516 lines
25 KiB
ReStructuredText
516 lines
25 KiB
ReStructuredText
.. highlightlang:: c
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.. _exceptionhandling:
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******************
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Exception Handling
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******************
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The functions described in this chapter will let you handle and raise Python
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exceptions. It is important to understand some of the basics of Python
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exception handling. It works somewhat like the Unix :cdata:`errno` variable:
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there is a global indicator (per thread) of the last error that occurred. Most
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functions don't clear this on success, but will set it to indicate the cause of
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the error on failure. Most functions also return an error indicator, usually
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*NULL* if they are supposed to return a pointer, or ``-1`` if they return an
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integer (exception: the :cfunc:`PyArg_\*` functions return ``1`` for success and
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``0`` for failure).
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When a function must fail because some function it called failed, it generally
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doesn't set the error indicator; the function it called already set it. It is
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responsible for either handling the error and clearing the exception or
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returning after cleaning up any resources it holds (such as object references or
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memory allocations); it should *not* continue normally if it is not prepared to
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handle the error. If returning due to an error, it is important to indicate to
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the caller that an error has been set. If the error is not handled or carefully
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propagated, additional calls into the Python/C API may not behave as intended
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and may fail in mysterious ways.
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The error indicator consists of three Python objects corresponding to the result
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of ``sys.exc_info()``. API functions exist to interact with the error indicator
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in various ways. There is a separate error indicator for each thread.
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.. % XXX Order of these should be more thoughtful.
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.. % Either alphabetical or some kind of structure.
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.. cfunction:: void PyErr_Print()
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Print a standard traceback to ``sys.stderr`` and clear the error indicator.
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Call this function only when the error indicator is set. (Otherwise it will
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cause a fatal error!)
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.. cfunction:: PyObject* PyErr_Occurred()
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Test whether the error indicator is set. If set, return the exception *type*
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(the first argument to the last call to one of the :cfunc:`PyErr_Set\*`
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functions or to :cfunc:`PyErr_Restore`). If not set, return *NULL*. You do not
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own a reference to the return value, so you do not need to :cfunc:`Py_DECREF`
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it.
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.. note::
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Do not compare the return value to a specific exception; use
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:cfunc:`PyErr_ExceptionMatches` instead, shown below. (The comparison could
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easily fail since the exception may be an instance instead of a class, in the
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case of a class exception, or it may the a subclass of the expected exception.)
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.. cfunction:: int PyErr_ExceptionMatches(PyObject *exc)
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Equivalent to ``PyErr_GivenExceptionMatches(PyErr_Occurred(), exc)``. This
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should only be called when an exception is actually set; a memory access
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violation will occur if no exception has been raised.
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.. cfunction:: int PyErr_GivenExceptionMatches(PyObject *given, PyObject *exc)
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Return true if the *given* exception matches the exception in *exc*. If *exc*
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is a class object, this also returns true when *given* is an instance of a
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subclass. If *exc* is a tuple, all exceptions in the tuple (and recursively in
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subtuples) are searched for a match. If *given* is *NULL*, a memory access
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violation will occur.
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.. cfunction:: void PyErr_NormalizeException(PyObject**exc, PyObject**val, PyObject**tb)
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Under certain circumstances, the values returned by :cfunc:`PyErr_Fetch` below
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can be "unnormalized", meaning that ``*exc`` is a class object but ``*val`` is
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not an instance of the same class. This function can be used to instantiate
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the class in that case. If the values are already normalized, nothing happens.
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The delayed normalization is implemented to improve performance.
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.. cfunction:: void PyErr_Clear()
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Clear the error indicator. If the error indicator is not set, there is no
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effect.
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.. cfunction:: void PyErr_Fetch(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)
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Retrieve the error indicator into three variables whose addresses are passed.
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If the error indicator is not set, set all three variables to *NULL*. If it is
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set, it will be cleared and you own a reference to each object retrieved. The
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value and traceback object may be *NULL* even when the type object is not.
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.. note::
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This function is normally only used by code that needs to handle exceptions or
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by code that needs to save and restore the error indicator temporarily.
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.. cfunction:: void PyErr_Restore(PyObject *type, PyObject *value, PyObject *traceback)
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Set the error indicator from the three objects. If the error indicator is
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already set, it is cleared first. If the objects are *NULL*, the error
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indicator is cleared. Do not pass a *NULL* type and non-*NULL* value or
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traceback. The exception type should be a class. Do not pass an invalid
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exception type or value. (Violating these rules will cause subtle problems
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later.) This call takes away a reference to each object: you must own a
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reference to each object before the call and after the call you no longer own
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these references. (If you don't understand this, don't use this function. I
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warned you.)
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.. note::
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This function is normally only used by code that needs to save and restore the
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error indicator temporarily; use :cfunc:`PyErr_Fetch` to save the current
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exception state.
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.. cfunction:: void PyErr_SetString(PyObject *type, const char *message)
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This is the most common way to set the error indicator. The first argument
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specifies the exception type; it is normally one of the standard exceptions,
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e.g. :cdata:`PyExc_RuntimeError`. You need not increment its reference count.
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The second argument is an error message; it is converted to a string object.
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.. cfunction:: void PyErr_SetObject(PyObject *type, PyObject *value)
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This function is similar to :cfunc:`PyErr_SetString` but lets you specify an
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arbitrary Python object for the "value" of the exception.
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.. cfunction:: PyObject* PyErr_Format(PyObject *exception, const char *format, ...)
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This function sets the error indicator and returns *NULL*. *exception* should be
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a Python exception (class, not an instance). *format* should be a string,
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containing format codes, similar to :cfunc:`printf`. The ``width.precision``
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before a format code is parsed, but the width part is ignored.
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.. % This should be exactly the same as the table in PyString_FromFormat.
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.. % One should just refer to the other.
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.. % The descriptions for %zd and %zu are wrong, but the truth is complicated
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.. % because not all compilers support the %z width modifier -- we fake it
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.. % when necessary via interpolating PY_FORMAT_SIZE_T.
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.. % %u, %lu, %zu should have "new in Python 2.5" blurbs.
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+-------------------+---------------+--------------------------------+
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| Format Characters | Type | Comment |
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+===================+===============+================================+
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| :attr:`%%` | *n/a* | The literal % character. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%c` | int | A single character, |
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| | | represented as an C int. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%d` | int | Exactly equivalent to |
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| | | ``printf("%d")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%u` | unsigned int | Exactly equivalent to |
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| | | ``printf("%u")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%ld` | long | Exactly equivalent to |
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| | | ``printf("%ld")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%lu` | unsigned long | Exactly equivalent to |
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| | | ``printf("%lu")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%zd` | Py_ssize_t | Exactly equivalent to |
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| | | ``printf("%zd")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%zu` | size_t | Exactly equivalent to |
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| | | ``printf("%zu")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%i` | int | Exactly equivalent to |
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| | | ``printf("%i")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%x` | int | Exactly equivalent to |
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| | | ``printf("%x")``. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%s` | char\* | A null-terminated C character |
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| | | array. |
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+-------------------+---------------+--------------------------------+
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| :attr:`%p` | void\* | The hex representation of a C |
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| | | pointer. Mostly equivalent to |
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| | | ``printf("%p")`` except that |
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| | | it is guaranteed to start with |
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| | | the literal ``0x`` regardless |
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| | | of what the platform's |
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| | | ``printf`` yields. |
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+-------------------+---------------+--------------------------------+
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An unrecognized format character causes all the rest of the format string to be
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copied as-is to the result string, and any extra arguments discarded.
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.. cfunction:: void PyErr_SetNone(PyObject *type)
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This is a shorthand for ``PyErr_SetObject(type, Py_None)``.
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.. cfunction:: int PyErr_BadArgument()
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This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
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*message* indicates that a built-in operation was invoked with an illegal
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argument. It is mostly for internal use.
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.. cfunction:: PyObject* PyErr_NoMemory()
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This is a shorthand for ``PyErr_SetNone(PyExc_MemoryError)``; it returns *NULL*
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so an object allocation function can write ``return PyErr_NoMemory();`` when it
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runs out of memory.
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.. cfunction:: PyObject* PyErr_SetFromErrno(PyObject *type)
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.. index:: single: strerror()
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This is a convenience function to raise an exception when a C library function
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has returned an error and set the C variable :cdata:`errno`. It constructs a
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tuple object whose first item is the integer :cdata:`errno` value and whose
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second item is the corresponding error message (gotten from :cfunc:`strerror`),
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and then calls ``PyErr_SetObject(type, object)``. On Unix, when the
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:cdata:`errno` value is :const:`EINTR`, indicating an interrupted system call,
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this calls :cfunc:`PyErr_CheckSignals`, and if that set the error indicator,
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leaves it set to that. The function always returns *NULL*, so a wrapper
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function around a system call can write ``return PyErr_SetFromErrno(type);``
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when the system call returns an error.
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.. cfunction:: PyObject* PyErr_SetFromErrnoWithFilename(PyObject *type, const char *filename)
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Similar to :cfunc:`PyErr_SetFromErrno`, with the additional behavior that if
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*filename* is not *NULL*, it is passed to the constructor of *type* as a third
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parameter. In the case of exceptions such as :exc:`IOError` and :exc:`OSError`,
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this is used to define the :attr:`filename` attribute of the exception instance.
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.. cfunction:: PyObject* PyErr_SetFromWindowsErr(int ierr)
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This is a convenience function to raise :exc:`WindowsError`. If called with
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*ierr* of :cdata:`0`, the error code returned by a call to :cfunc:`GetLastError`
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is used instead. It calls the Win32 function :cfunc:`FormatMessage` to retrieve
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the Windows description of error code given by *ierr* or :cfunc:`GetLastError`,
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then it constructs a tuple object whose first item is the *ierr* value and whose
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second item is the corresponding error message (gotten from
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:cfunc:`FormatMessage`), and then calls ``PyErr_SetObject(PyExc_WindowsError,
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object)``. This function always returns *NULL*. Availability: Windows.
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.. cfunction:: PyObject* PyErr_SetExcFromWindowsErr(PyObject *type, int ierr)
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Similar to :cfunc:`PyErr_SetFromWindowsErr`, with an additional parameter
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specifying the exception type to be raised. Availability: Windows.
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.. versionadded:: 2.3
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.. cfunction:: PyObject* PyErr_SetFromWindowsErrWithFilename(int ierr, const char *filename)
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Similar to :cfunc:`PyErr_SetFromWindowsErr`, with the additional behavior that
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if *filename* is not *NULL*, it is passed to the constructor of
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:exc:`WindowsError` as a third parameter. Availability: Windows.
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.. cfunction:: PyObject* PyErr_SetExcFromWindowsErrWithFilename(PyObject *type, int ierr, char *filename)
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Similar to :cfunc:`PyErr_SetFromWindowsErrWithFilename`, with an additional
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parameter specifying the exception type to be raised. Availability: Windows.
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.. versionadded:: 2.3
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.. cfunction:: void PyErr_BadInternalCall()
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This is a shorthand for ``PyErr_SetString(PyExc_TypeError, message)``, where
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*message* indicates that an internal operation (e.g. a Python/C API function)
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was invoked with an illegal argument. It is mostly for internal use.
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.. cfunction:: int PyErr_WarnEx(PyObject *category, char *message, int stacklevel)
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Issue a warning message. The *category* argument is a warning category (see
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below) or *NULL*; the *message* argument is a message string. *stacklevel* is a
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positive number giving a number of stack frames; the warning will be issued from
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the currently executing line of code in that stack frame. A *stacklevel* of 1
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is the function calling :cfunc:`PyErr_WarnEx`, 2 is the function above that,
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and so forth.
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This function normally prints a warning message to *sys.stderr*; however, it is
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also possible that the user has specified that warnings are to be turned into
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errors, and in that case this will raise an exception. It is also possible that
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the function raises an exception because of a problem with the warning machinery
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(the implementation imports the :mod:`warnings` module to do the heavy lifting).
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The return value is ``0`` if no exception is raised, or ``-1`` if an exception
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is raised. (It is not possible to determine whether a warning message is
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actually printed, nor what the reason is for the exception; this is
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intentional.) If an exception is raised, the caller should do its normal
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exception handling (for example, :cfunc:`Py_DECREF` owned references and return
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an error value).
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Warning categories must be subclasses of :cdata:`Warning`; the default warning
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category is :cdata:`RuntimeWarning`. The standard Python warning categories are
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available as global variables whose names are ``PyExc_`` followed by the Python
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exception name. These have the type :ctype:`PyObject\*`; they are all class
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objects. Their names are :cdata:`PyExc_Warning`, :cdata:`PyExc_UserWarning`,
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:cdata:`PyExc_UnicodeWarning`, :cdata:`PyExc_DeprecationWarning`,
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:cdata:`PyExc_SyntaxWarning`, :cdata:`PyExc_RuntimeWarning`, and
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:cdata:`PyExc_FutureWarning`. :cdata:`PyExc_Warning` is a subclass of
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:cdata:`PyExc_Exception`; the other warning categories are subclasses of
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:cdata:`PyExc_Warning`.
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For information about warning control, see the documentation for the
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:mod:`warnings` module and the :option:`-W` option in the command line
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documentation. There is no C API for warning control.
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.. cfunction:: int PyErr_WarnExplicit(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)
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Issue a warning message with explicit control over all warning attributes. This
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is a straightforward wrapper around the Python function
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:func:`warnings.warn_explicit`, see there for more information. The *module*
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and *registry* arguments may be set to *NULL* to get the default effect
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described there.
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.. cfunction:: int PyErr_CheckSignals()
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.. index::
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module: signal
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single: SIGINT
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single: KeyboardInterrupt (built-in exception)
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This function interacts with Python's signal handling. It checks whether a
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signal has been sent to the processes and if so, invokes the corresponding
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signal handler. If the :mod:`signal` module is supported, this can invoke a
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signal handler written in Python. In all cases, the default effect for
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:const:`SIGINT` is to raise the :exc:`KeyboardInterrupt` exception. If an
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exception is raised the error indicator is set and the function returns ``-1``;
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otherwise the function returns ``0``. The error indicator may or may not be
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cleared if it was previously set.
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.. cfunction:: void PyErr_SetInterrupt()
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.. index::
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single: SIGINT
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single: KeyboardInterrupt (built-in exception)
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This function simulates the effect of a :const:`SIGINT` signal arriving --- the
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next time :cfunc:`PyErr_CheckSignals` is called, :exc:`KeyboardInterrupt` will
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be raised. It may be called without holding the interpreter lock.
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.. % XXX This was described as obsolete, but is used in
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.. % thread.interrupt_main() (used from IDLE), so it's still needed.
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.. cfunction:: PyObject* PyErr_NewException(char *name, PyObject *base, PyObject *dict)
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This utility function creates and returns a new exception object. The *name*
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argument must be the name of the new exception, a C string of the form
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``module.class``. The *base* and *dict* arguments are normally *NULL*. This
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creates a class object derived from :exc:`Exception` (accessible in C as
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:cdata:`PyExc_Exception`).
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The :attr:`__module__` attribute of the new class is set to the first part (up
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to the last dot) of the *name* argument, and the class name is set to the last
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part (after the last dot). The *base* argument can be used to specify alternate
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base classes; it can either be only one class or a tuple of classes. The *dict*
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argument can be used to specify a dictionary of class variables and methods.
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.. cfunction:: void PyErr_WriteUnraisable(PyObject *obj)
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This utility function prints a warning message to ``sys.stderr`` when an
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exception has been set but it is impossible for the interpreter to actually
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raise the exception. It is used, for example, when an exception occurs in an
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:meth:`__del__` method.
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The function is called with a single argument *obj* that identifies the context
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in which the unraisable exception occurred. The repr of *obj* will be printed in
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the warning message.
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.. _standardexceptions:
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Standard Exceptions
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===================
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All standard Python exceptions are available as global variables whose names are
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``PyExc_`` followed by the Python exception name. These have the type
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:ctype:`PyObject\*`; they are all class objects. For completeness, here are all
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the variables:
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+------------------------------------+----------------------------+----------+
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| C Name | Python Name | Notes |
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+====================================+============================+==========+
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| :cdata:`PyExc_BaseException` | :exc:`BaseException` | (1), (4) |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_Exception` | :exc:`Exception` | \(1) |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_ArithmeticError` | :exc:`ArithmeticError` | \(1) |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_LookupError` | :exc:`LookupError` | \(1) |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_AssertionError` | :exc:`AssertionError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_AttributeError` | :exc:`AttributeError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_EOFError` | :exc:`EOFError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_EnvironmentError` | :exc:`EnvironmentError` | \(1) |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_FloatingPointError` | :exc:`FloatingPointError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_IOError` | :exc:`IOError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_ImportError` | :exc:`ImportError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_IndexError` | :exc:`IndexError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_KeyError` | :exc:`KeyError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_KeyboardInterrupt` | :exc:`KeyboardInterrupt` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_MemoryError` | :exc:`MemoryError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_NameError` | :exc:`NameError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_NotImplementedError` | :exc:`NotImplementedError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_OSError` | :exc:`OSError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_OverflowError` | :exc:`OverflowError` | |
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+------------------------------------+----------------------------+----------+
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| :cdata:`PyExc_ReferenceError` | :exc:`ReferenceError` | \(2) |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_RuntimeError` | :exc:`RuntimeError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_SyntaxError` | :exc:`SyntaxError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_SystemError` | :exc:`SystemError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_SystemExit` | :exc:`SystemExit` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_TypeError` | :exc:`TypeError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_ValueError` | :exc:`ValueError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_WindowsError` | :exc:`WindowsError` | \(3) |
|
|
+------------------------------------+----------------------------+----------+
|
|
| :cdata:`PyExc_ZeroDivisionError` | :exc:`ZeroDivisionError` | |
|
|
+------------------------------------+----------------------------+----------+
|
|
|
|
.. index::
|
|
single: PyExc_BaseException
|
|
single: PyExc_Exception
|
|
single: PyExc_ArithmeticError
|
|
single: PyExc_LookupError
|
|
single: PyExc_AssertionError
|
|
single: PyExc_AttributeError
|
|
single: PyExc_EOFError
|
|
single: PyExc_EnvironmentError
|
|
single: PyExc_FloatingPointError
|
|
single: PyExc_IOError
|
|
single: PyExc_ImportError
|
|
single: PyExc_IndexError
|
|
single: PyExc_KeyError
|
|
single: PyExc_KeyboardInterrupt
|
|
single: PyExc_MemoryError
|
|
single: PyExc_NameError
|
|
single: PyExc_NotImplementedError
|
|
single: PyExc_OSError
|
|
single: PyExc_OverflowError
|
|
single: PyExc_ReferenceError
|
|
single: PyExc_RuntimeError
|
|
single: PyExc_SyntaxError
|
|
single: PyExc_SystemError
|
|
single: PyExc_SystemExit
|
|
single: PyExc_TypeError
|
|
single: PyExc_ValueError
|
|
single: PyExc_WindowsError
|
|
single: PyExc_ZeroDivisionError
|
|
|
|
Notes:
|
|
|
|
(1)
|
|
This is a base class for other standard exceptions.
|
|
|
|
(2)
|
|
This is the same as :exc:`weakref.ReferenceError`.
|
|
|
|
(3)
|
|
Only defined on Windows; protect code that uses this by testing that the
|
|
preprocessor macro ``MS_WINDOWS`` is defined.
|
|
|
|
(4)
|
|
.. versionadded:: 2.5
|
|
|
|
|
|
Deprecation of String Exceptions
|
|
================================
|
|
|
|
.. index:: single: BaseException (built-in exception)
|
|
|
|
All exceptions built into Python or provided in the standard library are derived
|
|
from :exc:`BaseException`.
|
|
|
|
String exceptions are still supported in the interpreter to allow existing code
|
|
to run unmodified, but this will also change in a future release.
|
|
|