27. 文件的读取与写入

# Directory structure:
/home/alice/
  └── projects/
      └── script.py
 
# Running from the projects folder:
$ cd /home/alice/projects
$ python script.py
# CWD is /home/alice/projects
 
# Running from the parent folder:
$ cd /home/alice
$ python projects/script.py
# CWD is /home/alice (not where script.py is!)

import os
 
# 获取当前工作目录
current_dir = os.getcwd()
print(f"Current directory: {current_dir}")

Current directory: /home/alice/projects/file_demo

# 当前工作目录中的文件
filename = "student_grades.txt"
 
# 子目录中的文件（当前目录内的 data 文件夹）
filename = "data/student_grades.txt"
 
# 父目录中的文件
filename = "../shared_data.txt"

# 这在 Windows、macOS 和 Linux 上都能工作
filename = "data/reports/sales.txt"

# 打开文件用于读取（默认模式）
file = open("message.txt")

file = open("message.txt")
# ... 使用文件进行操作 ...
file.close()  # 释放资源并确保数据已保存

file = open("data.txt")
result = process_data(file)  # 如果这里抛出异常...
file.close()  # ...这一行就永远不会运行！

file = open("data.txt")
print(file.closed)  # Output: False
 
file.close()
print(file.closed)  # Output: True

file = open("data.txt")
file.close()
 
# This raises ValueError: I/O operation on closed file
content = file.read()

file = open("data.txt", "r")  # 或者直接 open("data.txt")

file = open("output.txt", "w")

file = open("log.txt", "a")

# 假设 data.txt 包含："Hello\nWorld\n"
 
# 读取模式 - 内容不变
file = open("data.txt", "r")
file.close()
# 文件仍然包含："Hello\nWorld\n"
 
# 写入模式 - 内容被清空
file = open("data.txt", "w")
file.write("New content\n")
file.close()
# 文件现在包含："New content\n"
 
# 追加模式 - 保留内容，添加新内容
file = open("data.txt", "a")
file.write("Added line\n")
file.close()
# 文件现在包含："New content\nAdded line\n"

# 显式指定 UTF-8 编码（虽然它通常就是默认值）
file = open("data.txt", "r", encoding="utf-8")

# 写入包含特殊字符的文件
file = open("names.txt", "w", encoding="utf-8")
file.write("José\n")
file.write("François\n")
file.write("Müller\n")
file.close()
 
# 再读回来
file = open("names.txt", "r", encoding="utf-8")
content = file.read()
file.close()
print(content)

José
François
Müller

with open("data.txt", "r") as file:
    content = file.read()
    print(content)
# 文件在这里会自动关闭

# 手动关闭 - 有风险
file = open("data.txt", "r")
content = file.read()
result = process(content)  # 如果这里抛出异常...
file.close()  # ...这一行不会运行
 
# 使用 with - 安全
with open("data.txt", "r") as file:
    content = file.read()
    result = process(content)  # 即使这里抛出异常...
# ...文件仍会自动关闭

with open("input.txt", "r") as infile, open("output.txt", "w") as outfile:
    content = infile.read()
    outfile.write(content.upper())
# 两个文件都在这里自动关闭

with open("data.txt", "r") as file:
    content = file.read()
    print("Inside with block:", file.closed)  # Output: Inside with block: False
 
print("Outside with block:", file.closed)  # Output: Outside with block: True
 
# This raises ValueError: I/O operation on closed file
more_content = file.read()

with open("message.txt", "r") as file:
    content = file.read()
    print(content)

Welcome to Python!
This is a text file.
It has multiple lines.

Welcome to Python!
This is a text file.
It has multiple lines.

with open("message.txt", "r") as file:
    first_ten = file.read(10)  # 读取前 10 个字符
    print(f"First 10 characters: '{first_ten}'")

First 10 characters: 'Welcome to'

with open("message.txt", "r") as file:
    line1 = file.readline()
    line2 = file.readline()
    line3 = file.readline()
    print(f"Line 1: {line1}")
    print(f"Line 2: {line2}")
    print(f"Line 3: {line3}")

Line 1: Welcome to Python!
 
Line 2: This is a text file.
 
Line 3: It has multiple lines.
 

with open("message.txt", "r") as file:
    line1 = file.readline().rstrip()
    line2 = file.readline().rstrip()
    print(f"Line 1: {line1}")
    print(f"Line 2: {line2}")

Line 1: Welcome to Python!
Line 2: This is a text file.

with open("message.txt", "r") as file:
    while True:
        line = file.readline()
        if line == "":  # 文件结束
            break
        print(line.rstrip())

with open("message.txt", "r") as file:
    for line in file:
        print(line.rstrip())

Welcome to Python!
This is a text file.
It has multiple lines.

with open("message.txt", "r") as file:
    lines = file.readlines()
 
print(f"Number of lines: {len(lines)}")
for i, line in enumerate(lines, start=1):
    print(f"Line {i}: {line.rstrip()}")

Number of lines: 3
Line 1: Welcome to Python!
Line 2: This is a text file.
Line 3: It has multiple lines.

with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is a new file.\n")

Hello, World!
This is a new file.

# 首先，创建一个包含一些内容的文件
with open("demo.txt", "w") as file:
    file.write("Original content\n")
 
# 现在再次以写入模式打开它
with open("demo.txt", "w") as file:
    file.write("New content\n")
 
# 读取文件看看它包含什么
with open("demo.txt", "r") as file:
    print(file.read())

New content

with open("shopping_list.txt", "w") as file:
    file.write("Apples\n")
    file.write("Bananas\n")
    file.write("Oranges\n")

students = ["Alice", "Bob", "Carol", "David"]
 
with open("students.txt", "w") as file:
    for student in students:
        file.write(student + "\n")

Alice
Bob
Carol
David

# 创建一个包含初始内容的文件
with open("log.txt", "w") as file:
    file.write("Program started\n")
 
# 之后，追加更多内容
with open("log.txt", "a") as file:
    file.write("Processing data\n")
    file.write("Processing complete\n")
 
# 读取文件查看所有内容
with open("log.txt", "r") as file:
    print(file.read())

Program started
Processing data
Processing complete

# WARNING: This will raise FileNotFoundError if data.txt doesn't exist
with open("data.txt", "r") as file:
    content = file.read()

FileNotFoundError: [Errno 2] No such file or directory: 'data.txt'

try:
    with open("data.txt", "r") as file:
        content = file.read()
        print(content)
except FileNotFoundError:
    print("Error: The file 'data.txt' was not found.")
    print("Please check the filename and try again.")

Error: The file 'data.txt' was not found.
Please check the filename and try again.

try:
    with open("/root/protected.txt", "r") as file:
        content = file.read()
except PermissionError:
    print("Error: You don't have permission to access this file.")

try:
    with open("my_folder", "r") as file:
        content = file.read()
except IsADirectoryError:
    print("Error: 'my_folder' is a directory, not a file.")

try:
    with open("data.txt", "r", encoding="utf-8") as file:
        content = file.read()
except UnicodeDecodeError:
    print("Error: The file encoding doesn't match UTF-8.")
    print("The file might use a different encoding.")

filename = input("Enter filename: ")
 
try:
    with open(filename, "r") as file:
        content = file.read()
        print(content)
except FileNotFoundError:
    print(f"Error: '{filename}' does not exist.")
except PermissionError:
    print(f"Error: You don't have permission to read '{filename}'.")
except IsADirectoryError:
    print(f"Error: '{filename}' is a directory, not a file.")
except UnicodeDecodeError:
    print(f"Error: '{filename}' contains invalid text encoding.")

filename = input("Enter filename: ")
 
try:
    with open(filename, "r") as file:
        content = file.read()
        print(content)
except FileNotFoundError:
    print(f"Error: '{filename}' not found.")
except PermissionError:
    print(f"Error: No permission to read '{filename}'.")
except IsADirectoryError:
    print(f"Error: '{filename}' is a directory.")
except UnicodeDecodeError:
    print(f"Error: '{filename}' has invalid encoding.")
except Exception as e:
    print(f"Unexpected error reading file: {e}")