Reading from a File

读取整个文件

with open('pi_digits.txt') as file_object:
    contents = file_object.read()
print(contents)

python会自动close(), 手动close会引发预想不到的问题

绝对路径

windows_path = 'C:\\path\\to\\file.txt'  
windows_path2 = 'C:/path/to/file.txt'
linux_path = '/home/ehmatthes/other_files/text_files/filename.txt'

逐行读取

with open('pi_digits.txt') as file_object:
    for line in file_object:
        print(line.rstrip()) # rstrip去除右边的换行符

可以看到每行末尾有换行符

将文件每行存入list，拼成字符串

file_name = 'pi_digits.txt'
with open(file_name) as file_object:
  file_list = file_object.readlines()
pi_string = ''
for line in file_list:
  pi_string += line.strip()
print(pi_string)

readlines()将每行存入list
你只能在with里读取file_object, 所以可以通过readlines()或着read()将数据读取到内存，在任意地方都可以访问
文件读取默认为字符串，可通过int()或float()转换为数字
python对处理文本的长度没有限制，只要内存够大

index()和replace()

index返回在字符串中位置

sentence = 'Python programming is fun.'
# Substring is searched in 'gramming is fun.'
print(sentence.index('ing', 10))
# Substring is searched in 'gramming is '
print(sentence.index('g is', 10, -4))
# Substring is searched in 'programming'
print(sentence.index('fun', 7, 18))   # 后2个参数是开始和结束位置

replace替换字符串中字符

song = 'cold, cold heart'
# replacing 'cold' with 'hurt'
print(song.replace('cold', 'hurt'))
song = 'Let it be, let it be, let it be, let it be'
# replacing only two occurences of 'let'
print(song.replace('let', "don't let", 2))  # 2代表替换的个数

Writing to a file

with open('programming.txt', 'w') as file_object:
    file_object.write("hello\n")
    file_object.write("I love programming\n")

• ‘w’ 代表以write模式打开
• 如果文件不存在会自动创建
• ‘w’ 模式会先清空文档，再返回file_object, 内容会被覆盖
• ‘a’ 模式不会清空原有内容，只会附加
• 只能写入字符串，不是字符串用str()转换
• 写入多行要用换行符 \n
• 模式：read mode (‘r’), write mode (‘w’), append mode (‘a’), or a mode that allowsyou to read and write to the file (‘r+’) ```python filename = ‘programming_poll.txt’

responses = []

while True: response = input(“Why do you like programming?”) responses.append(response)

continue_poll = input("Would you like to let someone else to respond? y/n")
if continue_poll != 'y':
    break

with open(filename, ‘a’) as f: for response in responses: f.write(f”{response}\n”)

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### Exceptions
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#### ZeroDivisionError
```python
print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit. ")
while True:
    first_number = input("\nFirst number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    if second_number == 'q':
        break
    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0")
    else:
        print(answer)

如果try成功，则执行else里的语句

FileNotFoundError

filename = 'alice.txt'
try:
    with open(filename, encoding='utf-8') as f:
        contents = f.read()
except FileNotFoundError:
    print(f"Sorry, the file {filename} does not exist.")

• 用 f 代表文件对象是传统
• encoding指定编码格式
• 如果encoding报错，还可以添加errors=’ignore’, 这样可能会丢失一些字符

  with open(filename, encoding='utf-8', errors='ignore') as f:

粗略计算一本书的字数

def word_counts(filename):
  """count the approximate number of words in a file"""
  try:
    with open(filename, encoding='utf-8') as f:
      contents = f.read()
  except FileNotFoundError:
    pass
  else:
    words = contents.split()
    number_words = len(words)
    print(f"The file {filename} has about {number_words} words.")
filenames = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt', 'little_women.txt']
for filename in filenames:
  word_counts(filename)

• split()将字符串分割，参数默认为空格
• 如果想except时什么都不做，用pass, pass也可以当作占位符

  count()

  粗略计算字符串出现的次数

  >>> line = "Row, row, row your boat"
  >>> line.count('row')
  2
  >>> line.lower().count('row')
  3

  Storing Data

  Using json.dump() and json.load()

  ```python import json

Load the username, if it has been stored previously.

Otherwise prompt for the username and store it.

filename = ‘username.json’ try: with open(filename) as f: username = json.load(f) # 读取 except FileNotFoundError: username = input(“What is your name? “) with open(filename, ‘w’) as f: json.dump(username, f) # 写入 print(f”We’ll remember you when you come back, {username}!”) else: print(f”Welcome back, {username}!”)

<a name="bDS1H"></a>
#### Refactoring重构
```python
import json
def get_stored_username():
    """Get stored username if available."""
    filename = 'username.json'
    try:
        with open(filename) as f:
          username = json.load(f)
    except FileNotFoundError:
        return None
    else:
        return username
def get_new_username():
    """Prompt for a new username."""
    username = input("What is your name? ")
    filename = 'username.json'
    with open(filename, 'w') as f:
        json.dump(username, f)
    return username
def greet_user():
    """Greet the user by name."""
    username = get_stored_username()
    if username:
        print(f"Welcome back, {username}")
    else:
        username = get_new_username()
        print(f"We'll remember you when you come back, {username}!")
greet_user()