Chapter 7 - Files and Exceptions

text file

contains data that has been encoded as text

binary file

contains data that has not been converted to text

open()

opens a file and returns it as a file object

file.close()

closes the file, after which no more reads or writes to the file are allowed

file.read()

returns the file contents as a string

file.readlines()

returns a list of strings, where the first element is the contents of the first line, the second is the contents of the second line, and so on

file.readline()

returns a single line at a time

file.write()

writes a string argument to a file

file.seek(offset)

change file’s current position

file.write(‘text’)

to write a string to the file

mode

indicates how a file is opened

“r”

open the file for reading

“w”

open the file for writing

• if the file doesn’t exist, then the file is created

• contents of an existing file are overwritten

“a”

open the file for appending

• if the file doesn’t exist, then the file is created

• writes are added to the end of existing file contents

What does the “+” modifier mean for modes?

it allows you to perform both reading and writing on the same file stream

“r+”

open for reading and writing without truncating the file; the pointer is at the beginning

“w+”

truncates the file to zero length or creates a new one for reading and writing

“a+”

open for. reading and writing, appends at the end, creates the file if it doesn’t exist

CSV (comma-separated values) file

a text file that contains data separated by a specific character

csv module

can be used to help read and write files in the csv format

output buffer

output to a file is buffered by the interpreter before being written to the computer’s hard disk, so there may be a delay between a call of write() and writing that data to the disk

file.flush()

manually forces the program to clear its internal memory buffer and write all pending data down to the operating system’s file buffer

exception

error that occurs while a program’s running, causing the program to abruptly halt

traceback

a lengthy error message that is shown in the sample run and gives information regarding the line number(s) that caused the exception

exception handling

process of responding to unexpected or unwanted events and errors during execution

exception handling format

• try:

• except:

• else:

• finally:

try:

wraps code that might cause an exception

except:

handles the specific exception raised

else:

executes if no exception occurs

finally:

executes regardless of exceptions; often used for cleanup

EOFError

input() hits an end-of-file condition (EOF) without reading any input

KeyError

a dictionary key is not found in the set of keys

ZeroDivisionError

divide by zero error

ValueError

invalid value (e.g. input mismatch)

IndexError

index is out of bounds

custom exception type

must define a new class that inherits from the built-in Exception class

custom exception type syntax

class MyCustomError(Exception):
     """Exception raised for specific application errors."""
     pass