File & I/O

---

What is I/O?

• 处理器访问任何寄存器和 Cache 等封装以外的数据资源都可以当成 I/O 操作，包括内存，磁盘，显卡等外部设备或者网络。

Why do you we non-volatile storage?

• store data after powering off your computer
• store a large mount of data for reuse
• share data among programs

---

What is streaming?

• build a pipeline to send data in and out
• python use streaming by default

file = open(...) # file is a streaming object, it is the pipline between code and the target file
pipeline = open(...) # makes more sense

---

Data Types of I/O

You can only read/write text or binary data from/into file.

• Text I/O: for text data

f = open("myfile.txt", "rt")

• Binary I/O: for binary data, buffered: music, video

f = open("myfile.jpg", "rb")

---

Buffered and UnBuffered I/O

• send a chunk of data rather than a piece of data
• Raw I/O: for binary data, not buffered, rarely use

f = open("myfile.jpg", "rb", buffering=0)

---

Input: Read

💡 You can only read data from `Read` mode (unless you use `+` parameter)

• r: Read. Default value

  • read data from the flie if the file exists
  • error if the file does not exist

  file = open("words.txt", "r")
  file = open("not-exist.txt") # throw error

  • how to read

  oneline = file.readline() # read one line
  
  lines = file.readlines() # store each line in an array [l1,l2,l3...ln]
  
  content = file.read() # read all content once
  
  f5 = file.readline(5) # read first 4 characters of the current line
  f5 = file.read(5) # read first 5 characters of all content
  
  for line in file: # for loop
  	print(line)
  
  next(file) # iterator method

  • manipulate the cursor (offset)

  # every time you run read txt file using any of above functionalities, the cursor will move downwards.
  # words.txt:
  # ->    1     |
  #       2     |
  #             ↓
  file.open('words.txt') # cursor at 0
  file.readline() # 1
  file.readline() # 2
  file.readline   # nothing!
  
  file.seek(0) # move curosr back to top
  file.tell() # print the current position of the cursor

  • It's not required to close the I/O if you are only reading data. But it is recommended.

  file.close()

  • If you want to auto close the file

  with open('words.txt') as file:
  	# your read actions here
    # you don't have to add file.close() any more

---

Output: Create, Write, Append

💡 You can only write data in `Create`, `Write` and `Append` mode unless you use the `+` parameter

• a: Append.

  • create the file if the file does not exist
  • adding data at the end of the file if the file already exists

  file = open("words.txt","a")
  
  file = open("not-exist.txt","a") # will automatically create not-exist.txt in current path

• w: Write.

  • create the file if the file does not exist.
  • clear content of an existing file and add data at the head of the file

  file.open("words.txt","w") # no error
  
  file.open("not-exist.txt","w") # will automatically create nont-exist.txt in current path

• x: Create

  • create the file if it does not exist
  • error if the file already exists

  file.open("file_to_create.txt","x") # will automatically create the file
  
  file.open("words.txt","x") # will throw error if the file already exsits

• how to write data

  file = open('words',"a")
  
  file.write("hello world") # without newline
  
  file.write("hello \n world") # with newline
  
  file.write(str(1)) # convert to string before writing number into file
  
  file.writeline("hello") # error, no methods for writeline
  
  file.writelines(["hello","world"]) # write any array into file

• It is required to use close to save data to file

  write() method only writes data into buffer, which is a temporay space in memory. The data will only be store into hard disk(flushing) only after you run close()

  file.close()

• It is very convenient and recommended to use with to auto close the file

  with open('words.txt') as file:
  	# your writing action here
  	# you don't have to add file.close() any more

---

Input/output Mode (Not Recommended)

Try to avoid the use of +

• make readable pipeline writable, but behave weired to write

file = open('words.txt','r+')
# replace different content from top to bottom, if exists, nothing happens
file.write('add something') # no error
file.close()

• make appendable pipeline readable, but behave weired to read

file = open('words.txt','a+')
file.read() # nothing showed since cursor will be automatically moved to the bottom
file.write('line\m') # append to the end
file.close()

• make writable/creatable pipeline readable

file = open('words.txt','w+')
file.read() # no error but no content
file.close()

---

Summary

• Basic Syntax

pipeline = open('words.txt','rt+')

• Auto-close Syntax

with open('words.txt','rt') as pipeline:
	# actions

💡 open会新建文件， close会保存文件