CIT 590Intro to Programming

Files etc

AnnouncementsFrom 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 email 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(http://www.google.com)• 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