CIT 590 Intro to Programming Files etc

Announcements From HW5 onwards (HW5, HW6,…) You can work alone. You can pick your own partner. You can also stick to the same partner for every single HW. Decision has to be communicated to us by Sunday evening 6pm. No communication = solo. Please do not send me saying you are working alone. Please put CIT590 in the subject For any given week, after 6pm on Sunday No changing of pairs allowed after that. Cannot go from solo to pair after that. Cannot go from pairs to solo after that either. You are allowed to make a fresh decision each week.

Agenda Files Try catch except A module to read html off a remote website (only works sometimes)

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 Read – reads the whole file in as a string Readline – read it line by line. Each line is read as a string Readlines – reads in all the lines as a list Using a while loop to read every single line of a file Write, writelines fileExperiments.py

Closing a file 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

Looping through the contents of a file Line = f.readLine() and then follow it up by while line: Instead of using a while loop with a readline, python provides the ability to do a for loop for x in f: That will go through the file line by line and each time a full line will be assigned to the variable x Then you can do fun things with x How does the HW4 code work?

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 but it is ‘yucky’ Csv files work as well Usually if you are dealing with a specific format, you will be using some extra packages

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

The file sort example from the book Good example for top down design What if you had to sort a massive file that cannot possibly be stored all in memory 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