File I/O in Python#

What is File I/O?#

File I/O (Input/Output) refers to reading data from and writing data to files. In Python, this is done using built-in functions.

Opening a File#

Syntax:

file = open('filename', 'mode')

Common modes: - ‘r’: Read (default mode) - ‘w’: Write (creates new or overwrites existing file) - ‘a’: Append (adds to the end of the file) - ‘b’: Binary mode (e.g., ‘rb’, ‘wb’) - ‘x’: Create a new file, fails if file exists

Reading from a File#

file = open('example.txt', 'r')
content = file.read()
file.close()

Other methods:

readline(): Reads one line at a time

with open('example.txt', 'r') as file:
    line1 = file.readline()
    line2 = file.readline()
    print("First line:", line1.strip())
    print("Second line:", line2.strip())

readlines(): Returns a list of all lines

with open('example.txt', 'r') as file:
    lines = file.readlines()
    print("All lines as list:")
    for i, line in enumerate(lines, 1):
        print(f"Line {i}: {line.strip()}")

Writing to a File#

file = open('example.txt', 'w')
file.write("Hello, World!\\n")
file.close()

Use ‘a’ mode to append data instead of overwriting.

Using `with` Statement#

Automatically manages file closing.

with open('example.txt', 'r') as file:
    content = file.read()

Working with Files Line-by-Line#

with open('example.txt', 'r') as file:
    for line in file:
        print(line.strip())

Checking File Existence (Optional)#

import os
if os.path.exists("example.txt"):
    print("File exists.")
else:
    print("File not found.")

File Methods Summary#

Below are commonly used file object methods in Python:

read(size=-1): Reads and returns up to size bytes. If size is omitted or negative, reads until end of file.

Example: .. code-block:: python

content = file.read(100) # Read first 100 bytes
write(string): Writes the specified string to the file. Only works in write or append mode.

Example: .. code-block:: python

file.write(“Hello, World!\n”)
close(): Closes the file. After closing, file operations will raise an error.

Example: .. code-block:: python

file.close()
seek(offset, whence=0): Moves the file pointer to the given byte offset. - offset: number of bytes - whence: optional; 0 (default) means start of file, 1 means current position, 2 means end of file

Example: .. code-block:: python

file.seek(0) # Move to the beginning of file
tell(): Returns the current file pointer position in bytes.

Example: .. code-block:: python

position = file.tell()

Binary File I/O#

Write a Binary File

data = bytes([120, 3, 255, 0, 100])

with open('example.bin', 'wb') as file:
    file.write(data)

Read a Binary File

with open('example.bin', 'rb') as file:
    content = file.read()
print("Binary content:", content)

Example in Some of Project#

Reading Annotation File

0--Parade/0_Parade_Parade_0_194.jpg
5
425 122 127 0 1 0 0 0 1
347 70 103 0 1 0 0 0 0
252 89 133 0 1 0 0 0 0
282 89 100 0 1 0 0 0 1
252 92 133 0 1 0 0 0 0

annotations = {}

with open('wider_face_val_bbx_gt.txt', 'r') as file:
    lines = [line.strip() for line in file if line.strip() != ""]

idx = 0
while idx < len(lines):
    image_path = lines[idx]
    idx += 1

    num_faces = int(lines[idx])
    idx += 1

    boxes = []
    for _ in range(num_faces):
        box = lines[idx].split()
        box = list(map(int, box[:4]))
        boxes.append(box)
        idx += 1

    annotations[image_path] = boxes