open() Function Complexity¶

The open() function opens a file and returns a file object, allowing reading, writing, and manipulation of file contents.

Complexity Reference¶

Operation	Time	Space	Notes
`open(filename)`	O(1)*	O(1)	System call; * may vary with filesystem
`file.read()`	O(n)	O(n)	n = file size
`file.readline()`	O(k)	O(k)	k = line length
`file.readlines()`	O(n)	O(n)	n = file size
`file.write(data)`	O(k)	O(1)*	k = data length; * buffered, actual I/O varies
`file.close()`	O(1)	O(1)	Flush and close
`for line in file:`	O(n)	O(1)	n = file size (lazy iteration)

Opening Files¶

Basic File Operations¶

# Open for reading - O(1) system call
file = open('data.txt', 'r')  # O(1)
content = file.read()         # O(n) - read entire file
file.close()                  # O(1)

# Or use context manager (recommended)
with open('data.txt', 'r') as file:  # O(1)
    content = file.read()             # O(n)
# File automatically closed - O(1)

File Modes¶

# Read mode (default) - O(1)
file = open('file.txt', 'r')      # O(1) - text mode

# Binary mode - O(1)
file = open('file.bin', 'rb')     # O(1) - binary read

# Write mode - O(1)
file = open('output.txt', 'w')    # O(1) - text write

# Append mode - O(1)
file = open('log.txt', 'a')       # O(1) - append to end

# Read/Write - O(1)
file = open('data.txt', 'r+')     # O(1) - read and write

Reading File Contents¶

Read Methods¶

with open('data.txt', 'r') as file:
    # Read entire file - O(n)
    content = file.read()  # O(file_size)

    # Read line by line - O(k) per line
    file.seek(0)  # O(1) - go to start
    line = file.readline()  # O(k) - k = line length

    # Read all lines into list - O(n)
    file.seek(0)
    lines = file.readlines()  # O(n) - creates list

    # Iterate lines (memory efficient) - O(1) per line
    file.seek(0)
    for line in file:  # O(1) per iteration, lazy
        process(line)

Reading Line by Line (Memory Efficient)¶

# Best for large files: iterate without loading all
with open('largefile.txt', 'r') as file:
    for line in file:  # O(1) per line, O(1) memory
        # Process one line at a time
        print(line.strip())

# vs. loading all at once - O(n) memory
with open('largefile.txt', 'r') as file:
    all_lines = file.readlines()  # O(n) memory!
    for line in all_lines:        # Still O(1) per iteration
        print(line.strip())

Writing to Files¶

Write Operations¶

# Write string - O(k) where k = string length
with open('output.txt', 'w') as file:
    file.write('Hello')  # O(5)
    file.write(' ')      # O(1)
    file.write('World')  # O(5)

# Write multiple lines - O(n)
lines = ['Line 1\n', 'Line 2\n', 'Line 3\n']
with open('output.txt', 'w') as file:
    file.writelines(lines)  # O(n)

# Write with formatting - O(n)
with open('output.txt', 'w') as file:
    for i, item in enumerate(items):
        file.write(f"{i}: {item}\n")  # O(k) per write

File Iteration¶

Iterating Lines¶

# Most efficient: lazy line iteration - O(1) memory per line
with open('data.txt', 'r') as file:
    for line in file:  # O(1) per iteration
        # Process line
        process(line)

# This is more efficient than:
with open('data.txt', 'r') as file:
    for line in file.readlines():  # O(n) memory upfront!
        process(line)

# For files with millions of lines:
# - Iteration: O(n) time, O(1) memory
# - readlines(): O(n) time, O(n) memory

File Positioning¶

Seek and Tell¶

with open('data.txt', 'rb') as file:
    # Get current position - O(1)
    pos = file.tell()  # Usually 0 at start

    # Read some bytes - O(k)
    data = file.