Errors and Exceptions

When an operation cannot be completed, Python raises an exception. An exception that is not handled propagates up the call stack and terminates the program, printing a traceback that lists the call sequence leading to the failure. Read a traceback from the bottom: the last line names the exception and its message, and the lines above show where it occurred.

The exceptions encountered most often are:

Exception	Cause
ValueError	Correct type, unusable value. int("abc")
TypeError	Wrong type. "a" + 1
KeyError	Dictionary key not present
IndexError	Sequence index out of range
FileNotFoundError	Opening a file that does not exist
ZeroDivisionError	Division by zero
AttributeError	Attribute or method not defined on the object

Handling exceptions

A try block runs code that may fail; an except clause names the exception to handle and the action to take. Only the named exception is caught, and only within the try block.

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skipping abc
45

Catch the narrowest exception that covers the expected failure. A bare except: catches everything, including KeyboardInterrupt and genuine defects such as a misspelled variable name, converting a visible failure into silent incorrect behaviour. If several exception types warrant the same response, name them as a tuple: except (ValueError, TypeError):.

Binding the exception with as gives access to its message, which is worth including in any diagnostic.

ValueError - invalid literal for int() with base 10: 'abc'

else and finally

An else block runs only if the try block raised nothing. It separates the code that might fail from the code that depends on its success, keeping the try block as small as possible. A finally block runs unconditionally and is used for cleanup.

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done

Raising exceptions

Raise an exception when a function receives input it cannot process. Failing immediately, at the point where the problem is detectable, produces a far more useful traceback than allowing an invalid value to propagate and fail later.

Choose the type that describes the problem: ValueError for an unusable value, TypeError for the wrong type.

error: sequence is empty

Task

Write safe_ratio(values). It receives a list of strings. Convert each to an integer, and return the mean of those that convert, rounded to two decimal places. Strings that do not represent a whole number are skipped. If none convert, return the string "no valid values".

Collect the valid numbers first and decide what to return afterwards; computing a mean requires knowing the count, which is not available until the loop ends.