Python open() Function

The open() function is a built-in function in Python used to open files. It opens a file and returns a file object for reading, writing, appending, and other file operations.

Function Syntax

open(file, mode='r', buffering=-1, 
    encoding=None, errors=None, newline=None,
    closefd=True, opener=None)

Parameter Details

file

Required parameter, file path and name to open. Can use relative or absolute paths.

# Relative paths
open('data.txt')
open('../parent/data.txt')

# Absolute paths Windows
open(r'C:\Users\name\data.txt')
open('C:\\Users\\name\\data.txt')
open('C:/Users/name/data.txt')
# Absolute paths Windows/Linux
open('/home/user/data.txt')

Can also be a file descriptor:

import os

fd = os.open('data.txt', os.O_RDONLY)
open(fd, 'r')

mode

Specifies the file opening mode. If not specified, defaults to read-only mode 'r'.

Mode	Description	When file exists	When file doesn't exist
'r'	Read-only (default)	Opens normally	Raises FileNotFoundError
'w'	Write-only	Clears file	Creates new file
'x'	Exclusive write	Raises FileExistsError	Creates new file
'a'	Append	Appends at file end	Creates new file
'b'	Binary	Used with other modes
't'	Text mode (default)	Used with other modes
'+'	Read-write mode	Used with other modes

Common examples:

# Text mode (default)
open('file.txt', 'r')    # Read-only text
open('file.txt', 'w')    # Write-only text
open('file.txt', 'a')    # Append text

# Binary mode
open('image.jpg', 'rb')  # Read image
open('image.jpg', 'wb')  # Save image

buffering

File buffer size, defaults to -1 (system default).

encoding

File encoding, defaults to None (system default encoding).

Common encodings:

# Common encodings
open('file.txt', 'r', encoding='utf-8')     # Most common
open('file.txt', 'r', encoding='gbk')       # Chinese Windows
open('file.txt', 'r', encoding='gb2312')    # Chinese
open('file.txt', 'r', encoding='ascii')     # ASCII
open('file.txt', 'r', encoding='latin-1')  # ISO-8859-1

errors

How to handle encoding errors.

# Raise error
open('file.txt', 'r', encoding='utf-8', errors='strict')
# Ignore invalid characters
open('file.txt', 'r', encoding='utf-8', errors='ignore')
# Replace with � or ?
open('file.txt', 'r', encoding='utf-8', errors='replace')
# Replace with \\xhh
open('file.txt', 'r', encoding='utf-8', errors='backslashreplace')

newline

In text mode, the newline character used when reading or writing files. Defaults to None (system default). Other options include '\n', '\r', and '\r\n'.

closefd

Whether to close the file descriptor when closing the file object. Defaults to True.

import os

# Get file descriptor
fd = os.open('data.txt', os.O_RDONLY)
print(f"File descriptor: {fd}")

# Use file descriptor
# closefd=False, Python won't close this descriptor
f = open(fd, 'r', closefd=False)
content = f.read()
f.close()  # Only closes file object, not file descriptor

# File descriptor still valid
print(f"File descriptor {fd} still valid: {os.fstat(fd)}")

# Need to close manually
os.close(fd)

opener

A callable object for opening files, defaults to None. Must return a file descriptor.

import os

def custom_opener(path, flags):
    print(f"Opening file: {path}, flags: {flags}")
    # Must return file descriptor
    return os.open(path, flags)

with open('test.txt', 'r', opener=custom_opener) as f:
    pass

Return Value

The open() function returns a file object for file operations like reading and writing. After operations, use close() to close the file object and release resources.

Common file object methods:

f = open('data.txt', 'r+', encoding='utf-8')
content = f.read()     # Read all content
line = f.readline()    # Read one line
lines = f.readlines()  # Read all lines into list
chunk = f.read(100)    # Read 100 characters
    
f.write('hello world!\n')    # Write content
f.writelines(['line1\n', 'line2\n'])  # Write multiple lines

position = f.tell() # Get current file pointer position
f.seek(0)           # Move to file beginning
f.seek(10)          # Move to 10th byte
f.seek(0, 2)        # Move to file end
    
f.flush()           # Write buffer to disk
print(f.name)       # File name
print(f.mode)       # Opening mode
print(f.closed)     # Whether closed
print(f.encoding)   # File encoding
f.close()           # Close file object

Context Manager

Use with with statement:

with open('test.txt', 'r') as f:
    content = f.read()
    print(content)
    # No need to manually call f.close()
    # Context manager handles it automatically

open() Function Examples

Read entire file at once:

# Not suitable for large files
with open('data.txt', 'r') as f:
    content = f.read()
    print(content)

Copy files:

# Copy image file
with open('source.jpg', 'rb') as src:
    with open('copy.jpg', 'wb') as dst:
        dst.write(src.read())

Process large files in chunks:

# Read large files chunk by chunk, memory-friendly
CHUNK_SIZE = 1024 * 1024  # 1MB
with open('large_file.dat', 'rb') as f:
    while True:
        chunk = f.read(CHUNK_SIZE)
        if not chunk:
            break
            
        # Process chunk...

Read line by line using iterator, suitable for large files:

with open('data.txt', 'r') as f:
    for line in f:
        # Lines include newline character
        print(line.strip())  # strip() removes leading/trailing whitespace

Write content to files:

# Overwrite completely
with open('data.txt', 'w') as f:
    f.write('Hello, World!\n')
    f.write('Second line\n')
    f.writelines(['Third line\n', 'Fourth line\n'])  # Write multiple lines

# Append writing
with open('log.txt', 'a') as f:
    for i in range(10):
        f.write(f"{i}\n")

Modify file content in binary mode:

with open('test.bin', 'r+b') as f:
    # Move file pointer to 10th byte (0-indexed)
    f.seek(9)  
    f.write(b'X')  # Modify to byte 'X'