reading-notes

FileIO and Exceptions

References

• Read & Write Files in Python
• Exceptions in Python
• VIDEO : Read & Write Files in Python Companion Video
• Reading & Writing Files in Python Quiz

Notes

Read & Write Files in Python

• What is a file?
  • A set of bytes used to store data
  • Modern files have three main parts:
    1. Header: metadata about the contents of the file
    2. Data: contents of the file
    3. End of File (EOF): special character that indicates the end of the file

    

• File Paths
  • All files require a file path
  • Three major parts of a file path:
    1. Folder Path: location on the file system where subsequent folders are separated by \ or /
    2. File Name: actual name of the file
    3. Extension: indicates file type
• Opening & Closing Files in Python
  • file = open('<file_name>')
  • file.close()
  • Two ways to implement:

    1. try, finally

       reader = open('dog_breeds.txt')
       try:
           # Further file processing goes here
       finally:
           reader.close()
       

    2. with

       with open('dog_breeds.txt', 'r') as reader:
       # Further file processing goes here
       

  • Optional arguments (mode) for open()
    • 'r' = open for reading (default)
    • 'w' = open for writing, truncating (overwriting) the file first
    • 'rb' or 'wb' = open in binary mode (read/write using byte data)
• Reading and Writing Opened Files
  • .read(size=-1)
    • This reads from the file based on the number of size bytes. If no argument is passed or None or -1 is passed, then the entire file is read.
  • .readline(size=-1)
    • This reads at most size number of characters from the line. This continues to the end of the line and then wraps back around. If no argument is passed or None or -1 is passed, then the entire line (or rest of the line) is read.
  • readlines()
    • Reads the remaining lines from the file object and returns them as a list.
    • NOTE: Returns a list object
  • write(string)
    • This writes the string to the file.
  • writelines(seq)
    • This writes the sequence to the file. No line endings are appended to each sequence item. It’s up to you to add the appropriate line ending(s).
• Tips & Tricks
  • __file__
    • Returns the path relative to where the initial Python script was called.
      • If you need the full system path, you can use os.getcwd() to get the current working directory of your executing code.
  • 'a' character

    • Append to a file or start writing at the end of an already populated file

      with open('dog_breeds.txt', 'a') as a_writer:
        a_writer.write('\nBeagle')
      

Python Exceptions

• Occur whenever syntactically correct Python code results in an error.
  • See list of built in exceptions

• Can create custom exceptions using raise

  x = 10
  if x > 5:
      raise Exception('x should not exceed 5. The value of x was: {}'.format(x))
    
  #Returns
  
  Traceback (most recent call last):
    File "<input>", line 4, in <module>
  Exception: x should not exceed 5. The value of x was: 10
  

• AssertionError
  • Can have the program throw an error when an assert condition is false
  • We assert that a condition exists, and if True the program will execute

  import sys
  assert ('linux' in sys.platform), "This code runs on Linux only."
  
  #Returns
  
  Traceback (most recent call last):
    File "<input>", line 2, in <module>
  AssertionError: This code runs on Linux only.
  

• try, except, else, finally
  • try says, “run this block of code if there are no exceptions”
  • except says, “run this block of code if there is an exception”
  • else says, “run this block of code next if there are no exceptions”
  • finally says, “after all of the above, then do this code”