Reading 3

Reading and Writing Python Files

What exactly is a file?

…a file is a continguous set of bytes used to store data.

### Parts of a Modern File

1. Header - the overall information about the file (name, size, type, etc)
2. Data - the meat and potatoes of the file
3. End of File (EOF) - a characted marking the end of the file.

Opening and Closing Files in Python

• Open - open(‘filename.ext’)
  • Once open always make sure to close the file when done.
  • Try and Finally help here:
• Closing

  reader = open('my_file.txt)
  try:
  # Do some things
  finally:
   reader.close()
  

  Ok Now What?

-.read() - reads entire file -.realine() - reads entire line -.readlines() - reads remaining lines from file -.write(string) - does what it says -.writelines(seq) - writes with no line endings, that is up to you

Exceptions

• raise can be used to throw an exception if a particular condition occurs.

  x = 3
  if x > 30:
  raise Exception('X shoul d exceed 30')
  

• try/except blocks are used to like try/catch in JS
  • ‘error’ may also be used as keyword and printed (except AssertionError as error: print(error) )
  • A try block will be exectued until the point an exception occurs, then skips any remaining code into the except block
  • an ‘else’ block may be used at the end, and is run if there are no exceptions
  • ‘finally’ blacks will run no matter the exception status