1. 15-110: Principles of Computing
File Processing
Lecture 17, November 06, 2018
Mohammad Hammoud
Carnegie Mellon University in Qatar

2. Today… Last Session: Today’s Session: Announcement:
Sequences- Part IV (Dictionaries)
Today’s Session:
Files:
Why Files
File Processing
File Functions
Examples
Announcement:
HA05 will be out on November 08. It is due on November 17 by midnight

3. Files
All data that we store in lists, tuples, and dictionaries within any program are lost when the program ends
These data structures are kept in volatile memory (i.e., RAM)
To save data for future accesses, we can use files, which are stored on non-volatile memory (usually on disk drives)
Files can contain any data type, but the easiest files to work with are those that contain text
You can think of a text file as a (possibly long) string that happens to be stored on disk!

4. File Processing
Python has several functions for creating, reading, updating, and deleting files
To create and/or open an existing file in Python, you can use the open() function as follows:
<variable> = open(<file_name>, <mode>)
<variable> is a file object that you can use in your program to process (i.e., read, write, update, or delete) the file

5. File Processing
<file_name> is a string that defines the name of the file on disk
<mode> can be only 1 of the following four modes:
"r“: This is the default mode; It allows opening a file for reading; An error will be generated if the file does not exist
"a“: It allows opening a file for appending; It creates the file if it does not exist
"w“: It allows opening a file for writing; It creates the file if it does not exist
"x“: It allows creating a specified file; It returns an error if the file exists

6. File Processing
In addition, you can specify if the file should be handled as a binary (e.g., image) or text file
"t“: This is the default value; It allows handling the file as a text file
"b“: It allows handling the file as a binary fine
Example:
f = open("file1.txt") is equivalent to f = open("file1.txt“, “rt”)
When opening a file for reading, make sure it exists, or otherwise you will get an error!

7. Reading a File
The open() function returns a file object, which has a read() method for reading the content of the file
f = open("file1.txt", "r") data = f.read()
print(data)
print(type(data))
The type of data is str, which you can parse as usual!

8. Reading a File
You can also specify how many characters to return from a file using read(x), where x is the first x characters in the file
f = open("file1.txt", "r") data = f.read(10)
print(data)
print(type(data))
This will return the first 10 characters in the file

9. Reading a File
You can also use the function readline() to read one line at a time
f = open("file1.txt", "r")
str1 = f.readline()
print(str1)
str2 = f.readline()
print(str2)
print(f.readline())
This file contains some information about sales
Total sales today = QAR100,000
Sales from PCs = QAR70,000
This is the content of the file

10. Reading a File
You can also use the function readline() to read one line at a time
f = open("file1.txt", "r")
str1 = f.readline()
print(str1)
str2 = f.readline()
print(str2)
print(f.readline())
This file contains some information about sales
Total sales today = QAR100,000
Sales from PCs = QAR70,000
Note that the string returned by readline() will always end with a newline, hence, two newlines were produced after each sentence, one by readline() and another by print()

11. Reading a File
You can also use the function readline() to read one line at a time
f = open("file1.txt", "r")
str1 = f.readline()
print(str1, end = “”)
str2 = f.readline()
print(str2, end = “”)
print(f.readline(), end = “”)
This file contains some information about sales
Total sales today = QAR100,000
Sales from PCs = QAR70,000
Only a solo newline will be generated by readline(), hence, the output on the right side

12. Looping Through a File
Like strings, lists, tuples, and dictionaries, you can also loop through a file line by line
f = open("file1.txt", "r")
for i in f:
print(i, end = "")
This file contains some information about sales
Total sales today = QAR100,000
Sales from PCs = QAR70,000
Again, each line returned will produce a newline, hence, the usage of the end keyword in the print function

13. Writing to a File
To write to a file, you can use the “w” or the “a” mode in the open() function alongside the write() function as follows:
f = open("file1.txt", "w")
f.write("This file contains some information about sales\n")
f.write("Total sales today = QAR100,000\n")
f.write("Sales from PCs = QAR70,000\n")
f.close()
Every time we run this code, the same above content will be written (NOT appended) to file1.txt since we are using the “w” mode
If, alternatively, we use the “a” mode, each time we run the code, the same above content will be appended to the end of file1.txt

14. Writing to a File
To write to a file, you can use the “w” or the “a” mode in the open() function alongside the write() function as follows:
f = open("file1.txt", "w")
f.write("This file contains some information about sales\n")
f.write("Total sales today = QAR100,000\n")
f.write("Sales from PCs = QAR70,000\n")
f.close()
Also, once we are finished writing to the file, we should always close it using the close() function. This will ensure that the written content are pushed from volatile memory (i.e., RAM) to non-volatile memory (i.e., disk), thus preserved

15. Writing to a File
You can also write information into a text file via the already familiar print() function as follows:
f = open("file1.txt", "w")
print("This file contains some information about sales", file = f)
print("Total sales today = QAR100,000", file = f)
print("Sales from PCs = QAR70,000", file = f)
f.close()
This behaves exactly like a normal print, except that the result will be sent to a file rather than to the screen

16. Deleting a File
To delete a file, you must first import the OS module, and then run its os.remove() function
You can also check if the file exists before you try to delete it
import os
os.remove("file1.txt")
import os
if os.path.exists("file1.txt"):
os.remove("file1.txt")
else:
print("The file does not exist")

17. Example: Copy a File import os def copyFile(name):
if type(name) is str and os.path.exists(name):
l = name.split(".")
new_name = l[0] + "_copy." + l[1]
f_source = open(name, "r")
f_destination = open(new_name, "w")
f_destination.write(f_source.read())
f_source.close()
f_destination.close()
else:
print("The file does not exist!")
copyFile("file1.txt")

18. Example: Word Count
A common utility on Unix/Linux systems is a small program called “wc” which, if called for any file, returns the numbers of lines, words, and characters contained therein
We can create our version of “wc” in Python as follows:
import os
def wordCount(name):
if type(name) is str and os.path.exists(name):
f = open(name)
l_c = 0
w_c = 0

19. Example: Word Count c_c = 0 for i in f: l_c = l_c + 1
list_of_words = i.split()
w_c = w_c + len(list_of_words)
for j in list_of_words:
c_c = c_c + len(j)
dic = {"Line Count": l_c, "Word Count": w_c, "Character Count": c_c}
return dic
else:
print("The file does not exist!")
print(wordCount("file1.txt"))

20. Next Class
Practicing & Discussing HW05 Problems …