Traceback in Python
Traceback is a python module that provides a standard interface to extract, format and print stack traces of a python program. When it prints the stack trace it exactly mimics the behaviour of a python interpreter. Useful when you want to print the stack trace at any step. They are usually seen when an exception occurs. Since a traceback gives all the information regarding the exception it becomes easier to track one and fix it.
General structure of a stack trace for an exception:
Traceback for most recent call Location of the program Line in the program where error was encountered Name of the error: relevant information about the exception
Traceback (most recent call last): File "C:/Python27/hdg.py", line 5, in value = A[5] IndexError: list index out of range
The module uses traceback objects, this is the object type that is stored in the sys.last_traceback variable and returned as the third item from sys.exc_info() .
Functions in the Module
- traceback.print_tb(tb, limit = None, file = None) : If limit is positive it prints upto limit stack trace entries from traceback object tb. Otherwise, print the last abs(limit) entries. If limit is omitted or None, all entries are printed. If file is omitted or None, the output goes to sys.stderr; otherwise it should be an open file or file-like object to receive the output.
- traceback.print_exception(etype, value, tb, limit = None, file = None, chain = True) : Prints exception information and stack trace entries from traceback object tb to file. If tb is not None, it prints a header Traceback (most recent call last): . It prints the exception etype and value after the stack trace. If type(value) is SyntaxError and value has the appropriate format, it prints the line where the syntax error occurred with a caret indicating the approximate position of the error.
- traceback.print_exc(limit = None, file = None, chain = True) : This is a shorthand for print_exception(*sys.exc_info(), limit, file, chain).
- traceback.print_last(limit = None, file = None, chain = True) : It works only after an exception has reached an interactive prompt. This is a shorthand for print_exception(sys.last_type, sys.last_value, sys.last_traceback, limit, file, chain).
- traceback.print_stack(f = None, limit = None, file = None) : If limit is positive, prints up to limit stack trace. Otherwise, print the last abs(limit) entries. If limit is omitted or None, all entries are printed. The optional f argument can be used to specify an alternate stack frame to start.
- traceback.extract_tb(tb, limit = None) : Returns a StackSummary object representing a list of “pre-processed” stack trace entries (FrameSummary object containing attributes filename, lineno, name, and line) extracted from the traceback object tb. It is useful for alternate formatting of stack traces.
- traceback.extract_stack(f = None, limit = None) : Extracts the raw traceback from the current stack frame. The return value has the same format as for extract_tb().
- traceback.format_list(extracted_list) : Given a list of tuples or FrameSummary objects returns a list of strings ready for printing. Each string in the resulting list corresponds to the item with the same index in the argument list. Each string ends in a newline; the strings may contain internal newlines as well.
- traceback.format_exception_only(etype, value) : Formats the exception part of a traceback. The arguments are the exception type and value such as given by sys.last_type and sys.last_value. It returns a list of strings each ending in a new line.
- traceback.format_exception(etype, value, tb, limit = None, chain = True) : Formats stack trace and exception information. The arguments have the same meaning as the corresponding arguments to print_exception(). It returns a list of strings each ending in new line and some have internal newlines too. When these lines are concatenated and printed they generate an exactly same output as print_exception().
- traceback.format_exc(limit = None, chain = True) : This is like print_exc(limit) except it returns a string instead of printing to a file.
- traceback.format_tb(tb, limit = None) : shorthand for format_list(extract_tb(tb, limit)).
- traceback.format_stack(f = None, limit = None) : shorthand for format_list(extract_stack(f, limit)).
- traceback.clear_frames(tb) : Clears the local variables of all stack frames in a traceback tb by calling clear() method of each frame object.
- traceback.walk_stack(f) : Walk a stack following f.f_back from the given frame, yielding the frame and line number for each frame. If f is None, the current stack is used. This helper is used with StackSummary.extract().
- traceback.walk_tb(tb) : Walk a traceback following tb_next yielding the frame and line number for each frame. This helper is used with StackSummary.extract().
Example : Program that prints the exception stack trace.