Reading and Writing Files

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:pytst.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 blahn”)
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?