Default Printing and State

Built-in exceptions also provide default print displays and state retention, which is often as much logic as user-defined classes require. Unless you redefine the constructors your classes inherit from them, any constructor arguments you pass to these classes are saved in the instance’s args tuple attribute and are automatically displayed when the instance is printed (an empty tuple and display string are used if no constructor arguments are passed).

This explains why arguments passed to built-in exception classes show up in error messages—any constructor arguments are attached to the instance and displayed when the instance is printed:

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>>> raise IndexError                    # Same as IndexError(): no arguments
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError

>>> raise IndexError('spam')            # Constructor argument attached, printed
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: spam

>>> I = IndexError('spam')              # Available in object attribute
>>> I.args
('spam',)

The same holds true for user-defined exceptions, because they inherit the constructor and display methods present in their built-in superclasses:

>>> class E(Exception): pass
...
>>> try:
...    raise E('spam')
... except E as X:
...    print(X, X.args)                 # Displays and saves constructor arguments
...
spam ('spam',)

>>> try:
...    raise E('spam', 'eggs', 'ham')
... except E as X:
...    print(X, X.args)
...
('spam', 'eggs', 'ham') ('spam', 'eggs', 'ham')

Note that exception instance objects are not strings themselves, but use the __str__ operator overloading protocol we studied in Chapter 29 to provide display strings when printed; to concatenate with real strings, perform manual conversions: str(X) + "string".

Although this automatic state and display support is useful by itself, for more specific display and state retention needs you can always redefine inherited methods such as __str__ and __init__ in Exception subclasses—the next section shows how.