14.2.2 Raising exceptions

>>> alist = [1, 2, 3]
>>> element = alist[7] 
Traceback (innermost last):
  File “<stdin>”, line 1, in ?
IndexError: list index out of range

>>> raise IndexError("Just kidding")
Traceback (innermost last):
  File “<stdin>”, line 1, in ?
IndexError: Just kidding

14.2.4 Defining exceptions

class MyError(Exception):
    pass

>>> raise MyError("Some information about what went wrong")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.MyError: Some information about what went wrong

try:
    raise MyError("Some information about what went wrong")
except MyError as error:
    print("Situation:", error)

try:
    raise MyError("Some information", "myFilename",3)
except MyError as error:
    print("Situation: problem {0} with file {1}: {2}".format((error.args[2],
        error.args[1], error.args[0]))

14.2.5 Debugging with assert

>>> x = (1, 2, 3)
>>> assert len(x)>5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError

14.2.7 Example

def save_to_file(filename) :
    try:
        save_text_to_file(filename)
        save_formats_to_file(filename)
        save_prefs_to_file(filename)
        .
        .
        .
    except IOError:
        ...handle the error...
def save_text_to_file(filename):
    ...lower-level call to write size of text...
    ...lower-level call to write actual text data...
    .
    .
    .

14.2.8 Example

def cell_value(string):
    try:
        return float(string)
    except ValueError:
        if string == "":
            return 0
        else:
            return None

def safe_apply(function, x, y, spreadsheet):
    try:
        return apply(function, (x, y, spreadsheet))
    except TypeError:
        return None

14.3 With

try:
    infile = open(filename)
    data = infile.read()
finally:
    infile.close()

with open(filename) as infile:
    data = infile.read()