Why does dis.dis(None) return output?

python

Solution

From the documentation:

dis.dis([bytesource])

Disassemble the bytesource object. bytesource can denote either a module, a class, a method, a function, or a code object. For a module, it disassembles all functions. For a class, it disassembles all methods. For a single code sequence, it prints one line per bytecode instruction. If no object is provided, it disassembles the last traceback.

Emphasis mine.

If you try it in a new interpreter there is no last traceback, so you get an error:

>>> import dis
>>> dis.dis(None)

Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    dis.dis(None)
  File "C:\Python27\lib\dis.py", line 23, in dis
    distb()
  File "C:\Python27\lib\dis.py", line 57, in distb
    raise RuntimeError, "no last traceback to disassemble"
RuntimeError: no last traceback to disassemble

But now if you try the same command it disassembles the `RuntimeError` you just got:

>>> dis.dis(None)
 53           0 LOAD_FAST                0 (tb)
              3 LOAD_CONST               2 (None)
              6 COMPARE_OP               8 (is)
              9 POP_JUMP_IF_FALSE       82

 54          12 SETUP_EXCEPT            13 (to 28)

 55          15 LOAD_GLOBAL              1 (sys)
             18 LOAD_ATTR                2 (last_traceback)
             21 STORE_FAST               0 (tb)
             24 POP_BLOCK           
             25 JUMP_FORWARD            26 (to 54)

 56     >>   28 DUP_TOP             
             29 LOAD_GLOBAL              3 (AttributeError)
             32 COMPARE_OP              10 (exception match)
             35 POP_JUMP_IF_FALSE       53
             38 POP_TOP             
             39 POP_TOP             
             40 POP_TOP             

 57          41 LOAD_GLOBAL              4 (RuntimeError)
             44 LOAD_CONST               1 ('no last traceback to disassemble')
    -->      47 RAISE_VARARGS            2
             50 JUMP_FORWARD             1 (to 54)

Problem

In particular, it outputs: ``` >>> dis.dis(None) 22 0 LOAD_FAST 0 (x) 3 LOAD_CONST 1 (None) 6 COMPARE_OP 8 (is) 9 POP_JUMP_IF_FALSE 23 23 12 LOAD_GLOBAL 1 (distb) 15 CALL_FUNCTION 0 18 POP_TOP 24 19 LOAD_CONST 1 (None) 22 RETURN_VALUE 25 >> 23 LOAD_GLOBAL 2 (isinstance) 26 LOAD_FAST 0 (x) 29 LOAD_GLOBAL 3 (types) 32 LOAD_ATTR 4 (InstanceType) 35 CALL_FUNCTION 2 38 POP_JUMP_IF_FALSE 53 26 41 LOAD_FAST 0 (x) 44 LOAD_ATTR 5 (__class__) 47 STORE_FAST 0 (x) 50 JUMP_FORWARD 0 (to 53) 27 >> 53 LOAD_GLOBAL 6 (hasattr) 56 LOAD_FAST 0 (x) 59 LOAD_CONST 2 ('im_func') 62 CALL_FUNCTION 2 65 POP_JUMP_IF_FALSE 80 28 68 LOAD_FAST 0 (x) 71 LOAD_ATTR 7 (im_func) 74 STORE_FAST 0 (x) 77 JUMP_FORWARD 0 (to 80) 29 >> 80 LOAD_GLOBAL 6 (hasattr) 83 LOAD_FAST 0 (x) 86 LOAD_CONST 3 ('func_code') 89 CALL_FUNCTION 2 92 POP_JUMP_IF_FALSE 107 30 95 LOAD_FAST 0 (x) 98 LOAD_ATTR 8 (func_code) 101 STORE_FAST 0 (x) 104 JUMP_FORWARD 0 (to 107) 31 >> 107 LOAD_GLOBAL 6 (hasattr) 110 LOAD_FAST 0 (x) 113 LOAD_CONST 4 ('__dict__') 116 CALL_FUNCTION 2 119 POP_JUMP_IF_FALSE 246 32 122 LOAD_FAST 0 (x) 125 LOAD_ATTR 9 (__dict__) 128 LOAD_ATTR 10 (items) 131 CALL_FUNCTION 0 134 STORE_FAST 1 (items) 33 137 LOAD_FAST 1 (items) 140 LOAD_ATTR 11 (sort) 143 CALL_FUNCTION 0 146 POP_TOP 34 147 SETUP_LOOP 174 (to 324) 150 LOAD_FAST 1 (items) 153 GET_ITER >> 154 FOR_ITER 85 (to 242) 157 UNPACK_SEQUENCE 2 160 STORE_FAST 2 (name) 163 STORE_FAST 3 (x1) 35 166 LOAD_GLOBAL 2 (isinstance) 169 LOAD_FAST 3 (x1) 172 LOAD_GLOBAL 12 (_have_code) 175 CALL_FUNCTION 2 178 POP_JUMP_IF_FALSE 154 36 181 LOAD_CONST 5 ('Disassembly of %s:') 184 LOAD_FAST 2 (name) 187 BINARY_MODULO 188 PRINT_ITEM 189 PRINT_NEWLINE 37 190 SETUP_EXCEPT 14 (to 207) 38 193 LOAD_GLOBAL 13 (dis) 196 LOAD_FAST 3 (x1) 199 CALL_FUNCTION 1 202 POP_TOP 203 POP_BLOCK 204 JUMP_FORWARD 28 (to 235) 39 >> 207 DUP_TOP 208 LOAD_GLOBAL 14 (TypeError) 211 COMPARE_OP 10 (exception match) 214 POP_JUMP_IF_FALSE 234 217 POP_TOP 218 STORE_FAST 4 (msg) 221 POP_TOP 40 222 LOAD_CONST 6 ('Sorry:') 225 PRINT_ITEM 226 LOAD_FAST 4 (msg) 229 PRINT_ITEM 230 PRINT_NEWLINE 231 JUMP_FORWARD 1 (to 235) >> 234 END_FINALLY 41 >> 235 PRINT_NEWLINE 236 JUMP_ABSOLUTE 154 239 JUMP_ABSOLUTE 154 >> 242 POP_BLOCK 243 JUMP_FORWARD 78 (to 324) 42 >> 246 LOAD_GLOBAL 6 (hasattr) 249 LOAD_FAST 0 (x) 252 LOAD_CONST 7 ('co_code') 255 CALL_FUNCTION 2 258 POP_JUMP_IF_FALSE 274 43 261 LOAD_GLOBAL 15 (disassemble) 264 LOAD_FAST 0 (x) 267 CALL_FUNCTION 1 270 POP_TOP 271 JUMP_FORWARD 50 (to 324) 44 >> 274 LOAD_GLOBAL 2 (isinstance) 277 LOAD_FAST 0 (x) 280 LOAD_GLOBAL 16 (str) 283 CALL_FUNCTION 2 286 POP_JUMP_IF_FALSE 302 45 289 LOAD_GLOBAL 17 (disassemble_string) 292 LOAD_FAST 0 (x) 295 CALL_FUNCTION 1 298 POP_TOP 299 JUMP_FORWARD 22 (to 324) 47 >> 302 LOAD_GLOBAL 14 (TypeError) 48 305 LOAD_CONST 8 ("don't know how to disassemble %s objects") 49 308 LOAD_GLOBAL 18 (type) 311 LOAD_FAST 0 (x) 314 CALL_FUNCTION 1 317 LOAD_ATTR 19 (__name__) 320 BINARY_MODULO --> 321 RAISE_VARARGS 2 >> 324 LOAD_CONST 1 (None) 327 RETURN_VALUE ``` Why?

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