Python File Handling

File handling lets your program save data to a file and read it back later.

Opening a File

The open() function takes a file name and a mode:

Mode	Meaning
`"r"`	Read (default). Error if the file does not exist
`"w"`	Write. Creates the file, or erases it if it exists
`"a"`	Append. Adds to the end of the file
`"x"`	Create. Error if the file already exists

Add "b" for binary files like images, for example "rb".

Writing a File

Example

python

with open("students.txt", "w") as f:
    f.write("Ali\n")
    f.write("Ayesha\n")
    f.write("Hamza\n")
print("File saved")

Output

File saved

The with statement closes the file for you when the block ends, even if an error happens. Always use it.

write() does not add a new line by itself. Put \n at the end of each line yourself.

Reading a File

Example

python

with open("students.txt", "r") as f:
    content = f.read()
print(content)

Output

Ali
Ayesha
Hamza

Method	Returns
`read()`	The whole file as one string
`readline()`	The next single line
`readlines()`	A list of all lines

Reading Line by Line

Looping over the file reads one line at a time. This works well even for very large files.

Example

python

with open("students.txt") as f:
    for number, line in enumerate(f, start=1):
        print(number, line.strip())

Output

1 Ali
2 Ayesha
3 Hamza

strip() removes the \n at the end of each line.

Appending to a File

Example

python

with open("students.txt", "a") as f:
    f.write("Sana\n")

with open("students.txt") as f:
    print(f.readlines())

Output

['Ali\n', 'Ayesha\n', 'Hamza\n', 'Sana\n']

A Small Practical Example

Save daily sales, then calculate the total:

Example

python

sales = [1500, 2300, 1800]

with open("sales.txt", "w") as f:
    for amount in sales:
        f.write(f"{amount}\n")

total = 0
with open("sales.txt") as f:
    for line in f:
        total += int(line)
print("Total sales: Rs", total)

Output

Total sales: Rs 5600

Checking and Deleting Files

Use the os module:

Example

python

import os

if os.path.exists("sales.txt"):
    os.remove("sales.txt")
    print("Deleted")
else:
    print("File not found")

Output

Deleted

Checking first avoids a FileNotFoundError.

Exercise

Which mode adds new text to the end of a file without erasing what is already there?

"w""r""a""x"