Types of Files

• There are two file types:
  • text: this is where data is stored in the form of strings
    • i.e. .txt, .py, .cpp
  • binary: this is where data is stored in the form of bytes
    • i.e. .jpg, .gif, .png, .exe

Opening a File (1)

• Files can be opened in Python using the open() function
• There are various opening modes:

Opening a File (2)

• Files can also be specified if they should be handled in a binary or text mode using:
  • t - for text mode (the default option)
  • b - for binary mode

exampleFile = open("filename.extension", "w")

Closing a File

• Files are closed in Python using the close() function
• You must always call this function when you have finished using or processing a file

exampleFile = open("myfile.txt", "w")
exampleFile.write("Hello 4061CEM")
exampleFile.close()

Reading Contents from a File

• Reading the contents of a file can be achieved in one of three ways:
  • read(): will read all the lines and return them line-by-line
  • read(n): will read \(n\) bytes from the file
  • readlines(): will return all strings as elements in a list

exampleFile = open("myfile.txt")
strRead = exampleFile.read()
print(strRead)
exampleFile.close()

Traceback (most recent call last):
  File "source.py", line 1, in <module>
    exampleFile = open("myfile.txt")
FileNotFoundError: [Errno 2] No such file or directory: 'myfile.txt'

Setting the File Cursor Position

• To read from a set position in a file, this can be achieved by using the seek() function
  • the function will also return the new position once it has finished reading the line

exampleFile = open("myfile.txt")
exampleFile.seek(4)
print(exampleFile.read())
exampleFile.close()

Writing to an Existing File

• Writing content to an existing file can be used with the open() function and one of the following modes:
  • a for append mode, which will append contents to the end of the file
  • w for write mode, which will overwrite existing content

exampleFile = open("myfile.txt", "w")
exampleFile.write("Oops, I overwrote the content of the file.\n")
exampleFile.close()

Using the with Keyword

• Reading a file can be achieved using the with keyword
  • a benefit is that it will take care of closing the file automatically
  • therefore, no requirement to call the close() function

with open('myfile.txt', 'w') as exampleFile:
    exampleFile.write("Hello 4061CEM\n")
    exampleFile.write("This is an exciting module.\n")

with open('myfile.txt', 'r') as exampleFile:
    for line in exampleFile:
        print(line)

Working with Directories (2)

Creating a Directory

• Creating a directory can be achieved using the mkdir() function

import os
os.mkdir("my_directory")

• Using a forwards slash (“/”) in the string will create a subdirectory
  • you must ensure that the top-level directory has already been created (“my_directory”)

os.mkdir("my_directory/sub_directory")

Working with Directories (3)

Deleting a Directory

• A directory can be deleted by using the rmdir() function
  • only empty directories can be deleted

import os
os.rmdir("my_directory")

Working with Directories (4)

Renaming a Directory

• The renaming of a directory can be achieved using the rename() function

import os
os.rename("my_directory", "a_new_name_directory")

Checking if a File Exists

• To check whether a file exists can be achieved using the isfile() function

import os
fileName = input("Enter a Filename:")
if os.path.isfile(fileName):
    print("File Exists")
else:
    print("File does not exist")

Deleting a File

• Deleting a file can be achieved using the remove() function

import os
os.remove("myfile.txt")

Displaying all Contents of a Folder

• To determine all folders and files of a directory can be achieved using the walk() function

import os
for dirPath, dirNames, filenames in os.walk('/'):
    print(f"Current Path: {dirPath}")
    print(f"Directories: {dirNames}")
    print(f"Files: {filenames}\n")

Running Executables

• The os module also contains a system() function which is useful to run shell commands from within Python

import os
os.system('dir')
os.system('python3 sample_script.py')

Structure of a CSV File (1)

• The first line of the file is considered to be the label, header or column name row
  • these values are often used to refer to the data for a particular column
• An example layout of a csv file can be something such as:

name,age,course
Ian,33,Computer Science
Terry,Unknown,Computer Science

• The first line consisting of: name, age, course are the labels for each column of data
• Subsequent lines following this are the data, with a comma (“,”) separating each value for the columns

Structure of a CSV File (2)

• You may picture the contents of a csv file as a table, for example:

Reading a CSV File in Python (1)

• There are two methods of reading a CSV file:
  • csv
  • DictReader

csv

• The csv module enables you to read the contents of a file
• Reads each line of the file and stores the data as a list
  • each item in the row being an item in the list
• Not the most functional way of reading the contents of a CSV file
  • does not utilise the labels or column headers of the file

DictReader

• An alternative method of reading a CSV file is using DictReader from the csv module
• Maps the content of the CSV file to a dictionary data type
• Provides a more useful method of importing data
  • you can read the whole CSV file and only access elements you want

Reading a CSV File in Python (2)

Method 1 - csv Example

import csv
with open("data.csv", "r") as csvFile:
    csvReader = csv.reader(csvFile, delimiter=",")
    for row in csvReader:
        print("\n\n", row)

Reading a CSV File in Python (3)

Method 2 - DictReader Example i

import csv
with open("data.csv", "r") as csvFile:
    csvReader = csv.DictReader(csvFile)
    for row in csvReader:
        print("\n\n", row)

Reading a CSV File in Python (4)

Method 2 - DictReader Example ii

• If you want to capture only the names in the file, you can adapt the code to be akin to:

import csv
names = []
with open("data.csv", "r") as csvFile:
    csvReader = csv.DictReader(csvFile)
    for row in csvReader:
        names.append(row["name"])
print("\n\n", names)

Writing to a CSV File in Python (1)

• There are two methods of creating a CSV file:
  • csv
  • DictWriter

csv

• The csv module also enables users to write to a CSV file
  • i.e. you want to add a new row to the contents in the file already

DictWriter

• An alternative method to write to a file is the DictWriter class
• Create a new dictionary object that has the details that you want to be written to the file
  • good practice is to use the same labels/keys as those used in the csv file
• Then you can easily write the data to the file by just calling the dictionary object in writerow()

Writing to a CSV File in Python (2)

Method 1 - csv Example

import csv
with open("data.csv", "a", newline='\n') as csvFile:
    csvWriter = csv.writer(csvFile, delimiter=",")
    csvWriter.writerow(['Daniel', 'Unknown', 'Ethical Hacking and Cyber Security'])

name,age,course
Ian,33,Computer Science
Terry,Unknown,Computer Science
Daniel,Unknown,Ethical Hacking and Cyber Security

Writing to a CSV File in Python (3)

Method 2 - DictWriter Example

import csv
aStudent = {"name": "Daniel",
            "age": "Unknown",
            "course": "Ethical Hacking and Cyber Security"}
with open("data.csv", "a", newline='\n') as csvFile:
    csvWriter = csv.DictWriter(csvFile, fieldnames=aStudent.keys())
    csvWriter.writerow(aStudent)

name,age,course
Ian,33,Computer Science
Terry,Unknown,Computer Science
Daniel,Unknown,Ethical Hacking and Cyber Security