Computer Science 1026A/B Chapter Notes - Chapter 7: Delimiter, Escape Character, Newline
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23 Oct 2017
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Chapter 7: Files & Exceptions
test files: include windows notepad, python source code, and html files
open a file: supply the file name and mode in which the file is to be opened
function >>> infield = open(“input.txt”, “r/w”) >>> chose r or w for either reading or writing
file object: object returned when open function is called
>>> stored in a variable
close a file: use the close method >>> infield.close()
fileName.readline() >>> obtains lines of text from a file
>>> only obtains strings, but can be converted to int/float; if converted “\n” will not be
printed as part of what the line contains (\n = new line)
write text to a file: fileName.write(“Hello, World!\n”) or print(“Hello, World!”, file=fileName) or
print(“Hello, World!”, end=“”, file=fileName)
\\ = \ >>> when entering the names of files since \ is an escape character on its own in python
•iterate over a file object to read the lines of text in a file by using the for loop
•unlike a container, once a file is read, you cannot iterate over the file again without closing
and then reopening it
line.rstrip() >>> removes the newline character (\n) from a line of text (no blank lines printed)
blank space = delimiter = white space
fileName.lstrip(chars) >>>characters in the string chars are removed
fileName.rstrip(chars) >>> characters in the string chars are removed from the right
fileName.strip(chars) >>>characters are removed from the front and end of s
line.split() >>> splits a string at each blank space and returns the list of substrings (arguments
can be passed in the brackets that will change where the string splits, rather than just spaces)
s.splitlines() >>> returns a list that contains lines of a string split using \n as a delimiter
fileName.read() >>> reads individual characters rather than a whole line (argument = # of ch to
read)
•only reads and returns newline (\n) characters that terminate the individual lines when
encountered
•programs that start from the command line receive the command line arguments in the argh
list defined in the sys module
exception handling: provides a flexible mechanism for passing control from the point of error
detection to a handler that can deal with the error
raise statement: signals an exceptional condition and raises an exception object
try block: contains statements that could cause an exception
except clause: the exception handler is placed in here
•once a try block is entered, the statements in a finally clause are guaranteed to be executed,
even if an exception is not raised
•raise an exception as soon as a problem is detected, and handle it only when it can be
handled
•never use a finally block after an except block; they should never be apart of the same try
block
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