Disassembler for Python bytecode

PythonProgrammingServer Side Programming

The dis module in Python standard library provides various functions useful for analysis of Python bytecode by disassembling it into a human-readable form. This helps to perform optimizations. Bytecode is a version-specific implementation detail of the interpreter.

dis() function

The function dis() generates disassembled representation of any Python code source i.e. module, class, method, function, or code object.

>>> def hello():
print ("hello world")
>>> import dis
>>> dis.dis(hello)
2    0 LOAD_GLOBAL 0 (print)
     3 LOAD_CONST 1 ('hello world')
     6 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
     9 POP_TOP
     10 LOAD_CONST 0 (None)
     13 RETURN_VALUE

Bytecode analysis API is defined in Bytecode class. Its constructor returns Bytecode object which is having following methods to analyse the bytecode

Bytecode()

This is the constructor. Analyse the bytecode corresponding to a function, generator, method, string of source code, or a code object. This is a convenience wrapper around many of the functions

>>> string=dis.Bytecode(hello)
>>> for x in string:
      print (x)
Instruction(opname = 'LOAD_GLOBAL', opcode = 116, arg = 0, argval = 'print', argrepr = 'print', offset = 0, starts_line = 2, is_jump_target = False)
Instruction(opname = 'LOAD_CONST', opcode = 100, arg = 1, argval = 'hello world', argrepr = "'hello world'", offset = 3, starts_line = None, is_jump_target = False)
Instruction(opname = 'CALL_FUNCTION', opcode = 131, arg = 1, argval = 1, argrepr = '1 positional, 0 keyword pair', offset = 6, starts_line = None, is_jump_target = False)
Instruction(opname = 'POP_TOP', opcode = 1, arg = None, argval = None, argrepr = '', offset = 9, starts_line = None, is_jump_target = False)
Instruction(opname = 'LOAD_CONST', opcode = 100, arg = 0, argval = None, argrepr = 'None', offset = 10, starts_line = None, is_jump_target = False)
Instruction(opname = 'RETURN_VALUE', opcode = 83, arg = None, argval = None, argrepr = '', offset = 13, starts_line = None, is_jump_target = False)

code_info()

This function returns information of Python code object.

>>> dis.code_info(hello)
"Name: hello\nFilename: <pyshell#2>\nArgument count: 0\nKw-only arguments: 0\nNumber of locals: 0\nStack size: 2\nFlags: OPTIMIZED, NEWLOCALS, NOFREE\nConstants:\n 0: None\n 1: 'hello world'\nNames:\n 0: print"

show_code()

This function prints detailed code information of Python module, function or class.

>>> dis.show_code(hello)
Name: hello
Filename: <pyshell#2>
Argument count: 0
Kw-only arguments: 0
Number of locals: 0
Stack size: 2
Flags: OPTIMIZED, NEWLOCALS, NOFREE
Constants:
   0: None
   1: 'hello world'
Names:
   0: print

disassemble()

This function disassembles a code object and gives output divided in the following columns −

  • the line number, for the first instruction of each line

  • the current instruction, indicated as -->,

  • a labelled instruction, indicated with >>,

  • the address of the instruction,

  • the operation code name,

  • operation parameters, and

  • interpretation of the parameters in parentheses.

>>> codeInString = 'a = 5\nb = 6\nsum = a + b \ nprint("sum = ",sum)'
>>> codeObejct = compile(codeInString, 'sumstring', 'exec')
>>> dis.disassemble(codeObejct)

Output

1    0 LOAD_CONST 0 (5)
     3 STORE_NAME 0 (a)
2    6 LOAD_CONST 1 (6)
     9 STORE_NAME 1 (b)
3    12 LOAD_NAME 0 (a)
     15 LOAD_NAME 1 (b)
     18 BINARY_ADD
     19 STORE_NAME 2 (sum)
4    22 LOAD_NAME 3 (print)
     25 LOAD_CONST 2 ('sum =')
     28 LOAD_NAME 2 (sum)
     31 CALL_FUNCTION 2 (2 positional, 0 keyword pair)
     34 POP_TOP
     35 LOAD_CONST 3 (None)
     38 RETURN_VALUE

get_instructions()

This function returns an iterator over the instructions in the supplied function, method, source code string or code object. The iterator generates a series of Instruction named tuples giving the details of each operation in the supplied code.

>>> it=dis.get_instructions(code)
>>> for i in it:
      print (i)
Instruction(opname = 'LOAD_CONST', opcode = 100, arg = 0, argval = <code object hello at 0x02A9BA70,
   file "<disassembly>", line 2>, argrepr = '<code object hello at 0x02A9BA70, file "<disassembly>",
   line 2>', offset = 0, starts_line = 2, is_jump_target = False)
Instruction(opname = 'LOAD_CONST', opcode = 100, arg = 1, argval = 'hello', argrepr = "'hello'", offset = 3, starts_line = None, is_jump_target = False)
Instruction(opname = 'MAKE_FUNCTION', opcode = 132, arg = 0, argval = 0, argrepr = '', offset = 6, starts_line = None, is_jump_target = False)
Instruction(opname = 'STORE_NAME', opcode = 90, arg = 0, argval = 'hello', argrepr = 'hello', offset = 9, starts_line = None, is_jump_target = False)
Instruction(opname = 'LOAD_CONST', opcode = 100, arg = 2, argval = None, argrepr = 'None', offset = 12, starts_line = None, is_jump_target = False)
Instruction(opname = 'RETURN_VALUE', opcode = 83, arg = None, argval = None, argrepr = '', offset = 15, starts_line = None, is_jump_target = False)

The functions information is in the form of tuple like object with the parameters as shown below −

Instruction
Details for a bytecode operation
opcode
numeric code for operation, corresponding to the opcode values listed below and the bytecode values in the Opcode collections.
opname
human readable name for operation
arg
numeric argument to operation (if any), otherwise None
argval
resolved arg value (if known), otherwise same as arg
argrepr
human readable description of operation argument
offset
start index of operation within bytecode sequence
starts_line
line started by this opcode (if any), otherwise None
is_jump_target
True if other code jumps to here, otherwise False
raja
Published on 27-Dec-2018 13:27:35
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