Python linecache Module: Random Access to File Lines

import linecache
 
# Get specific line (1-indexed)
line = linecache.getline('script.py', 10)
print(line)  # Line 10 of script.py
 
# Returns empty string if line doesn't exist
line = linecache.getline('script.py', 9999)
print(repr(line))  # ''

import linecache
 
# First call: reads and caches entire file
line1 = linecache.getline('data.txt', 1)
 
# Subsequent calls: returns from cache
line2 = linecache.getline('data.txt', 2)
line3 = linecache.getline('data.txt', 3)
 
# Much faster than repeatedly opening the file

import linecache
 
# Clear cache for specific file
linecache.checkcache('data.txt')
 
# Clear entire cache
linecache.clearcache()
 
# Update cache if file changed
linecache.updatecache('data.txt')

import linecache
 
# Get all lines (list)
lines = linecache.getlines('data.txt')
print(f"File has {len(lines)} lines")
 
# Lines include newline characters
for i, line in enumerate(lines, 1):
    print(f"{i}: {line}", end='')

import linecache
import os
 
# Read line from a module's source
line = linecache.getline(os.__file__, 1)
print(line)  # First line of os.py

import linecache
 
def show_error_context(filename, lineno, context=3):
    """Show lines around an error."""
    print(f"Error at {filename}:{lineno}")
    print("-" * 40)
    
    start = max(1, lineno - context)
    end = lineno + context + 1
    
    for n in range(start, end):
        line = linecache.getline(filename, n)
        if not line:
            break
        marker = ">>>" if n == lineno else "   "
        print(f"{marker} {n:4d}: {line}", end='')
 
show_error_context('script.py', 42)

import linecache
import traceback
import sys
 
def format_exception_with_context():
    exc_type, exc_value, exc_tb = sys.exc_info()
    
    print(f"Exception: {exc_type.__name__}: {exc_value}")
    print()
    
    for frame in traceback.extract_tb(exc_tb):
        filename, lineno, name, _ = frame
        print(f"  File {filename}, line {lineno}, in {name}")
        
        # Show surrounding lines
        for n in range(lineno - 1, lineno + 2):
            line = linecache.getline(filename, n)
            if line:
                prefix = ">" if n == lineno else " "
                print(f"    {prefix} {line}", end='')
        print()

import linecache
 
def get_log_entries(logfile, line_numbers):
    """Get specific log entries by line number."""
    entries = []
    for n in line_numbers:
        line = linecache.getline(logfile, n)
        if line:
            entries.append((n, line.rstrip()))
    return entries
 
# Get lines 100, 200, 300
entries = get_log_entries('app.log', [100, 200, 300])
for lineno, text in entries:
    print(f"Line {lineno}: {text}")

import linecache
 
def view_function_source(func):
    """Display source code of a function."""
    import inspect
    
    try:
        filename = inspect.getfile(func)
        lines, start = inspect.getsourcelines(func)
    except (TypeError, OSError):
        return "Source not available"
    
    output = []
    for i, line in enumerate(lines, start):
        output.append(f"{i:4d}: {line}")
    
    return "".join(output)

import linecache
 
def compare_lines(file1, file2, line_numbers):
    """Compare specific lines between two files."""
    differences = []
    
    for n in line_numbers:
        line1 = linecache.getline(file1, n).rstrip()
        line2 = linecache.getline(file2, n).rstrip()
        
        if line1 != line2:
            differences.append({
                'line': n,
                'file1': line1,
                'file2': line2,
            })
    
    return differences

import linecache
 
# linecache keeps files in memory
# Clear when done with large files
linecache.getlines('huge_file.txt')
# ... process lines ...
linecache.clearcache()  # Free memory

# linecache: cached, random access
linecache.getline('file.txt', 50)
 
# readlines: load all into list
with open('file.txt') as f:
    lines = f.readlines()
    line = lines[49]
 
# enumerate: sequential access
with open('file.txt') as f:
    for i, line in enumerate(f, 1):
        if i == 50:
            break

import linecache
 
# Get single line (1-indexed)
linecache.getline(filename, lineno)
 
# Get all lines
linecache.getlines(filename)
 
# Cache management
linecache.checkcache(filename)   # Refresh if changed
linecache.updatecache(filename)  # Force update
linecache.clearcache()           # Clear all
 
# Returns
# - Line with newline if exists
# - Empty string '' if not