Input and Output

Introduction to Software Engineering (CSSE 1001)

Author

Paul Vrbik

Published

March 1, 2026

Why Files?

When Python (i.e. Idle) launches it gets allocated space in RAM which is possible to fill up. This is a problem if we want to solve memory intensive problems (e.g. analyzing tweets, payroll, or scientific computing). We may also want to save our data with persistence.

In either case, it is necessary to instruct Python to use the disk memory one level up.

Type	Order	2016 MacBook Pro	Persistence
CPU Cache L2	KB	256KB	Requires power.
CPU Cache L3	MB	8MB	Requires power.
Random Access Memory	GB	16GB	Requires power.
Disk	GB/TB	256GB	Persistent.
Cloud	PB	Functionally Infinite.	Persistent.

Table 1: Memory Hierarchy

The following is the contents of the file hello.txt that is in a directory called data.

data/hello.txt

What a¶
wonderful¶
hello world.

Here ¶ is the a new-line character which normally is not displayed.

Open a file

To open a file assumed to be in the same directory as where your running Python do

file = open("file.dat", "<mode>")

Mode	Description
`r`	read
`w`	write
`a`	append

Note to “append” means write at end of file.

To determine and/or set your working directory see os.getcwd() and os.chdir().

Read a file

Open a file called hello.txt that is inside the directory data.

file = open("data/hello.txt", "r")

Read a single line from that file.

file.readline()

'What a\n'

And read a single line again.

file.readline()

'wonderful\n'

And again.

file.readline()

'hello world.'

Note the lack of newline character

file.readline()

''

Returns newlines indefinitely.

file.readline()

''

file = open("data/hello.txt", "r")  # open for read.
for line in file:
    print(line)

What a
¶
wonderful
¶
hello world.
¶

The extra ¶ are inserted by Python’s print (and are not actually displayed).

Observe what happens if we repeat the loop from the cell.

for line in file:
    print(line)

(Nothing happens). This is because the file-pointer file has already reached the end of the file the last loop.

We need to reset the file pointer to the beginning.

file = open("data/hello.txt", "r") 
for line in file:
    print(line)

What a

wonderful

hello world.

Closing Files

If you do not close your files they remain open and vulnerable to side-effects. That is, you may find some of your file is missing or extra bits in your file when you neglect to close after use.

file = open("data/hello.txt", "r")

for line in file:
    line

fileclose()

`with`

An alternative to close (which is better) is to use the with construct.

Using the with construct has the advantage of closing your file for you — even if the program crashes while it is executing its code block.

with open("data/hello.txt", "r") as file:
    for line in file:
        print(line)

What a

wonderful

hello world.

Exercises

Exercise 1 Write a function

def num_empty_lines(file: str) -> int:

that counts the number of empty lines (those that only contain \n) in a file.

Solution

def num_empty_lines(path: str) -> int:
    """
    Count 'empty' lines.
    """
    ans = 0
    with open(path, 'r') as file:
        for line in file:
            if line == '\n':
                ans = ans + 1
    return ans

>>> num_empty_lines("filename.txt")

Exercise 2 Write a function

from typing import TextIO  # new doc-hint type
def num_empty_lines(file_pointer: TextIO) -> int:

that counts the number of empty lines (those that only contain \n) in a file.

Note that this is indeed different than the last one which took a name and not pointer (to a file).

Solution

def num_empty_lines(file_pointer: str) -> int:
    """
    Count 'empty' lines.
    """
    ans = 0
    for line in file_pointer:
       if line == '\n':
            ans = ans + 1
    return ans

>>> fp = open("filename.txt")
>>> num_empty_lines(fp)

Suppose we want to read the numbers from the following file into a list.

data/numbers.dat

1¶
2¶
3¶
4¶
5¶
6

When reading from a file we are always reading strings.

Do not forget to cast your strings to a more appropriate types when necessary.

with open("data/numbers.dat", "r") as file:
    ans = []
    for line in file:
       ans.append(int(line))

ans

Exercise 3 Write a function that finds the highest-rated band in:

band-data.dat

Band,Rating,Plays  # this is called a header
Metallica,7,512
The Black Keys,8,193
Amy MacDonald,5,290
The Killers,9,402
U2,9,789
Portishead,6,116
Wiggles,2,1849
Coldplay,7,0
Taylor Swift,11,1234567
Drake,0,1

Solution

from typing import  TextIO  # additional type

def highest_rated(path: TextIO) -> str:
    current_most_popular_band = ""
    current_highest_rating = -float('inf')  # Guarantees line 11 satisfied
    
    with open(path, 'r') as file:
        file.readline()    # skip the header
        for line in file:
            band, rating, _ = line.split(',')   
            # don't give names to values you don't intend on using
            rating = int(rating)
            if rating > current_highest_rating:
                current_highest_rating = rating
                current_most_popular_band = band
    
    return current_most_popular_band

highest_rated("data/band-data.dat")

Here is the solution for the lowest_rated band.

def lowest_rated(path: TextIO) -> str:

    return "Drake"

(This is a joke.)

Exercise 4 Extend the previous answer to take an arbitrary file with a header of attributes like

name, grade, age

and write a function

def most(path: str, attribute: str) -> str:

that finds the maximum of that attribute.

Exercise 5 Write a function

def read_board(fname: str) -> list[list[int]:

that given a file containing a partially filled Suduko board (see next slide) returns a list of list of integers with None in unfilled spots.

"board.dat

685|13 | 47
7  |   | 1 
 1 |764| 5 
-----------
9  | 7 |5 4
8 1|  9| 72
4 3|  6|   
-----------
   |427|39 
 4 |9  | 68
1 7|   |4

Exercise 6 Write a function

def has_won(board: list[list[int]]) -> bool

that returns True when the Suduko board is solved (contains the digits \(1\) through \(9\) in each row, column, and \(3 \times 3\) grid.

Writing to Files

with open("numbers.dat", "w") as file:
    file.write("Hello World.\n") 
    # Write single string.
    file.writelines(["Hello\n", "World\n"])
    # Write a list of strings.

Warning

Opening a file for write will create the numbers.dat file or overwrite the old one if it exists. Overwriting a file has the same effect as deleting it.

Appending to Files

Appending will open a file without over-writing, instead appending to the end of the file.

A file is created if one does not exist.

with open("numbers.dat", "a") as file:
    file.write("Hello World.\n")

Summary

We can read strings from files and write strings to files. A file-pointer is moved every time a line from the file is read. Therefore, if we want to read a line twice the file must be opened twice.