is an object in your Python program that you create
• Can read from it and write to it in your program
• A filename (usually a string) states where to find the data on your disk drive
• Can be used to find/create a file
• Linux/Mac uses forward slashes (/)
• Ex: “/home/hfc/class/160/lectures/file_io.pptx”
• Windows uses backward slashes (\)
• Ex: “C:\Users\hfc\class\160\lectures\file_io.pptx”
• Ex: “Husky.png”�

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Types of filenames

• An absolute filename gives a specific location on disk:
• Starts with “/” (Unix) or “C:\” (Windows)
• Ex: “/home/hfc/class/160/lectures/file_io.pptx”, “C:\Users\hfc\class\160\lectures\file_io.pptx”
• Warning: code will fail to find file if you move/rename files or run program on a different computer
• A relative filename gives a location relative to the current working directory
• Ex: “lectures/file_io.pptx”, “images\Husky.png”
• Warning: code will fail to find the file unless you run your program from a directory that contains given contents

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Filename Examples

• Linux/Mac: This could all refer to the same file
• “home/hfc/class/160/homework3/images/Husky.png”
• “homework3/images/Husky.png”
• “images/Husky.png”
• “Husky.png”
• Windows: These could all refer to the same file
• “C:\Users\hfc\class\160\homework\images\Husky.png”
• “homework3\images\Husky.png”
• “images\Husky.png”
• “Husky.png”

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Current Working Directory

• Current working directory in Python - directory from which you ran Python program
• Code to determine current working directory:

import os

print(“Current working directory is”, os.getcwd())

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Opening Files

# Takes a filename and returns a file object.

# Fails if file cannot be found and opened.

# By default, file is opened for reading.

my_file = open(“datafile.dat”)

my_file = open(“data.file.dat”, “r”)

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# Opens file for writing.

# Creates given file if it does not exist. If file

# already exists, then it will be overwritten.

my_file = open(“datafile.dat”, “w”)

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# Writing/appending to already-existing file.

my_file = open(“datafile.dat”, “a”)

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Reading Files

# Takes a filename and returns a file object.

# Fails if file cannot be found and opened.

# By default, file is opened for reading.

my_file = open(“datafile.dat”)

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# Approach: Process one line at a time

for line_of_text in my_file:

…process line_of_text

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# Approach 2: Process entire file at once

all_data_as_big_string = my_file.read()

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# close the file when done reading

my_file.close()

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Reading File Example

# Reads in file one line at a time and

# prints contents of the file.

in_file = “student_info.txt”

my_file = open(in_file)

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for line_of_text in my_file:

print(line_of_text)

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my_file.close()

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Reading File Example

# Count number of words in text file.

in_file = “student_info.txt”

my_file = open(in_file)

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num_words = 0

for line_of_text in my_file:

word_list = line_of_text.split()

num_words += len(word_list)

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my_file.close()

print(“Total words in file:”, num_words)

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Reading a file multiple times

• Iterating a file uses it up - cannot read it more than once.

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my_file = open(“datafile.dat”)

for line_of_text in my_file:

…process line_of_text

# loop will not be executed

for line_of_text in my_file:

…process line_of_text

Reading a file multiple times

• Iterating a file uses it up - cannot read it more than once.
• Solution 1: Read file into list, then iterate over it.

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my_file = open(“datafile.dat”)

lines = []

for line_of_text in my_file:

lines.append(line_of_text)

for line_of_text in lines:

…process line_of_text

for line_of_text in lines:

…process line_of_text

Reading a file multiple times

• Iterating a file uses it up - cannot read it more than once.
• Solution 1: Read file into list, then iterate over it.
• Solution 2: Re-create file object (slower, but better if file does not fit in memory)
• Try to avoid reading files more than once. Reading files is slow.

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my_file = open(“datafile.dat”)

for line_of_text in my_file

…process line_of_text

my_file = open(“datafile.dat”)

for line_of_text in my_file:

…process line_of_text

Writing to Files

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# Replaces any existing file of this name

my_file = open(“datafile.dat”, “w”)

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# Similar to printing output, except must

# explicitly specify new lines using \n

my_file.write(“a bunch of data”)

my_file.write(“a line of text\n”)

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# Argument provided must be a string

my_file.write(4) # Error!

my_file.write(str(4))

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# close the file when done writing

my_file.close()