Python io Module: Core I/O Classes

from io import StringIO
 
# Create in-memory text file
buffer = StringIO()
buffer.write("Hello, ")
buffer.write("World!")
 
# Get contents
contents = buffer.getvalue()
print(contents)  # "Hello, World!"
 
# Read like a file
buffer.seek(0)
print(buffer.read())  # "Hello, World!"

from io import BytesIO
 
# Create in-memory binary file
buffer = BytesIO()
buffer.write(b"Binary data")
buffer.write(b"\x00\x01\x02")
 
# Get bytes
data = buffer.getvalue()
print(data)  # b'Binary data\x00\x01\x02'
 
# Read
buffer.seek(0)
print(buffer.read(6))  # b'Binary'

from io import StringIO, BytesIO
 
# Pre-populated StringIO
text_buffer = StringIO("Initial content")
print(text_buffer.read())  # "Initial content"
 
# Pre-populated BytesIO
binary_buffer = BytesIO(b"Initial bytes")
print(binary_buffer.read())  # b"Initial bytes"

from io import StringIO
 
buffer = StringIO("Line 1\nLine 2\nLine 3\n")
 
# Read all
buffer.read()
 
# Read line
buffer.seek(0)
buffer.readline()  # "Line 1\n"
 
# Iterate lines
buffer.seek(0)
for line in buffer:
    print(line, end='')
 
# Position
buffer.seek(0)      # Go to start
buffer.tell()       # Current position
buffer.seek(0, 2)   # Go to end
 
# Truncate
buffer.truncate(10)

from io import StringIO
import sys
 
# Capture print output
old_stdout = sys.stdout
sys.stdout = buffer = StringIO()
 
print("This goes to buffer")
print("So does this")
 
sys.stdout = old_stdout
captured = buffer.getvalue()
print(f"Captured: {captured}")

from io import StringIO
import csv
 
def parse_csv(file_obj):
    reader = csv.reader(file_obj)
    return list(reader)
 
# Test without actual file
csv_data = StringIO("a,b,c\n1,2,3\n4,5,6")
result = parse_csv(csv_data)
assert result == [['a', 'b', 'c'], ['1', '2', '3'], ['4', '5', '6']]

from io import BytesIO
from PIL import Image
 
# Create image in memory
img = Image.new('RGB', (100, 100), color='red')
buffer = BytesIO()
img.save(buffer, format='PNG')
 
# Get bytes for upload, storage, etc.
image_bytes = buffer.getvalue()

from io import BytesIO
import gzip
 
# Compress data in memory
data = b"Compress this data" * 100
buffer = BytesIO()
 
with gzip.GzipFile(fileobj=buffer, mode='wb') as f:
    f.write(data)
 
compressed = buffer.getvalue()
print(f"Original: {len(data)}, Compressed: {len(compressed)}")

from io import BytesIO
import requests
 
def download_to_buffer(url):
    response = requests.get(url, stream=True)
    buffer = BytesIO()
    
    for chunk in response.iter_content(chunk_size=8192):
        buffer.write(chunk)
    
    buffer.seek(0)
    return buffer
 
# Process without saving to disk
buffer = download_to_buffer("https://example.com/file.zip")

from io import StringIO
 
# Auto-closes (though not strictly necessary for memory buffers)
with StringIO() as buffer:
    buffer.write("content")
    content = buffer.getvalue()

from io import IOBase, RawIOBase, BufferedIOBase, TextIOBase
 
# IOBase: Abstract base for all I/O
# RawIOBase: Raw binary I/O
# BufferedIOBase: Buffered binary I/O  
# TextIOBase: Text I/O
 
# Check what type a file is
with open('file.txt', 'r') as f:
    print(isinstance(f, TextIOBase))  # True
 
with open('file.bin', 'rb') as f:
    print(isinstance(f, BufferedIOBase))  # True

from io import StringIO
 
# String: immutable, no file interface
text = "Hello"
# text.write("more")  # Error
 
# StringIO: mutable, file interface
buffer = StringIO("Hello")
buffer.write("more")  # Works
buffer.seek(0, 2)     # Seek to end
buffer.write(" text")

from io import StringIO
 
# Building strings: StringIO is efficient
buffer = StringIO()
for i in range(10000):
    buffer.write(f"Line {i}\n")
result = buffer.getvalue()
 
# Equivalent but often slower:
result = ""
for i in range(10000):
    result += f"Line {i}\n"
 
# Best for joining: use join
lines = [f"Line {i}\n" for i in range(10000)]
result = "".join(lines)

from io import StringIO, BytesIO
 
# StringIO: text
buf = StringIO()
buf = StringIO("initial")
buf.write("text")
buf.getvalue()       # Get all content
buf.read()           # Read from position
buf.readline()       # Read one line
buf.seek(0)          # Reset position
buf.tell()           # Current position
buf.truncate()       # Truncate at position
 
# BytesIO: binary
buf = BytesIO()
buf = BytesIO(b"initial")
buf.write(b"bytes")
buf.getvalue()       # Get all bytes
buf.read()           # Read from position