CIT 590 Intro to Programming Lecture 6

Vote in the doodle poll so we can use some fancy algorithm to pair you up You have to program in pairs I sadly still don’t know if we have an odd number of students or an even number. We don’t know the algorithm yet but it has to satisfy the obvious time constraint

Agenda Files Try catch except A module to read html off a remote website

Basic file operations f = open(‘myRandomFile.txt’) Open(“”) Default is the read only mode Open(“”, “w”) Open the file in the mode that you would like to use it Read - r Write - w Append – a Opening a file in the ‘r+’ mode if you want to read and write to it at the same time. Use this one with caution

Reading and writing to a file Read, readline, readlines Using a while loop to read every single line of a file Write, writelines Close() – why should I close a file Cleaning up The operating system should close the file, but that behavior is not entirely under your control Remember that opening a file for writing will remove the old values fileExperiments.py

Tell and seek Once I have read a file all the way through how do I go back to the start?? Or in general, how do I mark a point in the file that I now want to come back to. x = f.tell() #do some reading writing etc etc on f f.seek(x) to get back to where we were

What file formats are supported? Text files work best in that they do not come with headers and footers etc You can always read in binary Usually if you are dealing with a specific format, you will be using some extra packages

Looping through the contents of a file Instead of using a while loop with a readline, python provides the ability to do a for loop For line in f will read one single line of the file and assign it to the temp variable line. Loop until EOF (end of file)

The os module File management is generally being done at the operating system level In Python if you want to use command prompt like commands ‘import os’ os.listdir(‘C:\Python27’) os.rename(‘oldfilename’, ‘newfilename’)

Exceptions! Try except try: Something except error, e: do something with the error. Maybe print some message else: #it was a successful attempt. Now do the real work tryExceptForFiles.py

Standard input and output The sys module gives you access to some built in ‘files’ The console input is basically just sys.stdin Similarly the console output is sys.stdout Sys module has other built in functions like sys.argv to get the command line arguments. add10ToConsoleInput.py

The file sort example from the book Good example for top down design fileSort.py

Reading from a url Import urllib Urllib.urlopen( Only works on a subset of sites. Secure sites will usually not allow you to grab info that easily. My seas website isn’t particularly secure …. getAllTAs.py