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print_exc_plus : 8.6. Getting More Information from Tracebacks

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  1. import sys, traceback
  2. def print_exc_plus( ):
  3.     """ Print the usual traceback information, followed by a listing of
  4.         all the local variables in each frame.
  5.     """
  6.     tb = sys.exc_info( )[2]
  7.     while tb.tb_next:
  8.         tb = tb.tb_next
  9.     stack = [  ]
  10.     f = tb.tb_frame
  11.     while f:
  12.         stack.append(f)
  13.         f = f.f_back
  14.     stack.reverse( )
  15.     traceback.print_exc( )
  16.     print "Locals by frame, innermost last"
  17.     for frame in stack:
  18.         print
  19.         print "Frame %s in %s at line %s" % (frame.f_code.co_name,
  20.                                              frame.f_code.co_filename,
  21.                                              frame.f_lineno)
  22.         for key, value in frame.f_locals.items( ):
  23.             print "\t%20s = " % key,
  24.             # we must _absolutely_ avoid propagating exceptions, and str(value)
  25.             # COULD cause any exception, so we MUST catch any...:
  26.             try:
  27.                 print value
  28.             except:
  29.                 print "<ERROR WHILE PRINTING VALUE>"
  30.  
  31. data = ["1", "2", 3, "4"]     # Typo: we 'forget' the quotes on data[2]
  32. def pad4(seq):
  33.     """
  34.     Pad each string in seq with zeros up to four places. Note that there
  35.     is no reason to actually write this function; Python already
  36.     does this sort of thing much better.  It's just an example!
  37.     """
  38.     return_value = [  ]
  39.     for thing in seq:
  40.         return_value.append("0" * (4 - len(thing)) + thing)
  41.     return return_value
  42.  
  43.  
  44. Here's the (limited) information we get from a normal traceback.print_exc:
  45.  
  46. >>> try:
  47. ...     pad4(data)
  48. ... except:
  49. ...     traceback.print_exc( )
  50. ...
  51. Traceback (most recent call last):
  52.   File "<stdin>", line 2, in ?
  53.   File "<stdin>", line 9, in pad4
  54. TypeError: len( ) of unsized object
  55.  
  56. >>> try:
  57. ...     pad4(data)
  58. ... except:
  59. ...     print_exc_plus( )
  60. ...
  61. Traceback (most recent call last):
  62.   File "<stdin>", line 2, in ?
  63.   File "<stdin>", line 9, in pad4
  64. TypeError: len( ) of unsized object
  65. Locals by frame, innermost last
  66. Frame ? in <stdin> at line 4
  67.                          sys =  <module>
  68.                         pad4 =  <function>
  69.                 _ _builtins_ _ =  <module>
  70.                     _ _name_ _ =  _ _main_ _
  71.                         data =  ['1', '2', 3, '4']
  72.                      _ _doc_ _ =  None
  73.               print_exc_plus =  <function>
  74. Frame pad4 in <stdin> at line 9
  75.                        thing =  3
  76.                 return_value =  ['0001', '0002']
  77.                          seq =  ['1', '2', 3, '4']

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