python_doc_md/Advanced/c-api/exceptions.md

Exception Handling {#exceptionhandling}

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 POSIX errno{.interpreted-text role="c:data"} variable: there is a global indicator (per thread) of the last error that occurred. Most C API functions don't clear this on success, but will set it to indicate the cause of the error on failure. Most C API 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).

Concretely, the error indicator consists of three object pointers: the exception's type, the exception's value, and the traceback object. Any of those pointers can be NULL if non-set (although some combinations are forbidden, for example you can't have a non-NULL traceback if the exception type is NULL).

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.

:::: note ::: title Note :::

The error indicator is not the result of sys.exc_info{.interpreted-text role="func"}. The former corresponds to an exception that is not yet caught (and is therefore still propagating), while the latter returns an exception after it is caught (and has therefore stopped propagating). ::::

Printing and clearing

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

Print a standard traceback to sys.stderr and clear the error indicator. Unless the error is a SystemExit, in that case no traceback is printed and the 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 variable sys.last_exc{.interpreted-text role="data"} is set to the printed exception. For backwards compatibility, the deprecated variables sys.last_type{.interpreted-text role="data"}, sys.last_value{.interpreted-text role="data"} and sys.last_traceback{.interpreted-text role="data"} are also set to the type, value and traceback of this exception, respectively.

::: versionchanged 3.12 The setting of sys.last_exc{.interpreted-text role="data"} was added. :::

Alias for PyErr_PrintEx(1).

Call sys.unraisablehook{.interpreted-text role="func"} using the current exception and obj argument.

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 ~object.__del__{.interpreted-text role="meth"} 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. If obj is NULL, only the traceback is printed.

An exception must be set when calling this function.

::: versionchanged 3.4 Print a traceback. Print only traceback if obj is NULL. :::

::: versionchanged 3.8 Use sys.unraisablehook{.interpreted-text role="func"}. :::

Similar to PyErr_WriteUnraisable{.interpreted-text role="c:func"}, but the format and subsequent parameters help format the warning message; they have the same meaning and values as in PyUnicode_FromFormat{.interpreted-text role="c:func"}. PyErr_WriteUnraisable(obj) is roughly equivalent to PyErr_FormatUnraisable("Exception ignored in: %R", obj). If format is NULL, only the traceback is printed.

::: versionadded 3.13 :::

Print the standard traceback display of exc to sys.stderr, including chained exceptions and notes.

::: versionadded 3.12 :::

Raising exceptions

These functions help you set the current thread's error indicator. For convenience, some of these functions will always return a NULL pointer for use in a return statement.

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{.interpreted-text role="c:data"}. You need not create a new strong reference{.interpreted-text role="term"} to it (e.g. with Py_INCREF{.interpreted-text role="c:func"}). The second argument is an error message; it is decoded from 'utf-8'.

This function is similar to PyErr_SetString{.interpreted-text role="c:func"} but lets you specify an arbitrary Python object for the "value" of the exception.

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 PyUnicode_FromFormat{.interpreted-text role="c:func"}. format is an ASCII-encoded string.

Same as PyErr_Format{.interpreted-text role="c:func"}, but taking a va_list{.interpreted-text role="c:type"} argument rather than a variable number of arguments.

::: versionadded 3.5 :::

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

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.

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.

::: index single: strerror (C function) :::

This is a convenience function to raise an exception when a C library function has returned an error and set the C variable errno{.interpreted-text role="c:data"}. It constructs a tuple object whose first item is the integer errno{.interpreted-text role="c:data"} value and whose second item is the corresponding error message (gotten from !strerror{.interpreted-text role="c:func"}), and then calls PyErr_SetObject(type, object). On Unix, when the errno{.interpreted-text role="c:data"} value is !EINTR{.interpreted-text role="c:macro"}, indicating an interrupted system call, this calls PyErr_CheckSignals{.interpreted-text role="c:func"}, 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.

Similar to PyErr_SetFromErrno{.interpreted-text role="c:func"}, 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 OSError{.interpreted-text role="exc"} exception, this is used to define the !filename{.interpreted-text role="attr"} attribute of the exception instance.

Similar to PyErr_SetFromErrnoWithFilenameObject{.interpreted-text role="c:func"}, but takes a second filename object, for raising errors when a function that takes two filenames fails.

::: versionadded 3.4 :::

Similar to PyErr_SetFromErrnoWithFilenameObject{.interpreted-text role="c:func"}, but the filename is given as a C string. filename is decoded from the filesystem encoding and error handler{.interpreted-text role="term"}.

This is a convenience function to raise OSError{.interpreted-text role="exc"}. If called with ierr of 0, the error code returned by a call to !GetLastError{.interpreted-text role="c:func"} is used instead. It calls the Win32 function !FormatMessage{.interpreted-text role="c:func"} to retrieve the Windows description of error code given by ierr or !GetLastError{.interpreted-text role="c:func"}, then it constructs a OSError{.interpreted-text role="exc"} object with the ~OSError.winerror{.interpreted-text role="attr"} attribute set to the error code, the ~OSError.strerror{.interpreted-text role="attr"} attribute set to the corresponding error message (gotten from !FormatMessage{.interpreted-text role="c:func"}), and then calls PyErr_SetObject(PyExc_OSError, object). This function always returns NULL.

::: availability Windows. :::

Similar to PyErr_SetFromWindowsErr{.interpreted-text role="c:func"}, with an additional parameter specifying the exception type to be raised.

::: availability Windows. :::

Similar to PyErr_SetFromWindowsErr{.interpreted-text role="c:func"}, with the additional behavior that if filename is not NULL, it is decoded from the filesystem encoding (os.fsdecode{.interpreted-text role="func"}) and passed to the constructor of OSError{.interpreted-text role="exc"} as a third parameter to be used to define the !filename{.interpreted-text role="attr"} attribute of the exception instance.

::: availability Windows. :::

Similar to PyErr_SetExcFromWindowsErr{.interpreted-text role="c:func"}, with the additional behavior that if filename is not NULL, it is passed to the constructor of OSError{.interpreted-text role="exc"} as a third parameter to be used to define the !filename{.interpreted-text role="attr"} attribute of the exception instance.

::: availability Windows. :::

Similar to PyErr_SetExcFromWindowsErrWithFilenameObject{.interpreted-text role="c:func"}, but accepts a second filename object.

::: availability Windows. :::

::: versionadded 3.4 :::

Similar to PyErr_SetFromWindowsErrWithFilename{.interpreted-text role="c:func"}, with an additional parameter specifying the exception type to be raised.

::: availability Windows. :::

This is a convenience function to raise ImportError{.interpreted-text role="exc"}. msg will be set as the exception's message string. name and path, both of which can be NULL, will be set as the ImportError{.interpreted-text role="exc"}'s respective name and path attributes.

::: versionadded 3.3 :::

Much like PyErr_SetImportError{.interpreted-text role="c:func"} but this function allows for specifying a subclass of ImportError{.interpreted-text role="exc"} to raise.

::: versionadded 3.6 :::

Set file, line, and offset information for the current exception. If the current exception is not a SyntaxError{.interpreted-text role="exc"}, then it sets additional attributes, which make the exception printing subsystem think the exception is a SyntaxError{.interpreted-text role="exc"}.

::: versionadded 3.4 :::

Similar to PyErr_SyntaxLocationObject{.interpreted-text role="c:func"}, but also sets the end_lineno and end_col_offset information for the current exception.

::: versionadded 3.10 :::

Like PyErr_SyntaxLocationObject{.interpreted-text role="c:func"}, but filename is a byte string decoded from the filesystem encoding and error handler{.interpreted-text role="term"}.

::: versionadded 3.2 :::

Like PyErr_SyntaxLocationEx{.interpreted-text role="c:func"}, but the col_offset parameter is omitted.

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.

Get the source line in filename at line lineno. filename should be a Python str{.interpreted-text role="class"} object.

On success, this function returns a Python string object with the found line. On failure, this function returns NULL without an exception set.

Similar to PyErr_ProgramTextObject{.interpreted-text role="c:func"}, but filename is a const char *{.interpreted-text role="c:expr"}, which is decoded with the filesystem encoding and error handler{.interpreted-text role="term"}, instead of a Python object reference.

Issuing warnings

Use these functions to issue warnings from C code. They mirror similar functions exported by the Python warnings{.interpreted-text role="mod"} module. They normally print 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 they will raise an exception. It is also possible that the functions raise an exception because of a problem with the warning machinery. 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{.interpreted-text role="c:func"} owned references and return an error value).

Issue a warning message. The category argument is a warning category (see below) or NULL; the message argument is a UTF-8 encoded string. stack_level 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 stack_level of 1 is the function calling PyErr_WarnEx{.interpreted-text role="c:func"}, 2 is the function above that, and so forth.

Warning categories must be subclasses of PyExc_Warning{.interpreted-text role="c:data"}; PyExc_Warning{.interpreted-text role="c:data"} is a subclass of PyExc_Exception{.interpreted-text role="c:data"}; the default warning category is PyExc_RuntimeWarning{.interpreted-text role="c:data"}. The standard Python warning categories are available as global variables whose names are enumerated at standardwarningcategories{.interpreted-text role="ref"}.

For information about warning control, see the documentation for the warnings{.interpreted-text role="mod"} module and the -W{.interpreted-text role="option"} option in the command line documentation. There is no C API for warning control.

Issue a warning message with explicit control over all warning attributes. This is a straightforward wrapper around the Python function warnings.warn_explicit{.interpreted-text role="func"}; see there for more information. The module and registry arguments may be set to NULL to get the default effect described there.

::: versionadded 3.4 :::

Similar to PyErr_WarnExplicitObject{.interpreted-text role="c:func"} except that message and module are UTF-8 encoded strings, and filename is decoded from the filesystem encoding and error handler{.interpreted-text role="term"}.

Function similar to PyErr_WarnEx{.interpreted-text role="c:func"}, but use PyUnicode_FromFormat{.interpreted-text role="c:func"} to format the warning message. format is an ASCII-encoded string.

::: versionadded 3.2 :::

Similar to PyErr_WarnExplicit{.interpreted-text role="c:func"}, but uses PyUnicode_FromFormat{.interpreted-text role="c:func"} to format the warning message. format is an ASCII-encoded string.

::: versionadded 3.2 :::

Function similar to PyErr_WarnFormat{.interpreted-text role="c:func"}, but category is ResourceWarning{.interpreted-text role="exc"} and it passes source to !warnings.WarningMessage{.interpreted-text role="class"}.

::: versionadded 3.6 :::

Querying the error indicator

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{.interpreted-text role="c:func"}). If not set, return NULL. You do not own a reference to the return value, so you do not need to Py_DECREF{.interpreted-text role="c:func"} it.

The caller must have an attached thread state{.interpreted-text role="term"}.

:::: note ::: title Note :::

Do not compare the return value to a specific exception; use PyErr_ExceptionMatches{.interpreted-text role="c:func"} 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.) ::::

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.

Return true if the given exception matches the exception type 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 exception types in the tuple (and recursively in subtuples) are searched for a match.

Return the exception currently being raised, clearing the error indicator at the same time. Return NULL if the error indicator is not set.

This function is used by code that needs to catch exceptions, or code that needs to save and restore the error indicator temporarily.

For example:

{
   PyObject *exc = PyErr_GetRaisedException();

   /* ... code that might produce other errors ... */

   PyErr_SetRaisedException(exc);
}

::: seealso PyErr_GetHandledException{.interpreted-text role="c:func"}, to save the exception currently being handled. :::

::: versionadded 3.12 :::

Set exc as the exception currently being raised, clearing the existing exception if one is set.

:::: warning ::: title Warning :::

This call steals a reference to exc, which must be a valid exception. ::::

::: versionadded 3.12 :::

::: deprecated 3.12

Use PyErr_GetRaisedException{.interpreted-text role="c:func"} instead. :::

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 ::: title Note :::

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

For example:

{
   PyObject *type, *value, *traceback;
   PyErr_Fetch(&type, &value, &traceback);

   /* ... code that might produce other errors ... */

   PyErr_Restore(type, value, traceback);
}

::::

::: deprecated 3.12

Use PyErr_SetRaisedException{.interpreted-text role="c:func"} instead. :::

Set the error indicator from the three objects, type, value, and traceback, clearing the existing exception if one is set. 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 ::: title Note :::

This function is normally only used by legacy code that needs to save and restore the error indicator temporarily. Use PyErr_Fetch{.interpreted-text role="c:func"} to save the current error indicator. ::::

::: deprecated 3.12

Use PyErr_GetRaisedException{.interpreted-text role="c:func"} instead, to avoid any possible de-normalization. :::

Under certain circumstances, the values returned by PyErr_Fetch{.interpreted-text role="c:func"} 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.

:::: note ::: title Note :::

This function does not implicitly set the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute on the exception value. If setting the traceback appropriately is desired, the following additional snippet is needed:

if (tb != NULL) {
  PyException_SetTraceback(val, tb);
}

::::

Retrieve the active exception instance, as would be returned by sys.exception{.interpreted-text role="func"}. This refers to an exception that was already caught, not to an exception that was freshly raised. Returns a new reference to the exception or NULL. Does not modify the interpreter's exception state.

:::: note ::: title Note :::

This function is not normally used by code that wants to handle exceptions. Rather, it can be used when code needs to save and restore the exception state temporarily. Use PyErr_SetHandledException{.interpreted-text role="c:func"} to restore or clear the exception state. ::::

::: versionadded 3.11 :::

Set the active exception, as known from sys.exception(). This refers to an exception that was already caught, not to an exception that was freshly raised. To clear the exception state, pass NULL.

:::: note ::: title Note :::

This function is not normally used by code that wants to handle exceptions. Rather, it can be used when code needs to save and restore the exception state temporarily. Use PyErr_GetHandledException{.interpreted-text role="c:func"} to get the exception state. ::::

::: versionadded 3.11 :::

Retrieve the old-style representation of the exception info, as known from sys.exc_info{.interpreted-text role="func"}. This refers to an exception that was already caught, not to an exception that was freshly raised. Returns new references for the three objects, any of which may be NULL. Does not modify the exception info state. This function is kept for backwards compatibility. Prefer using PyErr_GetHandledException{.interpreted-text role="c:func"}.

:::: note ::: title Note :::

This function is not normally used by code that wants to handle exceptions. Rather, it can be used when code needs to save and restore the exception state temporarily. Use PyErr_SetExcInfo{.interpreted-text role="c:func"} to restore or clear the exception state. ::::

::: versionadded 3.3 :::

Set the exception info, as known from sys.exc_info(). This refers to an exception that was already caught, not to an exception that was freshly raised. This function steals the references of the arguments. To clear the exception state, pass NULL for all three arguments. This function is kept for backwards compatibility. Prefer using PyErr_SetHandledException{.interpreted-text role="c:func"}.

:::: note ::: title Note :::

This function is not normally used by code that wants to handle exceptions. Rather, it can be used when code needs to save and restore the exception state temporarily. Use PyErr_GetExcInfo{.interpreted-text role="c:func"} to read the exception state. ::::

::: versionadded 3.3 :::

::: versionchanged 3.11 The type and traceback arguments are no longer used and can be NULL. The interpreter now derives them from the exception instance (the value argument). The function still steals references of all three arguments. :::

Signal Handling

::: index pair: module; signal single: SIGINT (C macro) single: KeyboardInterrupt (built-in exception) :::

Handle external interruptions, such as signals or activating a debugger, whose processing has been delayed until it is safe to run Python code and/or raise exceptions.

For example, pressing Ctrl-C{.interpreted-text role="kbd"} causes a terminal to send the signal.SIGINT{.interpreted-text role="py:data"} signal. This function executes the corresponding Python signal handler, which, by default, raises the KeyboardInterrupt{.interpreted-text role="exc"} exception.

!PyErr_CheckSignals{.interpreted-text role="c:func"} should be called by long-running C code frequently enough so that the response appears immediate to humans.

Handlers invoked by this function currently include:

• Signal handlers, including Python functions registered using the signal{.interpreted-text role="mod"} module.

  Signal handlers are only run in the main thread of the main interpreter.

  (This is where the function got the name: originally, signals were the only way to interrupt the interpreter.)

• Running the garbage collector, if necessary.

• Executing a pending remote debugger <remote-debugging>{.interpreted-text role="ref"} script.

If any handler raises an exception, immediately return -1 with that exception set. Any remaining interruptions are left to be processed on the next PyErr_CheckSignals(){.interpreted-text role="c:func"} invocation, if appropriate.

If all handlers finish successfully, or there are no handlers to run, return 0.

::: versionchanged 3.12 This function may now invoke the garbage collector. :::

::: versionchanged 3.14 This function may now execute a remote debugger script, if remote debugging is enabled. :::

::: index pair: module; signal single: SIGINT (C macro) single: KeyboardInterrupt (built-in exception) :::

Simulate the effect of a !SIGINT{.interpreted-text role="c:macro"} signal arriving. This is equivalent to PyErr_SetInterruptEx(SIGINT).

:::: note ::: title Note :::

This function is async-signal-safe. It can be called without an attached thread state{.interpreted-text role="term"} and from a C signal handler. ::::

::: index pair: module; signal single: KeyboardInterrupt (built-in exception) :::

Simulate the effect of a signal arriving. The next time PyErr_CheckSignals{.interpreted-text role="c:func"} is called, the Python signal handler for the given signal number will be called.

This function can be called by C code that sets up its own signal handling and wants Python signal handlers to be invoked as expected when an interruption is requested (for example when the user presses Ctrl-C to interrupt an operation).

If the given signal isn't handled by Python (it was set to signal.SIG_DFL{.interpreted-text role="py:const"} or signal.SIG_IGN{.interpreted-text role="py:const"}), it will be ignored.

If signum is outside of the allowed range of signal numbers, -1 is returned. Otherwise, 0 is returned. The error indicator is never changed by this function.

:::: note ::: title Note :::

This function is async-signal-safe. It can be called without an attached thread state{.interpreted-text role="term"} and from a C signal handler. ::::

::: versionadded 3.10 :::

This utility function specifies a file descriptor to which the signal number is written as a single byte whenever a signal is received. fd must be non-blocking. 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{.interpreted-text role="func"} in Python, but without any error checking. fd should be a valid file descriptor. The function should only be called from the main thread.

::: versionchanged 3.5 On Windows, the function now also supports socket handles. :::

Exception Classes

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{.interpreted-text role="exc"} (accessible in C as PyExc_Exception{.interpreted-text role="c:data"}).

The ~type.__module__{.interpreted-text role="attr"} 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.

Same as PyErr_NewException{.interpreted-text role="c:func"}, 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.

::: versionadded 3.2 :::

Return non-zero if ob is an exception class, zero otherwise. This function always succeeds.

Return ~PyTypeObject.tp_name{.interpreted-text role="c:member"} of the exception class ob.

This is a soft deprecated{.interpreted-text role="term"} macro including the base fields for an exception object.

This was included in Python's C API by mistake and is not designed for use in extensions. For creating custom exception objects, use PyErr_NewException{.interpreted-text role="c:func"} or otherwise create a class inheriting from PyExc_BaseException{.interpreted-text role="c:data"}.

Exception Objects

Return true if op is an instance of BaseException{.interpreted-text role="class"}, false otherwise. This function always succeeds.

Equivalent to Py_TYPE(op) <Py_TYPE>{.interpreted-text role="c:func"}.

Return the traceback associated with the exception as a new reference, as accessible from Python through the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute. If there is no traceback associated, this returns NULL.

Set the traceback associated with the exception to tb. Use Py_None to clear it.

Return the context (another exception instance during whose handling ex was raised) associated with the exception as a new reference, as accessible from Python through the ~BaseException.__context__{.interpreted-text role="attr"} attribute. If there is no context associated, this returns NULL.

Set the context associated with the exception to ctx. Use NULL to clear it. There is no type check to make sure that ctx is an exception instance. This steals a reference to ctx.

Return the cause (either an exception instance, or None, set by raise ... from ...) associated with the exception as a new reference, as accessible from Python through the ~BaseException.__cause__{.interpreted-text role="attr"} attribute.

Set the cause associated with the exception to cause. Use NULL to clear it. There is no type check to make sure that cause is either an exception instance or None. This steals a reference to cause.

The ~BaseException.__suppress_context__{.interpreted-text role="attr"} attribute is implicitly set to True by this function.

Return ~BaseException.args{.interpreted-text role="attr"} of exception ex.

Set ~BaseException.args{.interpreted-text role="attr"} of exception ex to args.

Implement part of the interpreter's implementation of !except*{.interpreted-text role="keyword"}. orig is the original exception that was caught, and excs is the list of the exceptions that need to be raised. This list contains the unhandled part of orig, if any, as well as the exceptions that were raised from the !except*{.interpreted-text role="keyword"} clauses (so they have a different traceback from orig) and those that were reraised (and have the same traceback as orig). Return the ExceptionGroup{.interpreted-text role="exc"} that needs to be reraised in the end, or None if there is nothing to reraise.

::: versionadded 3.12 :::

Unicode Exception Objects {#unicodeexceptions}

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

Create a UnicodeDecodeError{.interpreted-text role="class"} object with the attributes encoding, object, length, start, end and reason. encoding and reason are UTF-8 encoded strings.

Recursion Control {#recursion}

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). They are also not needed for tp_call implementations because the call protocol <call>{.interpreted-text role="ref"} takes care of recursion handling.

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

The function then checks if the stack limit is reached. If this is the case, a RecursionError{.interpreted-text role="exc"} is set and a nonzero value is returned. Otherwise, zero is returned.

where should be a UTF-8 encoded string such as " in instance check" to be concatenated to the RecursionError{.interpreted-text role="exc"} message caused by the recursion depth limit.

::: seealso The PyUnstable_ThreadState_SetStackProtection{.interpreted-text role="c:func"} function. :::

::: versionchanged 3.9 This function is now also available in the limited API <limited-c-api>{.interpreted-text role="ref"}. :::

Ends a Py_EnterRecursiveCall{.interpreted-text role="c:func"}. Must be called once for each successful invocation of Py_EnterRecursiveCall{.interpreted-text role="c:func"}.

::: versionchanged 3.9 This function is now also available in the limited API <limited-c-api>{.interpreted-text role="ref"}. :::

Properly implementing ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} for container types requires special recursion handling. In addition to protecting the stack, ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} also needs to track objects to prevent cycles. The following two functions facilitate this functionality. Effectively, these are the C equivalent to reprlib.recursive_repr{.interpreted-text role="func"}.

Called at the beginning of the ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} implementation to detect cycles.

If the object has already been processed, the function returns a positive integer. In that case the ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} implementation should return a string object indicating a cycle. As examples, dict{.interpreted-text role="class"} objects return {...} and list{.interpreted-text role="class"} objects return [...].

The function will return a negative integer if the recursion limit is reached. In that case the ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} implementation should typically return NULL.

Otherwise, the function returns zero and the ~PyTypeObject.tp_repr{.interpreted-text role="c:member"} implementation can continue normally.

Ends a Py_ReprEnter{.interpreted-text role="c:func"}. Must be called once for each invocation of Py_ReprEnter{.interpreted-text role="c:func"} that returns zero.

Get the recursion limit for the current interpreter. It can be set with Py_SetRecursionLimit{.interpreted-text role="c:func"}. The recursion limit prevents the Python interpreter stack from growing infinitely.

This function cannot fail, and the caller must hold an attached thread state{.interpreted-text role="term"}.

::: seealso sys.getrecursionlimit{.interpreted-text role="py:func"} :::

Set the recursion limit for the current interpreter.

This function cannot fail, and the caller must hold an attached thread state{.interpreted-text role="term"}.

::: seealso sys.setrecursionlimit{.interpreted-text role="py:func"} :::

Exception and warning types {#standardexceptions}

All standard Python exceptions and warning categories are available as global variables whose names are PyExc_ followed by the Python exception name. These have the type PyObject*{.interpreted-text role="c:expr"}; they are all class objects.

For completeness, here are all the variables:

Exception types

C name Python name

---

       `BaseException`{.interpreted-text role="exc"}
       `BaseExceptionGroup`{.interpreted-text role="exc"}
       `Exception`{.interpreted-text role="exc"}
       `ArithmeticError`{.interpreted-text role="exc"}
       `AssertionError`{.interpreted-text role="exc"}
       `AttributeError`{.interpreted-text role="exc"}
       `BlockingIOError`{.interpreted-text role="exc"}
       `BrokenPipeError`{.interpreted-text role="exc"}
       `BufferError`{.interpreted-text role="exc"}
       `ChildProcessError`{.interpreted-text role="exc"}
       `ConnectionAbortedError`{.interpreted-text role="exc"}
       `ConnectionError`{.interpreted-text role="exc"}
       `ConnectionRefusedError`{.interpreted-text role="exc"}
       `ConnectionResetError`{.interpreted-text role="exc"}
       `EOFError`{.interpreted-text role="exc"}
       `FileExistsError`{.interpreted-text role="exc"}
       `FileNotFoundError`{.interpreted-text role="exc"}
       `FloatingPointError`{.interpreted-text role="exc"}
       `GeneratorExit`{.interpreted-text role="exc"}
       `ImportCycleError`{.interpreted-text role="exc"}
       `ImportError`{.interpreted-text role="exc"}
       `IndentationError`{.interpreted-text role="exc"}
       `IndexError`{.interpreted-text role="exc"}
       `InterruptedError`{.interpreted-text role="exc"}
       `IsADirectoryError`{.interpreted-text role="exc"}
       `KeyError`{.interpreted-text role="exc"}
       `KeyboardInterrupt`{.interpreted-text role="exc"}
       `LookupError`{.interpreted-text role="exc"}
       `MemoryError`{.interpreted-text role="exc"}
       `ModuleNotFoundError`{.interpreted-text role="exc"}
       `NameError`{.interpreted-text role="exc"}
       `NotADirectoryError`{.interpreted-text role="exc"}
       `NotImplementedError`{.interpreted-text role="exc"}
       `OSError`{.interpreted-text role="exc"}
       `OverflowError`{.interpreted-text role="exc"}
       `PermissionError`{.interpreted-text role="exc"}
       `ProcessLookupError`{.interpreted-text role="exc"}
       `PythonFinalizationError`{.interpreted-text role="exc"}
       `RecursionError`{.interpreted-text role="exc"}
       `ReferenceError`{.interpreted-text role="exc"}
       `RuntimeError`{.interpreted-text role="exc"}
       `StopAsyncIteration`{.interpreted-text role="exc"}
       `StopIteration`{.interpreted-text role="exc"}
       `SyntaxError`{.interpreted-text role="exc"}
       `SystemError`{.interpreted-text role="exc"}
       `SystemExit`{.interpreted-text role="exc"}
       `TabError`{.interpreted-text role="exc"}
       `TimeoutError`{.interpreted-text role="exc"}
       `TypeError`{.interpreted-text role="exc"}
       `UnboundLocalError`{.interpreted-text role="exc"}
       `UnicodeDecodeError`{.interpreted-text role="exc"}
       `UnicodeEncodeError`{.interpreted-text role="exc"}
       `UnicodeError`{.interpreted-text role="exc"}
       `UnicodeTranslateError`{.interpreted-text role="exc"}
       `ValueError`{.interpreted-text role="exc"}
       `ZeroDivisionError`{.interpreted-text role="exc"}

::: versionadded 3.3 PyExc_BlockingIOError{.interpreted-text role="c:data"}, PyExc_BrokenPipeError{.interpreted-text role="c:data"}, PyExc_ChildProcessError{.interpreted-text role="c:data"}, PyExc_ConnectionError{.interpreted-text role="c:data"}, PyExc_ConnectionAbortedError{.interpreted-text role="c:data"}, PyExc_ConnectionRefusedError{.interpreted-text role="c:data"}, PyExc_ConnectionResetError{.interpreted-text role="c:data"}, PyExc_FileExistsError{.interpreted-text role="c:data"}, PyExc_FileNotFoundError{.interpreted-text role="c:data"}, PyExc_InterruptedError{.interpreted-text role="c:data"}, PyExc_IsADirectoryError{.interpreted-text role="c:data"}, PyExc_NotADirectoryError{.interpreted-text role="c:data"}, PyExc_PermissionError{.interpreted-text role="c:data"}, PyExc_ProcessLookupError{.interpreted-text role="c:data"} and PyExc_TimeoutError{.interpreted-text role="c:data"} were introduced following 3151{.interpreted-text role="pep"}. :::

::: versionadded 3.5 PyExc_StopAsyncIteration{.interpreted-text role="c:data"} and PyExc_RecursionError{.interpreted-text role="c:data"}. :::

::: versionadded 3.6 PyExc_ModuleNotFoundError{.interpreted-text role="c:data"}. :::

::: versionadded 3.11 PyExc_BaseExceptionGroup{.interpreted-text role="c:data"}. :::

OSError aliases

The following are a compatibility aliases to PyExc_OSError{.interpreted-text role="c:data"}.

::: versionchanged 3.3 These aliases used to be separate exception types. :::

C name Python name Notes

---

       `OSError`{.interpreted-text role="exc"}   
       `OSError`{.interpreted-text role="exc"}   
       `OSError`{.interpreted-text role="exc"}   [\[win\]](#win){.citation}

Notes:

Warning types {#standardwarningcategories}

C name Python name

---

       `Warning`{.interpreted-text role="exc"}
       `BytesWarning`{.interpreted-text role="exc"}
       `DeprecationWarning`{.interpreted-text role="exc"}
       `EncodingWarning`{.interpreted-text role="exc"}
       `FutureWarning`{.interpreted-text role="exc"}
       `ImportWarning`{.interpreted-text role="exc"}
       `PendingDeprecationWarning`{.interpreted-text role="exc"}
       `ResourceWarning`{.interpreted-text role="exc"}
       `RuntimeWarning`{.interpreted-text role="exc"}
       `SyntaxWarning`{.interpreted-text role="exc"}
       `UnicodeWarning`{.interpreted-text role="exc"}
       `UserWarning`{.interpreted-text role="exc"}

::: versionadded 3.2 PyExc_ResourceWarning{.interpreted-text role="c:data"}. :::

::: versionadded 3.10 PyExc_EncodingWarning{.interpreted-text role="c:data"}. :::

Tracebacks

Type object for traceback objects. This is available as types.TracebackType{.interpreted-text role="class"} in the Python layer.

Return true if op is a traceback object, false otherwise. This function does not account for subtypes.

Replace the ~BaseException.__traceback__{.interpreted-text role="attr"} attribute on the current exception with a new traceback prepending f to the existing chain.

Calling this function without an exception set is undefined behavior.

This function returns 0 on success, and returns -1 with an exception set on failure.

Write the traceback tb into the file f.

This function returns 0 on success, and returns -1 with an exception set on failure.

::: {#citations}

[win]{#win .citation-label}

: !PyExc_WindowsError{.interpreted-text role="c:var"} is only defined on Windows; protect code that uses this by testing that the preprocessor macro MS_WINDOWS is defined. :::