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Python File Operations

• Files are data containers on disk, with text files (.txt, .csv) being human-readable and binary files (.jpg, .docx) having special formats.

• File paths can be absolute ("/home/student/documents/diary.txt") or relative ("../pictures/vacation.jpg").

• open("file", "r") opens for reading, "w" for writing (overwrite), "a" for appending, "a+" for append (create if not exist), "r+" for read+write.

• file.read() reads entire file, file.readline() reads one line, file.readlines() reads all lines as list.

• file.write("string") writes a string, file.writelines(list) writes lines from a list.

• Always use file.close() to close files and free resources.

• with open('file.txt', 'r') as f: automatically closes file after code block execution.

• Nested with handles multiple files: with open('src.txt', 'r') as src: with open('dst.txt', 'w') as dst: dst.write(src.read()).

• import os for file system operations. os.path.exists("path") checks path existence.

• os.path.isfile("path") checks if path is a file. os.path.isdir("path") checks if path is a directory.

• os.path.getsize("file.txt") returns file size in bytes.

• os.mkdir("dirname") creates a directory. os.makedirs("path") creates nested directories.

• os.listdir("directory/") returns list of names in that directory.

• os.rename("old.txt", "new.txt") renames files or directories.

• Encoding converts text to numbers. ASCII supports 128 English characters. UTF-8 supports all languages (1-4 bytes, Python 3 default). GBK is Chinese legacy encoding (2 bytes per Chinese character).

• Specify encoding when opening files: open('file.txt', 'r', encoding='utf-8').

• Handle encoding errors with errors='ignore' to skip characters or errors='replace' to replace with ?.

• ord('A') returns 65, chr(65) returns ‘A’.

• "string".replace('old', 'new') replaces substrings.

• Caesar cipher encrypts by shifting: chr(ord(char) + shift).

• Caesar cipher decrypts by shifting back: chr(ord(char) - shift).