Errors and Exceptions

When Python cannot continue, it raises an exception. An uncaught exception stops the program and prints a traceback.

Common types:

Exception	Cause
ValueError	Right type, wrong value. int("abc")
TypeError	Wrong type. "a" + 1
KeyError	Missing dictionary key
IndexError	Index past the end of a list
FileNotFoundError	Opening a file that does not exist
ZeroDivisionError	Dividing by zero

Catching

values = ["120", "abc", "45"]
for value in values:
    try:
        print(int(value))
    except ValueError:
        print("skipping", value)

Catch the specific exception you expect. A bare except: hides real bugs, including typos in your own code.

else and finally

else runs when no exception was raised. finally always runs.

try:
    depth = int("120")
except ValueError:
    print("bad value")
else:
    print("parsed", depth)
finally:
    print("done")

Raising

Raise an exception when a function is given something it cannot handle.

def gc_content(sequence):
    if not sequence:
        raise ValueError("sequence is empty")
    gc = sequence.count("G") + sequence.count("C")
    return gc / len(sequence)


try:
    gc_content("")
except ValueError as error:
    print("error:", error)

Task

Write safe_ratio(values). It receives a list of strings. Convert each to an integer, sum them, and return the sum divided by the count, rounded to 2 decimals.

Skip any string that is not a whole number. If nothing valid remains, return the string "no valid values".