1. Reading and Writing Files

2. Using Files for Data Storage
Allows data to be retained between program runs
It’s lost when a local variable goes out of scope or when the program terminates
Computers store files on secondary storage devices
hard disks, optical disks, flash drives and magnetic tapes.
Data maintained in files is persistent data because it exists beyond the duration of program execution.
Used in many applications:
Word processing, Image Editors, Databases, Spreadsheets, Compilers, Games and Web Browsers
File: a set of data stored on a computer, often on a disk drive
Programs can read from and/or write to files
Steps:
Open the file
Use the file (read from, write to, or both)
Close the file

3. Data Hierarchy
The smallest data item in a computer can assume the value 0 or 1.
Such data items are called bits
Python uses Unicode characters that are composed of two bytes, each composed of eight bits(lang)
Fields are composed of characters or bytes.
Data items processed by computers form a data hierarchy that becomes larger and more complex in structure as we progress from bits to characters to fields, and so on.
Typically, several fields compose a record (implemented as a class in Java).
A file is a group of related records.  Database

4. Opening a File Stream
The first part of read/write a file is to create a stream to the file.
A stream is a one way path for information to travel
Code: varName = open(fileName, mode)
Ex.
fileWrite = open(“test.txt”,”w”)
input=open(“c:\\py\\tst.txt”,”r”)

5. Writing Files
To write to a file use the file stream name like a variable with the write function.
Code: write(str)
Ex.
output = open(“city.txt”)
output.write(“blah blah blah\n”)
Note: Loops can be used to write numerous values to a file

6. Reading Files
Reading a file is similar to writing a file in that you use the file stream name followed by one of the read functions.
Code: read() – reads all characters from a file
read(#) – read a number of characters
readline() – reads all characters up to and including \n
readlines() – reads all lines

7. Reading Data From a File
Two main ways to read data from a file:
1. While loop that iterates to the end of the file
Code: line = infile.readline()
while line != ‘’:
#process line here
line = infile.readline()
2. A enhanced for loop
Code: for varname in fileName
#process the line here

8. Closing a File Stream Clean up is a necessary and essential step.
Just like when you make a mess, life will go on but there are implications of not cleaning up.
Ensure you close a stream after you have processed all of your data.
Code: fileStream.close()

9. Reading and Writing Files Example
def main(): # Prompt the user to enter filenames f1 = input("Enter a source file: ").strip() f2 = input("Enter a target file: ").strip() # Open files for input and output infile = open(f1, "r") outfile = open(f2, "w") # Copy from input file to output file countLines = countChars = 0 for line in infile: countLines += 1 countChars += len(line) outfile.write(line) print(countLines, "lines and", countChars, "chars copied") infile.close() # Close the input file outfile.close() # Close the output file main() # Call the main function

10. Checkpoint 1. What are the 3 phases to a file stream?
2. What are some different ways to setup your file stream?
3. What are the key differences between a read and write file stream?