Python Programming Tutorial 15

 Error Handling

Anytime we work with user input, we should make sure not only the data is valid and within the range we want, like what we did in the previous lesson, but we also need to make sure that input is not going to make our program crash. In order to do that, we are going to use the try and except statements. So let's understand why the program crashed. 

Let's create a new variable called number, and let's assign a user input converted to a float and type a string to see what happens. 

This leads to an error, as Python is unable to convert this value into a string. So in order to solve this problem, we are going to use try and accept statements. 

 

Let's just copy and paste our program in a new file, and instead of converting to float immediately, we can ask for user input first. 

Instead of this: 

number = float ( input("Type a number: ") )

We are going to use this instead:

number =input("Type a number: ")

Then, we are going to use a try statement. This works the same way as the conditionals. The code after the try function is going to be inside the try statement. 

number = input("Type a number:")

try:

      number = float(numbers)

      print("The number is:", number)

What I tried to do here is that i tried to get the conversion. If we are able to get to the next line it means we were able to do this without any problems. If we see the print message, it means we made the conversion without any problem. 

If we have any errors inside the try block, instead of crashing we can just jump to the except function. And inside the except function we can print invalid number.

except:

    

    print("Invalid number")

Entire code:

number =input("Type a number: ")

try:
      number = float(number)
      print("The number is:", number)
except:
    
    print("Invalid number")

Okay, so let's run this code and see what happens. 

First, let's try a valid number and see if it will print the float conversion in the format 20.0.

Output:

>>> 
== RESTART: C:\Users\ameny\OneDrive\Desktop\Python.course\Error handling.py ==
Type a number: 20
The number is: 20.0
>>> 

And it worked. Now, let's try an invalid number: a string.

>>> 
== RESTART: C:\Users\ameny\OneDrive\Desktop\Python.course\Error handling.py ==
Type a number: test
Invalid number
>>> 

And now instead of crashing, it just printed a message. We could have done anything we wanted inside the except block. So this is how we do error handling in Python. 

See you inside our next tutorial. Goodluck.

 

 

dgbds

Python Programming Tutorial 14.

 Data Validation

In this lesson we are going to talk about data validation, and we are going to use the program we created for the grades of the student. 

..................................................................................................................................................................

I'm referring to this data:

grade1 = float ( input("Type the grade of the first test: ") )    

grade2 = float ( input("Type the grade of the second test: ") )

absences = int ( input("Type the number of absences: ") )

total_classes = int ( input("Type the the total number of classes: ") )

avg_grade = (grade1 + grade2) / 2

attendance = (total_classes - absences) / total_classes

print("Average grade: ", round(avg_grade,2) )

print("Attendance rate: ", str(round((attendance * 100),2))+'%' )

if (avg_grade >= 6 or attendance >= 0.8):

        print("The student has been approved.")

elif(avg_grade < 6 and attendance < 0.8):

    print("The student has failed due to an average grade less than 6.0 and an attendance rate less than 80%.")

elif(attendance >= 0.8):

     print("The student has failed due to an average grade less than 6.0.")

else:

    print("The student has failed due to an attendance rate less than 80%")

..................................................................................................................................................................

Let's talk about some problems that might occur in this program. 

In this data, there's no data validation, so even grades from 0 to 10, we can type a 100 by accident, and the program is going to accept it. 

Another program is that if I type a string, our program crashes, which is really tragic. Before we see how we can do basic data validation, let's visualize this: 

And we had an error after running this code and answering with the above information. In our next lesson, we will keep our program crashing by doing error handling.

Data validation

So before asking for user input, we are going to create a variable called "data_valid" and assign to it a value of False. { data_valid  =  False }.

So let's start a while loop with this information: 

Now, the highlighted text is going to return true because data_valid is equal to false, so we are going to go inside this while loop. Afterwards, we will request the grade and perform the validation using an if statement.

 

data_valid = False

while data_valid == False:

        grade1 = float ( input("Type the grade of the first test: ") )

        if grade1 < 0 or grade1 > 10:

                print("Grade should be between 0 and 10")

                continue

        else:

                data_valid = True

This is what it will look like. So we just went inside the while loop and asked for the grade, and we made the validation with an if condition. You do notice that we use the'or'operator, so if one of the tests results true, the whole test is going to result true. If this happens, it means we have an invalid input, and in this case we are going to print the range of the grade. Then we are going to continue in this loop; using the continue function, the continue will jump out of this loop and start over again. Then, we complete with an else statement because if this if grade1 < 0 or grade1 > 10 is not true, it means we have valid input, so in this case, we are going to make data_valid = true. This is going to end the loop, and when it tries to run again, this test data_valid == False is going to return in false, so it is not going to go inside the loop and it's going to continue with our program. Now we are going to do the exact same thing for grade 2.

Let's copy all the data, and set the value of data_valid to false again.

data_valid = False

while data_valid == False:

        grade1 = float ( input("Type the grade of the first test: ") )

        if grade1 < 0 or grade1 > 10:

                print("Grade should be between 0 and 10")

                continue

        else:

                data_valid = True

data_valid = False

while data_valid == False:

        grade2 = float ( input("Type the grade of the second test: ") )

        if grade2 < 0 or grade2 > 10:

                print("Grade should be between 0 and 10")

                continue

        else:

                data_valid = True

This is how our code will look like. Before we continue, let's test it to see what happens.

I first tried negative ten, and it didn't work, it said grade should be between zero and 10. I tried eleven, and it still didn't work. Now, our program only accepts numbers between zero and ten, so let's try eight for the first test and minus ten for the second test. 

It still didn't work. 

Alright, let's continue. For the absences in total classes, lets do something similar, but let's change the order and ask for the total classes first. Let's copy the data one more time. 

 data_valid = False

while data_valid == False:

        total_classes = int ( input("Type the the total number of classes: ") )

        if total_classes <= 0:

                print("The number of classes can't be zero or less")

                continue

        else:

                data_valid = True

We asked for the total number of classes, and we tested if total classes were less than 0. We can have a negative number of classes here, but we won't put any limitation on the number of classes. Actually, this is going to be a bit different since we can have zero classes, and hence the less than or equal to sign was used. So we printed or told the user that the number of classes can't be zero or less.

 

Now let's do the same for the absences. 

We won't accept absences less than zero. We can't accept zero and we can't accept the number of absences greater than the total number of classes. 

data_valid = False

while data_valid == False:

        absences = int ( input("Type the number of absences: ") )

        if absences <= 0 or absences > total_classes:

                print("The number of absences can't be less than or greater than the number of total classes")

                continue

        else:

                data_valid = True

Alright, let's run this and see. 

The validation is working fine. Well this is better but our program crashes when we type in a string, so in our next lesson, we are going to do some error handling. 

Lines of code used: 

data_valid = False

while data_valid == False:
        grade1 = float ( input("Type the grade of the first test: ") )
        if grade1 < 0 or grade1 > 10:
                print("Grade should be between 0 and 10")
                continue
        else:
                data_valid = True

data_valid = False

while data_valid == False:
        grade2 = float ( input("Type the grade of the second test: ") )
        if grade2 < 0 or grade2 > 10:
                print("Grade should be between 0 and 10")
                continue
        else:
                data_valid = True
                
data_valid = False

while data_valid == False:
        total_classes = int ( input("Type the the total number of classes: ") )
        if total_classes <= 0:
                print("The number of classes can't be zero or less")
                continue
        else:
                data_valid = True

data_valid = False

while data_valid == False:
        absences = int ( input("Type the number of absences: ") )
        if absences <= 0 or absences > total_classes:
                print("The number of absences can't be less than or greater than the number of total classes")
                continue
        else:
                data_valid = True


avg_grade = (grade1 + grade2) / 2
attendance = (total_classes - absences) / total_classes

print("Average grade: ", round(avg_grade,2) )
print("Attendance rate: ", str(round((attendance * 100),2))+'%' )

if (avg_grade >= 6 or attendance >= 0.8):
        print("The student has been approved.")
elif(avg_grade < 6 and attendance < 0.8):
    print("The student has failed due to an average grade less than 6.0 and an attendance rate less than 80%.")
elif(attendance >= 0.8):
     print("The student has failed due to an average grade less than 6.0.")
else:
    print("The student has failed due to an attendance rate less than 80%")
     
        

Goodluck see you next time.

     

        

Exercise 2: Loops

2. Create a guess game with the names of the colors. At each round pick a random color from a list and let a user guess it. When he does it, ask if he wants to play again. Keep playing until the user types "no."

I think this is going to be a very nice challenge. Good luck, and when you finish, come back here to see the solution. 

Solution to exercise.

Code:

import random

colors = ["white", "black", "yellow", "blue", "pink", "purple", "grey", "indigo"]

while True:
    color = colors[random.randint(0, len(colors) - 1)]
    guess = input("I'm thinking about a color, can you guess it: ")

    while True:
        if color == guess.lower():
            print("You guessed it! I was thinking about", color)
            break
        else:
            guess = input("Nope. Try again: ")

    play_again = input("Let's play again? Type 'no' to quit: ")

    if play_again.lower() == 'no':
        print("It was fun, thanks for playing!")
        break

Output:

>>> 
==== RESTART: C:\Users\ameny\OneDrive\Desktop\Python.course\Ex.loops2.py ====
I'm thinking about a color, can you guess it: red
Nope. Try again: white
Nope. Try again: blue
Nope. Try again: green
Nope. Try again: yellow
Nope. Try again: pink
Nope. Try again: grey
Nope. Try again: indigo
Nope. Try again: black
You guessed it! I was thinking about black
Let's play again? Type 'no' to quit: 

You can hit enter to see if it repeats, then you try again.

Let's try typing no to see what happens:

>>> 
==== RESTART: C:\Users\ameny\OneDrive\Desktop\Python.course\Ex.loops2.py ====
I'm thinking about a color, can you guess it: red
Nope. Try again: white
Nope. Try again: blue
Nope. Try again: green
Nope. Try again: yellow
Nope. Try again: pink
Nope. Try again: grey
Nope. Try again: indigo
Nope. Try again: black
You guessed it! I was thinking about black
Let's play again? Type 'no' to quit: no
It was fun, thanks for playing!
>>> 

Alright let me know in the comment section if you have further questions. See you in our next tutorial.

dsssds

Exercise 1: Loops

 1. Create a program that asks the user for 8 names of people and store them in a list. When all the names have been given, pick a random one and print it.

I think this is going to be a very nice challenge. Good luck,  and when you finish, come back here to see the solution. 

Okay let's delve into the solution of this problem.

Code:

import random

people = []

for x in range(0,8):
    person = input("Type the name of the person: ")
    people.append(person)

index = random.randint(0,7)

random_person = people[index]

print("Picking a random person: ", random_person)

#Let's see if this is working

Output:

>>> 
 RESTART: C:/Users/ameny/OneDrive/Desktop/Python.course/Exercise 1 on loops.py 
Type the name of the person: John
Type the name of the person: Paul 
Type the name of the person: Kofi
Type the name of the person: William
Type the name of the person: Kwame
Type the name of the person: KING
Type the name of the person: Hank
Type the name of the person: Walt
Picking a random person:  KING
>>> 

And it worked perfectly. You have the solution for excercise number 1. Let me know if you have further questions.