3.6 Standard Module traceback

This module provides a standard interface to extract, format and print stack traces of Python programs. It exactly mimics the behavior of the Python interpreter when it prints a stack trace. This is useful when you want to print stack traces under program control, e.g. in a ``wrapper'' around the interpreter.

The module uses traceback objects -- this is the object type that is stored in the variables sys.exc_traceback and sys.last_traceback and returned as the third item from sys.exc_info().

The module defines the following functions:

print_tb (traceback[, limit[, file]])

Print up to limit stack trace entries from traceback. 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.

extract_tb (traceback[, limit])

Return a list of up to limit ``pre-processed'' stack trace entries extracted from traceback. It is useful for alternate formatting of stack traces. If limit is omitted or None, all entries are extracted. A ``pre-processed'' stack trace entry is a quadruple (filename, line number, function name, line text) representing the information that is usually printed for a stack trace. The line text is a string with leading and trailing whitespace stripped; if the source is not available it is None.

print_exception (type, value, traceback[, limit[, file]])

Print exception information and up to limit stack trace entries from traceback to file. This differs from print_tb() in the following ways: (1) if traceback is not None, it prints a header "Traceback (innermost last):"; (2) it prints the exception type and value after the stack trace; (3) if type 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.

print_exc ([limit[, file]])

This is a shorthand for `print_exception(sys.exc_type, sys.exc_value, sys.exc_traceback, limit, file)'. (In fact, it uses sys.exc_info() to retrieve the same information in a thread-safe way.)

print_last ([limit[, file]])

This is a shorthand for `print_exception(sys.last_type, sys.last_value, sys.last_traceback, limit, file)'.

print_stack ([f[, limit[, file]]])

This function prints a stack trace from its invocation point. The optional f argument can be used to specify an alternate stack frame to start. The optional limit and file arguments have the same meaning as for print_exception().

extract_tb (tb[, limit])

Return a list containing the raw (unformatted) traceback information extracted from the traceback object tb. The optional limit argument has the same meaning as for print_exception(). The items in the returned list are 4-tuples containing the following values: filename, line number, function name, and source text line. The source text line is stripped of leading and trailing whitespace; it is None when the source text file is unavailable.

extract_stack ([f[, limit]])

Extract the raw traceback from the current stack frame. The return value has the same format as for extract_tb(). The optional f and limit arguments have the same meaning as for print_stack().

format_list (list)

Given a list of tuples as returned by extract_tb() or extract_stack(), return 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, for those items whose source text line is not None.

format_exception_only (type, value)

Format 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. The return value is a list of strings, each ending in a newline. Normally, the list contains a single string; however, for SyntaxError exceptions, it contains several lines that (when printed) display detailed information about where the syntax error occurred. The message indicating which exception occurred is the always last string in the list.

format_exception (type, value, tb[, limit])

Format a stack trace and the exception information. The arguments have the same meaning as the corresponding arguments to print_exception(). The return value is a list of strings, each ending in a newline and some containing internal newlines. When these lines are contatenated and printed, exactly the same text is printed as does print_exception().

format_tb (tb[, limit])

A shorthand for format_list(extract_tb(tb, limit)).

format_stack ([f[, limit]])

A shorthand for format_list(extract_stack(f, limit)).

tb_lineno (tb)

This function returns the current line number set in the traceback object. This is normally the same as the tb.tb_lineno field of the object, but when optimization is used (the -O flag) this field is not updated correctly; this function calculates the correct value.

A simple example follows:

import sys, traceback

def run_user_code(envdir):
    source = raw_input(">>> ")
    try:
        exec source in envdir
    except:
        print "Exception in user code:"
        print '-'*60
        traceback.print_exc(file=sys.stdout)
        print '-'*60

envdir = {}
while 1:
    run_user_code(envdir)