Slide 1

Chapter 5

Files and

Exceptions I

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What is a file?

•A file is a collection of data that is storedon secondary storage like a disk or athumb drive

•accessing a file means establishing aconnection between the file and theprogram and moving data between the two

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Two types of files

Files come in two general types:

•text files. Files where control characterssuch as "/n" are translated. This aregenerally human readable

•binary files. All the information is takendirectly without translation. Not readableand contains non-readable info.

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File Objects or stream

•When opening a file, you create a fileobject or file stream that is a connectionbetween the file information on disk andthe program.

•The stream contains a buffer of theinformation from the file, and provides theinformation to the program

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Buffering

•Reading from a disk is very slow. Thus thecomputer will read a lot of data from a filein the hopes that, if you need the data inthe future, it will be buffered in the fileobject.

•This means that the file object contains acopy of information from the file called acache (pronounced "cash")

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Making a file object

my_file = open("my_file.txt", "r")

•my_file is the file object. It contains thebuffer of information. The open functioncreates the connection between the disk fileand the file object. The first quoted string isthe file name on disk, the second is the modeto open it (here,"r" means to read)

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Where is the disk file?

•When opened, the name of the file cancome in one of two forms:

•"file.txt" assumes the file name isfile.txt and it is located in the currentprogram directory

•"c:\bill\file.txt" is the fullyqualified file name and includes thedirectory information

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Different modes

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Careful with write modes

•Be careful if you open a file with the 'w'mode. It sets an existing file’s contents tobe empty, destroying any existing data.

•The 'a' mode is nicer, allowing you towrite to the end of an existing file withoutchanging the existing contents

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Text files use strings

•If you are interacting with text files (whichis all we will do in this book), rememberthat everything is a string

–everything read is a string

–if you write to a file, you can only write a string

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writing to a file

Once you have created a file object, openedfor reading, you can use the print command

•you add file=file to the printcommand

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When the program is finished with a file, weclose the file

•flush the buffer contents from thecomputer to the file

•tear down the connection to the file

•close is a method of a file obj

file_obj.close()

•All files should be closed!

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Code Listing 5.1

Reverse file lines

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ExceptionsFirst Cut

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How to deal with problems

•Most modern languages provide methodsto deal with ‘exceptional’ situations

•Gives the programmer the option to keepthe user from having the program stopwithout warning

•Again, this is not about fundamental CS,but about doing a better job as aprogrammer

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What counts as exceptional

•errors. indexing past the end of a list,trying to open a nonexistent file, fetching anonexistent key from a dictionary, etc.

•events. search algorithm doesn’t find avalue (not really an error), mail messagearrives, queue event occurs

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exceptions (2)

•ending conditions. File should be closed atthe end of processing, list should besorted after being filled

•weird stuff. For rare events, keep fromclogging your code with lots of ifstatements.

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Error Names

Errors have specific names, and Pythonshows them to us all the time.

You can recreate an error to find the

correct name. Spelling counts!

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a kind of non-local control

Basic idea:

•keep watch on a particular section of code

•if we get an exception, raise/throw thatexception (let it be known)

•look for a catcher that can handle that kindof exception

•if found, handle it, otherwise let Pythonhandle it (which usually halts the program)

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Doing better with input

•In general, we have assumed that theinput we receive is correct (from a file,from the user).

•This is almost never true. There is alwaysthe chance that the input could be wrong

•Our programs should be able to handlethis.

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Worse yet, input is evil

• "Writing Secure Code”, by Howard andLeBlanc

– “All input is evil until proven otherwise”

•Most security holes in programs are basedon assumptions programmers make aboutinput

•Secure programs protect themselves fromevil input

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Rule 7

All input is evil, until proven otherwise

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General form, version 1

try:

suite

except a_particular_error:

suite

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try suite

•the try suite contains code that we wantto monitor for errors during its execution.

•if an error occurs anywhere in that trysuite, Python looks for a handler that candeal with the error.

•if no special handler exists, Pythonhandles it, meaning the program halts andwith an error message as we have seenso many times 

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except suite

•an except suite (perhaps multipleexcept suites) is associated with a trysuite.

•each exception names a type of exceptionit is monitoring for.

•if the error that occurs in the try suitematches the type of exception, then thatexcept suite is activated.

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try/except group

•if no exception in the try suite, skip allthe try/except to the next line of code

•if an error occurs in a try suite, look forthe right exception

•if found, run that except suite and thenskip past the try/except group to thenext line of code

•if no exception handling found, give theerror to Python

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Code Listing 5.2

Find a line in a file

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Counting Poker Hands

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Reminder, rules so far

1.Think before you program!

2.A program is a human-readable essay on problemsolving that also happens to execute on a computer.

3.The best way to imporve your programming andproblem solving skills is to practice!

4.A foolish consistency is the hobgoblin of little minds

5.Test your code, often and thoroughly

6.If it was hard to write, it is probably hard to read. Add acomment.

7.All input is evil, unless proven otherwise.