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Python Basics. Part 2

Introduction
The very basics of text files
with statement and exception handling
11. ValueError
Summary

Instruction

Great! Naturally, when an exception is thrown, we can do more than just print a custom error message. Consider this example:

while True:
  try:
    x = int(input('Provide a number: '))
    break
  except ValueError:
    print('That is not a number! Try again. ')

In the example above, we ask the user to provide a number using the input() function within a while True loop. input() provides a string from the user, so we want to convert it to a number using int(). Python Documentation states that int() throws a ValueError exception if the argument can't be converted to an integer.

In our case, two options are possible:

  • If int() converts user input successfully, the next line of code is executed, which is break: we escape the otherwise infinite while loop.
  • If int() fails and throws a ValueError, the try block is immediately interrupted. Then, break is never executed. Instead, the except block prints an error message, and the whole loop repeats in another iteration.

Exercise

Inside a loop, keep asking the user to provide a filename until it's a valid one and the file can be read.

When the filename is incorrect, print:

Could not read the file. Try another one.

When the filename is correct, simply print its contents.

You can test the correctness of the program for the animals.txt.

Stuck? Here's a hint!

Use a while True loop, as shown in the example. Use a try-except block, and catch any IOError.