Python Debugging and Testing – Day 8th Python Series

As we all know when we write a program, there are two types of errors that we might get. So we need to debug them, and that’s today’s topic: Python Debugging and Testing.

1. Debugging syntax errors
2. Debugging logical errors

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• Introduction to Programming – Day1
• Introduction to Algorithms – Day 2
• Python operators and variables – Day 3
• Python Control Structures – Day 4
• Functions in Python – Day 5 of Python Series

1.Debugging syntax error in python

Eclipse detects the syntax errors while we type the code or when we try to run code.

It underlines the errors in the code.

2. Debugging logical errors in python

Logical errors are hard to find since eclipse can not find logical errors. We have to find out logical errors by executing the code line by line.

Observe the output of this program

N =10
A = 30
B = 50
n = A+B
print(N)

Output – 10

You might be wondering that how can the value be 10, it should be (30+50) =80.

But you are wrong here.

These are the common mistakes most novice programmer does.

That is why we need to debug the program.

Let’s start debugging program by taking the example below

Example:

i=1
n=10
while(i<=n):
    if(i%2==0):
        print(i)
        i+=1
    else:
        continue
    i+=1

Follow the steps to debug the code:

1. Copy the above code and paste it in Eclipse IDE
2. Change the perspective to Debug, for that navigate to Window>perspective>open perspactive>debug
3. Use the debugger tool in order to see what is happening in the program.

For that we want to break the execution at some point, for that double click on the line you want.

A green tick will be shown when you select a line.

4. Click on the Debug button

This is the layout of the debug perspective below. You can now observe that the program has paused at line number 6, since we had created a breakpoint at that line.

You can go back to programming layout anytime by clicking on the Programming Assignment button on top right.

Let’s observe the program below. The top right portion of the window has a tab called Variables. This shows a list of all variables and their current value. Click on a variable name to see its value displayed below it.

If you want to execute one line at a time from the breakpoint, you can use the step over function.

As we continue to step over we observe that control has gone back to line 6. At line 6, value of “i” is still 1. Continue to step over once again and observe the value of “i”. We observe that value of “i” remains as 1, thereby resulting in infinte loop

Now we got the Bug, we can now fix our code.

Modified code:

i=1
n=10
while(i<=n):
    if(i%2==0):
        print(i)
    i+=1

If we have to write test cases and test a program written for the above requirement, instead of writing the test cases and testing it manually, we can use some tools.

pytest can be used to write test cases and automatically test a program.

Once the test cases are written, we need to execute those test cases against the program to identify the test cases that have passed or failed. If there are any failed test cases, we should debug the program for any logical errors, correct it and test again as shown below.

Let’s configure PyTest

1. Create a pyDev project in Eclipse. Create two files, solution.py and test_solution.py inside a pyDev package in the project.

2. To configure pytest, click on Run -> Run Configurations

3. Double click on Python unit test.

4. Choose the Main Tab and click on Browse to choose the Project.

5. Choose your project

Then click on Argument

Enable “Override PyUnit preferences for this Launch?” and choose Pytest runner from the drop-down.

Enter the following text in the highlighted portion in the screenshot:

--tb=short 
--capture=no

Click apply then run.

That is it!

If you are still getting any problem in python debugging and testing then comment below with your problem we will surely reply within 12 hours.

FAQs

Frequently Asked Questions

How do you debug Python?

We have covered this topic scroll up to see in detail.

How do I enable breakpoints in eclipse?

To set a breakpoint double click on a line where you want to break execution.

What is debugger?

Debugging is the routine process of locating and removing bugs, errors or abnormalities from programs

Questions covered in this article.