Module 10 - File Access Flashcards
What do you call a program that runs all the time and keeps at least some of its data in permanent storage?
A persistent program
example: operating systems and web servers
How do you read a line from a text file? Write the sample code.
use open function and assign to variable:
fin = open (‘words.txt’)
then use readline method:
fin.readline ( )
What is a text file?
A sequence of characters stored on a permanent medium like a hard drive, flash memory, or CD-ROM
How do you write a file?
You have to open it with mode ‘w’ as a second parameter:
fout = open(‘output.txt’, ‘w’)
What are the two types of files? What is the difference?
Text file - contains data that has been encoded as text
Binary file - contains data that has not been converted to text
What are the two types of access to data stored in a file?
Sequential access - file read sequentially from beginning to end, can’t skip ahead
Direct access - can jump directly to any piece of data in the file
What is a filename extension and what does it indicate?
Short sequence of characters at the end of filename (after the period)
Indicate the type of data stored in the file
What’s a file object? What’s the point of it?
Object associated with a specific file
Provides a way for a program to work with the file: file object referenced by a variable
What’s the ‘open’ function used for and what does it do? What’s the syntax?
Used to open a file
It creates a file object and associates it with a file on the disk
file_object = open (filename, mode)
What are the different modes of the ‘open’ function?
Mode is the second parameter in the function
Reading only - ‘r’
Writing - ‘w’
Appending - ‘a’
What happens if the open function receives a filename that does not contain a path?
It assumes that file is in the same directory as the program
If a program is running and a file is created, where is it created? Can you alter this?
In the same directory as the program
Can specify alternative path and file name in the open function argument - prefix the path string literal with the letter r
What is a method?
A function that belongs to an object
Performs operations using that object
How do you write data to a file?
File object’s write method
file_variable.write(string)
How do you close a file?
File object’s close method
file_variable.close( )
What’s the return value of the write method?
The number of characters that were added (i.e., ‘24’)
What does ‘read’ do? How does it work?
Reads entire file contents into memory
Works only if file has been opened for reading
Contents return as a string
What’s ‘read position’ do?
Marks the location of the next item to be read from a file
What happens to the data if you open an existing file in ‘w’ mode?
It is overwritten
Use ‘a’ (append) to add to end of current contents
What happens if you try to append a file that does not exist?
The ‘a’ mode will create a new file
T or F: Numbers must be converted to strings before they are written to a file
True
Use str function
Then use int or float functions to convert back (to perform math)
How can you use a while loop to read a file?
Use the readline method
while line != “ “
How can you use a for loop to read a file?
for line in file_object:
___statements
What is a record?
Set of data that describes one item
What is a field?
Single piece of data within a record
How do you write and read a record to/from a file?
Write record to sequential access file by writing the fields one after the other
Read record from sequential access file by reading each field until record complete
What is the difference between write and writelines methods?
WriteLine() provides a new line character \n at the end of the string
How do you remove the \n character at the end of the writelines method?
rstrip method
What is the difference between read and readlines methods?
Reads entire file contents into memory vs. reads a line from the file (line returned as string ending in ‘\n’)
Open the file hostdata.txt for reading.
open (‘hostdata.txt’, ‘r’)
DON’T FORGET QUOTES!
How do you create a file object from a file?
Use the open function
It creates a file object, assigns it to a variable and associates it to the file
A file named numbers.txt contains an unknown number of lines, each consisting of a single integer. Write some code that computes the sum of all these integers, and stores this sum in a variable name sum.
fin=open(‘numbers.txt’,’r’)
sum=0
for line in fin:
____sum+=int(line)
- create file object (open function)
- initialize sum variable
- create for loop (it will iterate over every line in the file)
- add each line to the sum variable (remember to CONVERT to integer!!)
