Exception Handling — Python 2.7.18 documentation

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# Exception Handling

The functions described in this chapter will let you handle and raise Python exceptions. It is important to understand some of the basics of Python exception handling. It works somewhat like the Unix `errno` variable: there is a global indicator (per thread) of the last error that occurred. Most functions don’t clear this on success, but will set it to indicate the cause of the error on failure. Most functions also return an error indicator, usually NULL if they are supposed to return a pointer, or `-1` if they return an integer (exception: the `PyArg_*()` functions return `1` for success and `0` for failure).

When a function must fail because some function it called failed, it generally doesn’t set the error indicator; the function it called already set it. It is responsible for either handling the error and clearing the exception or returning after cleaning up any resources it holds (such as object references or memory allocations); it should not continue normally if it is not prepared to handle the error. If returning due to an error, it is important to indicate to the caller that an error has been set. If the error is not handled or carefully propagated, additional calls into the Python/C API may not behave as intended and may fail in mysterious ways.

The error indicator consists of three Python objects corresponding to the Python variables `sys.exc_type`, `sys.exc_value` and `sys.exc_traceback`. API functions exist to interact with the error indicator in various ways. There is a separate error indicator for each thread.

void `PyErr_PrintEx`(int set_sys_last_vars)

  Print a standard traceback to `sys.stderr` and clear the error indicator. Unless the error is a `SystemExit`. In that case the no traceback is printed and Python process will exit with the error code specified by the `SystemExit` instance.

  Call this function only when the error indicator is set. Otherwise it will cause a fatal error!

  If set_sys_last_vars is nonzero, the variables `sys.last_type`, `sys.last_value` and `sys.last_traceback` will be set to the type, value and traceback of the printed exception, respectively.

void `PyErr_Print`()

  Alias for `PyErr_PrintEx(1)`.

PyObject* `PyErr_Occurred`()
  Return value: Borrowed reference.

  Test whether the error indicator is set. If set, return the exception type (the first argument to the last call to one of the `PyErr_Set*()` functions or to `PyErr_Restore()`). If not set, return NULL. You do not own a reference to the return value, so you do not need to `Py_DECREF()` it.

  Note

  Do not compare the return value to a specific exception; use `PyErr_ExceptionMatches()` instead, shown below. (The comparison could easily fail since the exception may be an instance instead of a class, in the case of a class exception, or it may be a subclass of the expected exception.)

int `PyErr_ExceptionMatches`(PyObject *exc)

  Equivalent to `PyErr_GivenExceptionMatches(PyErr_Occurred(), exc)`. This should only be called when an exception is actually set; a memory access violation will occur if no exception has been raised.

int `PyErr_GivenExceptionMatches`(PyObject *given, PyObject *exc)

  Return true if the given exception matches the exception in exc. If exc is a class object, this also returns true when given is an instance of a subclass. If exc is a tuple, all exceptions in the tuple (and recursively in subtuples) are searched for a match.

void `PyErr_NormalizeException`(PyObject**exc, PyObject**val, PyObject**tb)

  Under certain circumstances, the values returned by `PyErr_Fetch()` below can be “unnormalized”, meaning that `*exc` is a class object but `*val` is not an instance of the same class. This function can be used to instantiate the class in that case. If the values are already normalized, nothing happens. The delayed normalization is implemented to improve performance.

void `PyErr_Clear`()

  Clear the error indicator. If the error indicator is not set, there is no effect.

void `PyErr_Fetch`(PyObject **ptype, PyObject **pvalue, PyObject **ptraceback)

  Retrieve the error indicator into three variables whose addresses are passed. If the error indicator is not set, set all three variables to NULL. If it is set, it will be cleared and you own a reference to each object retrieved. The value and traceback object may be NULL even when the type object is not.

  Note

  This function is normally only used by code that needs to handle exceptions or by code that needs to save and restore the error indicator temporarily.

void `PyErr_Restore`(PyObject *type, PyObject *value, PyObject *traceback)

  Set the error indicator from the three objects. If the error indicator is already set, it is cleared first. If the objects are NULL, the error indicator is cleared. Do not pass a NULL type and non-NULL value or traceback. The exception type should be a class. Do not pass an invalid exception type or value. (Violating these rules will cause subtle problems later.) This call takes away a reference to each object: you must own a reference to each object before the call and after the call you no longer own these references. (If you don’t understand this, don’t use this function. I warned you.)

  Note

  This function is normally only used by code that needs to save and restore the error indicator temporarily; use `PyErr_Fetch()` to save the current exception state.

void `PyErr_SetString`(PyObject *type, const char *message)

  This is the most common way to set the error indicator. The first argument specifies the exception type; it is normally one of the standard exceptions, e.g. `PyExc_RuntimeError`. You need not increment its reference count. The second argument is an error message; it is converted to a string object.

void `PyErr_SetObject`(PyObject *type, PyObject *value)

  This function is similar to `PyErr_SetString()` but lets you specify an arbitrary Python object for the “value” of the exception.

PyObject* `PyErr_Format`(PyObject *exception, const char *format, ...)
  Return value: Always NULL.

  This function sets the error indicator and returns NULL. exception should be a Python exception class. The format and subsequent parameters help format the error message; they have the same meaning and values as in `PyString_FromFormat()`.

void `PyErr_SetNone`(PyObject *type)

  This is a shorthand for `PyErr_SetObject(type, Py_None)`.

int `PyErr_BadArgument`()

  This is a shorthand for `PyErr_SetString(PyExc_TypeError, message)`, where message indicates that a built-in operation was invoked with an illegal argument. It is mostly for internal use.

PyObject* `PyErr_NoMemory`()
  Return value: Always NULL.

  This is a shorthand for `PyErr_SetNone(PyExc_MemoryError)`; it returns NULL so an object allocation function can write `return PyErr_NoMemory();` when it runs out of memory.

PyObject* `PyErr_SetFromErrno`(PyObject *type)
  Return value: Always NULL.

  This is a convenience function to raise an exception when a C library function has returned an error and set the C variable `errno`. It constructs a tuple object whose first item is the integer `errno` value and whose second item is the corresponding error message (gotten from `strerror()`), and then calls `PyErr_SetObject(type, object)`. On Unix, when the `errno` value is `EINTR`, indicating an interrupted system call, this calls `PyErr_CheckSignals()`, and if that set the error indicator, leaves it set to that. The function always returns NULL, so a wrapper function around a system call can write `return PyErr_SetFromErrno(type);` when the system call returns an error.

PyObject* `PyErr_SetFromErrnoWithFilenameObject`(PyObject *type, PyObject *filenameObject)

  Similar to `PyErr_SetFromErrno()`, with the additional behavior that if filenameObject is not NULL, it is passed to the constructor of type as a third parameter. In the case of exceptions such as `IOError` and `OSError`, this is used to define the `filename` attribute of the exception instance.

PyObject* `PyErr_SetFromErrnoWithFilename`(PyObject *type, const char *filename)
  Return value: Always NULL.

  Similar to `PyErr_SetFromErrnoWithFilenameObject()`, but the filename is given as a C string.

PyObject* `PyErr_SetFromWindowsErr`(int ierr)
  Return value: Always NULL.

  This is a convenience function to raise `WindowsError`. If called with ierr of `0`, the error code returned by a call to `GetLastError()` is used instead. It calls the Win32 function `FormatMessage()` to retrieve the Windows description of error code given by ierr or `GetLastError()`, then it constructs a tuple object whose first item is the ierr value and whose second item is the corresponding error message (gotten from `FormatMessage()`), and then calls `PyErr_SetObject(PyExc_WindowsError, object)`. This function always returns NULL. Availability: Windows.

PyObject* `PyErr_SetExcFromWindowsErr`(PyObject *type, int ierr)
  Return value: Always NULL.

  Similar to `PyErr_SetFromWindowsErr()`, with an additional parameter specifying the exception type to be raised. Availability: Windows.

  New in version 2.3.

PyObject* `PyErr_SetFromWindowsErrWithFilenameObject`(int ierr, PyObject *filenameObject)

  Similar to `PyErr_SetFromWindowsErr()`, with the additional behavior that if filenameObject is not NULL, it is passed to the constructor of `WindowsError` as a third parameter. Availability: Windows.

PyObject* `PyErr_SetFromWindowsErrWithFilename`(int ierr, const char *filename)
  Return value: Always NULL.

  Similar to `PyErr_SetFromWindowsErrWithFilenameObject()`, but the filename is given as a C string. Availability: Windows.

PyObject* `PyErr_SetExcFromWindowsErrWithFilenameObject`(PyObject *type, int ierr, PyObject *filename)

  Similar to `PyErr_SetFromWindowsErrWithFilenameObject()`, with an additional parameter specifying the exception type to be raised. Availability: Windows.

  New in version 2.3.

PyObject* `PyErr_SetExcFromWindowsErrWithFilename`(PyObject *type, int ierr, const char *filename)
  Return value: Always NULL.

  Similar to `PyErr_SetFromWindowsErrWithFilename()`, with an additional parameter specifying the exception type to be raised. Availability: Windows.

  New in version 2.3.

void `PyErr_BadInternalCall`()

  This is a shorthand for `PyErr_SetString(PyExc_SystemError, message)`, where message indicates that an internal operation (e.g. a Python/C API function) was invoked with an illegal argument. It is mostly for internal use.

int `PyErr_WarnEx`(PyObject *category, char *message, int stacklevel)

  Issue a warning message. The category argument is a warning category (see below) or NULL; the message argument is a message string. stacklevel is a positive number giving a number of stack frames; the warning will be issued from the currently executing line of code in that stack frame. A stacklevel of 1 is the function calling `PyErr_WarnEx()`, 2 is the function above that, and so forth.

  This function normally prints a warning message to sys.stderr; however, it is also possible that the user has specified that warnings are to be turned into errors, and in that case this will raise an exception. It is also possible that the function raises an exception because of a problem with the warning machinery (the implementation imports the `warnings` module to do the heavy lifting). The return value is `0` if no exception is raised, or `-1` if an exception is raised. (It is not possible to determine whether a warning message is actually printed, nor what the reason is for the exception; this is intentional.) If an exception is raised, the caller should do its normal exception handling (for example, `Py_DECREF()` owned references and return an error value).

  Warning categories must be subclasses of `PyExc_Warning`; `PyExc_Warning` is a subclass of `PyExc_Exception`; the default warning category is `PyExc_RuntimeWarning`. The standard Python warning categories are available as global variables whose names are enumerated at Standard Warning Categories.

  For information about warning control, see the documentation for the `warnings` module and the `-W` option in the command line documentation. There is no C API for warning control.

int `PyErr_Warn`(PyObject *category, char *message)

  Issue a warning message. The category argument is a warning category (see below) or NULL; the message argument is a message string. The warning will appear to be issued from the function calling `PyErr_Warn()`, equivalent to calling `PyErr_WarnEx()` with a stacklevel of 1.

  Deprecated; use `PyErr_WarnEx()` instead.

int `PyErr_WarnExplicit`(PyObject *category, const char *message, const char *filename, int lineno, const char *module, PyObject *registry)

  Issue a warning message with explicit control over all warning attributes. This is a straightforward wrapper around the Python function `warnings.warn_explicit()`, see there for more information. The module and registry arguments may be set to NULL to get the default effect described there.

int `PyErr_WarnPy3k`(char *message, int stacklevel)

  Issue a `DeprecationWarning` with the given message and stacklevel if the `Py_Py3kWarningFlag` flag is enabled.

  New in version 2.6.

int `PyErr_CheckSignals`()

  This function interacts with Python’s signal handling. It checks whether a signal has been sent to the processes and if so, invokes the corresponding signal handler. If the `signal` module is supported, this can invoke a signal handler written in Python. In all cases, the default effect for `SIGINT` is to raise the `KeyboardInterrupt` exception. If an exception is raised the error indicator is set and the function returns `-1`; otherwise the function returns `0`. The error indicator may or may not be cleared if it was previously set.

void `PyErr_SetInterrupt`()

  This function simulates the effect of a `SIGINT` signal arriving — the next time `PyErr_CheckSignals()` is called, `KeyboardInterrupt` will be raised. It may be called without holding the interpreter lock.

int `PySignal_SetWakeupFd`(int fd)

  This utility function specifies a file descriptor to which a `'\0'` byte will be written whenever a signal is received. It returns the previous such file descriptor. The value `-1` disables the feature; this is the initial state. This is equivalent to `signal.set_wakeup_fd()` in Python, but without any error checking. fd should be a valid file descriptor. The function should only be called from the main thread.

  New in version 2.6.

PyObject* `PyErr_NewException`(char *name, PyObject *base, PyObject *dict)
  Return value: New reference.

  This utility function creates and returns a new exception class. The name argument must be the name of the new exception, a C string of the form `module.classname`. The base and dict arguments are normally NULL. This creates a class object derived from `Exception` (accessible in C as `PyExc_Exception`).

  The `__module__` attribute of the new class is set to the first part (up to the last dot) of the name argument, and the class name is set to the last part (after the last dot). The base argument can be used to specify alternate base classes; it can either be only one class or a tuple of classes. The dict argument can be used to specify a dictionary of class variables and methods.

PyObject* `PyErr_NewExceptionWithDoc`(char *name, char *doc, PyObject *base, PyObject *dict)
  Return value: New reference.

  Same as `PyErr_NewException()`, except that the new exception class can easily be given a docstring: If doc is non-NULL, it will be used as the docstring for the exception class.

  New in version 2.7.

void `PyErr_WriteUnraisable`(PyObject *obj)

  This utility function prints a warning message to `sys.stderr` when an exception has been set but it is impossible for the interpreter to actually raise the exception. It is used, for example, when an exception occurs in an `__del__()` method.

  The function is called with a single argument obj that identifies the context in which the unraisable exception occurred. If possible, the repr of obj will be printed in the warning message.

## Unicode Exception Objects

The following functions are used to create and modify Unicode exceptions from C.

PyObject* `PyUnicodeDecodeError_Create`(const char *encoding, const char *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

  Create a `UnicodeDecodeError` object with the attributes encoding, object, length, start, end and reason.

PyObject* `PyUnicodeEncodeError_Create`(const char *encoding, const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

  Create a `UnicodeEncodeError` object with the attributes encoding, object, length, start, end and reason.

PyObject* `PyUnicodeTranslateError_Create`(const Py_UNICODE *object, Py_ssize_t length, Py_ssize_t start, Py_ssize_t end, const char *reason)

  Create a `UnicodeTranslateError` object with the attributes object, length, start, end and reason.

PyObject* `PyUnicodeDecodeError_GetEncoding`(PyObject *exc)
PyObject* `PyUnicodeEncodeError_GetEncoding`(PyObject *exc)

  Return the encoding attribute of the given exception object.

PyObject* `PyUnicodeDecodeError_GetObject`(PyObject *exc)
PyObject* `PyUnicodeEncodeError_GetObject`(PyObject *exc)
PyObject* `PyUnicodeTranslateError_GetObject`(PyObject *exc)

  Return the object attribute of the given exception object.

int `PyUnicodeDecodeError_GetStart`(PyObject *exc, Py_ssize_t *start)
int `PyUnicodeEncodeError_GetStart`(PyObject *exc, Py_ssize_t *start)
int `PyUnicodeTranslateError_GetStart`(PyObject *exc, Py_ssize_t *start)

  Get the start attribute of the given exception object and place it into *start. start must not be NULL. Return `0` on success, `-1` on failure.

int `PyUnicodeDecodeError_SetStart`(PyObject *exc, Py_ssize_t start)
int `PyUnicodeEncodeError_SetStart`(PyObject *exc, Py_ssize_t start)
int `PyUnicodeTranslateError_SetStart`(PyObject *exc, Py_ssize_t start)

  Set the start attribute of the given exception object to start. Return `0` on success, `-1` on failure.

int `PyUnicodeDecodeError_GetEnd`(PyObject *exc, Py_ssize_t *end)
int `PyUnicodeEncodeError_GetEnd`(PyObject *exc, Py_ssize_t *end)
int `PyUnicodeTranslateError_GetEnd`(PyObject *exc, Py_ssize_t *end)

  Get the end attribute of the given exception object and place it into *end. end must not be NULL. Return `0` on success, `-1` on failure.

int `PyUnicodeDecodeError_SetEnd`(PyObject *exc, Py_ssize_t end)
int `PyUnicodeEncodeError_SetEnd`(PyObject *exc, Py_ssize_t end)
int `PyUnicodeTranslateError_SetEnd`(PyObject *exc, Py_ssize_t end)

  Set the end attribute of the given exception object to end. Return `0` on success, `-1` on failure.

PyObject* `PyUnicodeDecodeError_GetReason`(PyObject *exc)
PyObject* `PyUnicodeEncodeError_GetReason`(PyObject *exc)
PyObject* `PyUnicodeTranslateError_GetReason`(PyObject *exc)

  Return the reason attribute of the given exception object.

int `PyUnicodeDecodeError_SetReason`(PyObject *exc, const char *reason)
int `PyUnicodeEncodeError_SetReason`(PyObject *exc, const char *reason)
int `PyUnicodeTranslateError_SetReason`(PyObject *exc, const char *reason)

  Set the reason attribute of the given exception object to reason. Return `0` on success, `-1` on failure.

## Recursion Control

These two functions provide a way to perform safe recursive calls at the C level, both in the core and in extension modules. They are needed if the recursive code does not necessarily invoke Python code (which tracks its recursion depth automatically).

int `Py_EnterRecursiveCall`(const char *where)

  Marks a point where a recursive C-level call is about to be performed.

  If `USE_STACKCHECK` is defined, this function checks if the OS stack overflowed using `PyOS_CheckStack()`. In this is the case, it sets a `MemoryError` and returns a nonzero value.

  The function then checks if the recursion limit is reached. If this is the case, a `RuntimeError` is set and a nonzero value is returned. Otherwise, zero is returned.

  where should be a string such as `" in instance check"` to be concatenated to the `RuntimeError` message caused by the recursion depth limit.

void `Py_LeaveRecursiveCall`()

  Ends a `Py_EnterRecursiveCall()`. Must be called once for each successful invocation of `Py_EnterRecursiveCall()`.

## Standard Exceptions

All standard Python exceptions are available as global variables whose names are `PyExc_` followed by the Python exception name. These have the type `PyObject*`; they are all class objects. For completeness, here are all the variables:

C Name | Python Name | Notes
--- | --- | ---
`PyExc_BaseException` | `BaseException` | (1), (4)
`PyExc_Exception` | `Exception` | (1)
`PyExc_StandardError` | `StandardError` | (1)
`PyExc_ArithmeticError` | `ArithmeticError` | (1)
`PyExc_AssertionError` | `AssertionError` |
`PyExc_AttributeError` | `AttributeError` |
`PyExc_BufferError` | `BufferError` |
`PyExc_EnvironmentError` | `EnvironmentError` | (1)
`PyExc_EOFError` | `EOFError` |
`PyExc_FloatingPointError` | `FloatingPointError` |
`PyExc_GeneratorExit` | `GeneratorExit` |
`PyExc_ImportError` | `ImportError` |
`PyExc_IndentationError` | `IndentationError` |
`PyExc_IndexError` | `IndexError` |
`PyExc_IOError` | `IOError` |
`PyExc_KeyError` | `KeyError` |
`PyExc_KeyboardInterrupt` | `KeyboardInterrupt` |
`PyExc_LookupError` | `LookupError` | (1)
`PyExc_MemoryError` | `MemoryError` |
`PyExc_NameError` | `NameError` |
`PyExc_NotImplementedError` | `NotImplementedError` |
`PyExc_OSError` | `OSError` |
`PyExc_OverflowError` | `OverflowError` |
`PyExc_ReferenceError` | `ReferenceError` | (2)
`PyExc_RuntimeError` | `RuntimeError` |
`PyExc_StopIteration` | `StopIteration` |
`PyExc_SyntaxError` | `SyntaxError` |
`PyExc_SystemError` | `SystemError` |
`PyExc_SystemExit` | `SystemExit` |
`PyExc_TabError` | `TabError` |
`PyExc_TypeError` | `TypeError` |
`PyExc_UnboundLocalError` | `UnboundLocalError` |
`PyExc_UnicodeDecodeError` | `UnicodeDecodeError` |
`PyExc_UnicodeEncodeError` | `UnicodeEncodeError` |
`PyExc_UnicodeError` | `UnicodeError` |
`PyExc_UnicodeTranslateError` | `UnicodeTranslateError` |
`PyExc_VMSError` | `VMSError` | (5)
`PyExc_ValueError` | `ValueError` |
`PyExc_WindowsError` | `WindowsError` | (3)
`PyExc_ZeroDivisionError` | `ZeroDivisionError` |

Notes:

1. This is a base class for other standard exceptions.

2. This is the same as `weakref.ReferenceError`.

3. Only defined on Windows; protect code that uses this by testing that the preprocessor macro `MS_WINDOWS` is defined.

4. New in version 2.5.

5. Only defined on VMS; protect code that uses this by testing that the preprocessor macro `__VMS` is defined.

## Standard Warning Categories

All standard Python warning categories are available as global variables whose names are `PyExc_` followed by the Python exception name. These have the type `PyObject*`; they are all class objects. For completeness, here are all the variables:

C Name | Python Name | Notes
--- | --- | ---
`PyExc_Warning` | `Warning` | (1)
`PyExc_BytesWarning` | `BytesWarning` |
`PyExc_DeprecationWarning` | `DeprecationWarning` |
`PyExc_FutureWarning` | `FutureWarning` |
`PyExc_ImportWarning` | `ImportWarning` |
`PyExc_PendingDeprecationWarning` | `PendingDeprecationWarning` |
`PyExc_RuntimeWarning` | `RuntimeWarning` |
`PyExc_SyntaxWarning` | `SyntaxWarning` |
`PyExc_UnicodeWarning` | `UnicodeWarning` |
`PyExc_UserWarning` | `UserWarning` |

Notes:

1. This is a base class for other standard warning categories.

## String Exceptions

Changed in version 2.6: All exceptions to be raised or caught must be derived from `BaseException`. Trying to raise a string exception now raises `TypeError`.