I719 Fundamentals of Python/lecture3v3

= Lecture 3 =

Input, Handling Errors, Unit Testing

Input from console

You can prompt the user for input using a function called input.

on python2, this is called raw_input

input is a function that stops execution until the user hits 'enter', and the value they enter is the output of the the input function.

Task 1

Write a program that takes two numbers from the user and prints the numbers added together.

"""
Addition calculator

Write a script that takes in two numbers, and adds them together.
"""

number_1 = int(input('Write a number: '))
number_2 = int(input('Write a number: '))
print(number_1 + number_2)

try and except

try creates a block that can run code that may raise an error. If an error is raised, the code in except will run. If an error is NOT raised, then the code in else will run

number_1 = input('Write a number: ')
number_2 = input('Write a number: ')

try:
    number_1 = int(number_1)
    number_2 = int(number_2)

except ValueError:
    print('You must enter valid integers')
else:
    print(number_1 + number_2)

Exceptions

class NotIntegerError(Exception):
    pass


number_1 = input('Write a number: ')
number_2 = input('Write a number: ')

if not number_1.isdigit():
    raise NotIntegerError('{0} is not a number'.format(number_1))

if not number_2.isdigit():
    raise NotIntegerError('{0} is not a number'.format(number_2))

number_1 = int(number_1)
number_2 = int(number_2)
print(number_1 + number_2)

Unit testing

See https://wiki.itcollege.ee/index.php/I719_Fundamentals_of_Python/testing

Arguments from commandline

The standard library has sys which give us access to the command line arguments executed.

import sys
 
if '-h' in sys.argv or '--help' in sys.argv:
    print('Sorry I can\'t help you :(')

But argparse from the standard library is much easier if we want to use the commandline arguments.

Opening a file, printing every line

The file path is passed into the script in the command line. run this script using python3 my_script.py --help and a help message will show. run this script with python3 my_script path/to/my/file.txt to read file.txt line by line.

NOTE: you must run this from the commandline, not from your IDE

Task 2

Count the occurence of words in the file.

example output

{'banana': 1, 'carrot': 9, 'pumpkin': 2, 'cabbage': 7}

example solution

import argparse

parser = argparse.ArgumentParser(description='Count unique words in a file')
parser.add_argument('path')

args = parser.parse_args()


my_file = open(args.path, 'r')

result = {}
for line in my_file:
    clean_line = line.strip()
    result[clean_line] = result.get(clean_line, 0) + 1

my_file.close()
print(result)