Difference between revisions of "Python-3.4.3/C4/Handling-Errors-and-Exceptions/English"

Objectives

• Understand errors and exceptions
• Handle errors and exceptions

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3
• IPython 5.1.0

Prerequisite

• use functions and
• carry-out testing and debugging

If not, see the relevant Python tutorials on this website.

• syntax errorsand
• exceptions

Syntax Error

For example:

if True print inside brackets inside double quotes done

It will give Syntax Error since colon is missing after True.

Exceptions

Python generates an exception that can be handled, which avoids the program to crash.

For example:

1/0

It will give ZeroDivisionError exception.

Open the terminal.

ipython3

From here onwards remember to press the Enter key after typing every command on the terminal.

if True print("done")

Highlight the arrow mark

if True print inside brackets inside double quotes done

The output shows SyntaxError.

It displays an ‘arrow’ pointing at the earliest point in the line where the error was detected.

In our case, the error is caused by missing colon after True in If clause

1 / 0

1 / 0

Python throws an exception called ZeroDivisionError.

Even though the expression is syntactically correct, we cannot divide a number by zero.

a = input("Enter a number: ")

Enter ac

num = int(a)

Type

a = input inside brackets inside double quotes Enter an integer

I will enter ac as input.

Now type, num is equal to int inside brackets a

This is because, we are trying to convert the string to integer.

For this we have try and except clause in Python.

try..except..else

try:

statement1

statement2

First, the statements between the try and except keywords is executed.

except exception name:

exception handling statement(s)

If the statements cause any exception, then except clause is executed if exception name matches.

And the execution continues after the try statement.

statement(s) when no exception

after statement(s)

a = input("Enter an integer: ")

ac

try:

num = int(a)

except ValueError:

print ("Wrong input")

Press Enter twice

output:

Wrong input

Give ac as input.

Now we will type the try and except blocks.

Press Enter twice to get the output.

In the previous example,

We encountered a problem because we tried to convert the string ‘ac’ to integer.

Here conversion of string value to an integer is given inside try block.

ValueError exception is raised and so the output is displayed as Wrong input.

This whole process is called debugging.

23

try:

num = int(b)

except ValueError:

print ("Wrong input")

else:

print(“No exception “)

Press Enter twice

Highlight No exception

Type as shown.

I will give input as 23.

Now type as shown.

This try… except statement has an optional else clause.

It is useful if the try clause does not raise an exception.

def test():

prod = 1

for i in range(0, 10):

prod *= i / (i - 5)

print(prod)

Save it as file mymodule.py

Create a file mymodule.py with the following code.

The variable ‘i’ is iterated from 0 to 9.

from mymodule import test

test()

Type,

from mymodule import test

test open and close brackets

Interpreter gives us ZeroDivisionError because there is a division by zero error.

%debug

Highlight the arrow mark

Highlight ipdb>

Percentage debug

The code which caused the error is shown by an arrow.

The prompt has changed to ipdb which is the ipython debugger mode.

ipdb> i

5

We can check values of variables to inspect what went wrong.

For example, on typing ‘i’, we get 5 as output.

This means that the error was caused when the value of i became 5.

To exit from the ipdb prompt, press q and press Enter.

Summary

In this tutorial, we have learnt about

• Errors and exceptions.
• Handling exception using try and except.
• Using percentage debug for debugging in ipython.

Self assessment questions

1. How do you start the debugger on ipython?

Solutions

1. We start the debugger on ipython by saying, percentage debug

Acknowledgment

For more details, visit this website.

Thank you.