Eroman: Created page with " = 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..."

2017-04-20T22:42:07Z

Created page with " = Lecture 3 = Input, Handling Errors, Unit Testing == Input from console == You can prompt the user for input using a function called <code>input</code>. on python2, this..."

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= Lecture 3 =

Input, Handling Errors, Unit Testing

== Input from console ==

You can prompt the user for input using a function called <code>input</code>.

on python2, this is called <code>raw_input</code>

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

== Task 1 ==

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

<pre>"""
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)
</pre>
== <code>try</code> and <code>except</code> ==

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

<source lang="python">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)</source>
== Exceptions ==

<source lang="python">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)</source>
== 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.

<source lang="python">import sys

if '-h' in sys.argv or '--help' in sys.argv:
print('Sorry I can\'t help you :(')</source>
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 <code>python3 my_script.py --help</code> and a help message will show. run this script with <code>python3 my_script path/to/my/file.txt</code> to read <code>file.txt</code> line by line.

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

<source lang="python"></source>
== Task 2 ==

Count the occurence of words in the file.

example output

<pre>{'banana': 1, 'carrot': 9, 'pumpkin': 2, 'cabbage': 7}</pre>
example solution

<source lang="python">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)</source>

I719 Fundamentals of Python/lecture3v3 - Revision history

Eroman: Created page with " = 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..."

Eroman: Created page with " = 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..."