4.7. Catching exceptions using try and except¶
Earlier we saw a code segment where we used the input and
int functions to read and parse an integer number entered
by the user. We also saw how treacherous doing this could be:
>>> prompt = "What is the air velocity of an unladen swallow?\n"
>>> speed = input(prompt)
What is the air velocity of an unladen swallow?
What do you mean, an African or a European swallow?
>>> int(speed)
ValueError: invalid literal for int() with base 10:
>>>
When we are executing these statements in the Python interpreter, we get a new prompt from the interpreter, think “oops”, and move on to our next statement.
However if you place this code in a Python script and this error occurs, your script immediately stops in its tracks with a traceback. It does not execute the following statement.
Here is a sample program to convert a Fahrenheit temperature to a Celsius temperature:
Example:
Enter Fahrenheit Temperature:72
22.22222222222222
If we execute this code and give it invalid input, it simply fails with an unfriendly error message:
Enter Fahrenheit Temperature:fred
Traceback (most recent call last):
File "fahren.py", line 2, in <module>
fahr = float(inp)
ValueError: could not convert string to float: 'fred'
There is a conditional execution structure built into Python to handle
these types of expected and unexpected errors called “try / except”. The
idea of try and except is that you know that
some sequence of instruction(s) may have a problem and you want to add
some statements to be executed if an error occurs. These extra
statements (the except block) are ignored if there is no error.
You can think of the try and except feature in
Python as an “insurance policy” on a sequence of statements.
We can rewrite our temperature converter as follows:
Python starts by executing the sequence of statements in the
try block. If all goes well, it skips the
except block and proceeds. If an exception occurs in the
try block, Python jumps out of the try block
and executes the sequence of statements in the except
block.
Example:
Enter Fahrenheit Temperature:72
22.22222222222222
If we execute this code and give it invalid input, it executes the statements in the except block:
Enter Fahrenheit Temperature:fred
Please enter a number
Handling an exception with a try statement is called
catching an exception. In this example, the
except clause prints an error message. In general, catching
an exception gives you a chance to fix the problem, or try again, or at
least end the program gracefully.
The following program should get a temperature in Fahrenheit from the user, then print out the temperature in Celsius. If the input is not a number, it should ask the user to enter a number again. Be sure to indent correctly!