7.1 File I/O

Files store data persistently. Variables in memory disappear when a program ends, but data written to a file can remain. File I/O means input from files and output to files.

open()

Open a file before operating on it. open() returns a file object, and later reading, writing, and closing happen through that object.

file = open("text.txt", "r", encoding="UTF-8")
content = file.read()
file.close()

Common file modes:

• r: read only; the file must exist.
• w: write only; creates a new file and overwrites existing content.
• a: append; writes new content at the end.
• r+: read and write; the file must exist.

Prefer with open(...) as file because the with statement closes the file automatically when the block ends.

with open("text.txt", "r", encoding="UTF-8") as file:
    content = file.read()

Read Methods

Common file reading methods:

• read(): read the entire file and return a string.
• readline(): read one line and return a string.
• readlines(): read all lines and return a list of strings.

Parsing Words

The program below reads quotes.txt, removes punctuation, and writes each word into words.txt. In this example, quotes.txt contains:

Talk is cheap. Show me the code.
Code never lies, comments sometimes do.
Stay Hungry Stay Foolish.

import string

with open("words.txt", "w", encoding="UTF-8") as output_file:
    with open("quotes.txt", "r", encoding="UTF-8") as input_file:
        for line in input_file:
            words = line.split()
            for word in words:
                word = word.strip(string.punctuation)
                output_file.write(word + "\n")

with blocks can be nested. The outer block opens the output file, and the inner block reads the input file.

After the program runs, words.txt becomes:

Talk
is
cheap
Show
me
the
code
Code
never
lies
comments
sometimes
do
Stay
Hungry
Stay
Foolish