Python Notes (8) - File Input/Ou

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The note covers how to apply what just learned to a real-world application: writing data to a file.

Python notes of open courses @Codecademy.

File I/O (Input/Output)

• f = open("output.txt", "w")
  • This told Python to open output.txt in "w" mode ("w" stands for "write").
  • It stored the result of this operation in a file object, f.
• Four modes
  • "w": write-only mode
  • "r": read-only mode
  • "r+": read and write mode
  • "a": append mode, which adds any new data you write to the file to the end of the file.
• .write(): writes to a file.
  • It takes a string argument. str() might be used.
• .read(): reads from a file.
• .close(): You must close the file after writing.

Advanced Functions

• .readline(): reading between the lines
  • If you open a file and call .readline() on the file object, you'll get the first line of the file; subsequent calls to .readline() will return successive lines.
• Buffering Data
  • During the I/O process, data is buffered: this means that it is held in a temporary location before being written to the file.
  • Python doesn't flush the buffer.
  • If we don't close a file, our data will get stuck in the buffer.
• with and as

  • A way to get Python to automatically close files.

  • File objects contain a special pair of built-in methods: __enter__() and __exit__(). When a file object's __exit__() method is invoked, it automatically closes the file.

  • Using with and as to invoke the __exit__() method, the syntax looks like this:

    with open("file", "mode") as variable:
        # Read or write to the file
    

• .closed
  • Python file objects have a closed attribute which is True when the file is closed and False otherwise.
  • By checking file_object.closed, we'll know whether our file is closed and can call close() on it if it's still open.

External Resources

• Python Files I/O