Styjun

How to get list of function parameters in python from code object.

import xdis.std as dis
ops = list(dis.Bytecode("""def f(a, b):
 return 1"""))
print(ops)
code_obj = ops[0]
print(list(dis.Bytecode(code_obj.argval)))

The above code uses xdis module to decompile code which needs to be installed via pip. However the code object is a normal one which you would expect from the default dis module in python.
I want to know how to get the list of function parameters in order. In this case, a and b

This is the output in interactive python -

>>> import xdis.std as dis
>>> ops = list(dis.Bytecode("""def f(a, b):
... return 1"""))
>>> print(ops)
[Instruction(opname='LOAD_CONST', opcode=100, optype='const', inst_size=2, arg=0, argval=<code object f at 0x7a1a5c4f60, file "<disassembly>", line 1>, argrepr='<code object f at 0x7a1a5c4f60, file "<disassembly>", line 1>', has_arg=True, offset=0, starts_line=1, is_jump_target=False), Instruction(opname='LOAD_CONST', opcode=100, optype='const', inst_size=2, arg=1, argval='f', argrepr="'f'", has_arg=True, offset=2, starts_line=None, is_jump_target=False), Instruction(opname='MAKE_FUNCTION', opcode=132, optype=None, inst_size=2, arg=0, argval=0, argrepr='', has_arg=True, offset=4, starts_line=None, is_jump_target=False), Instruction(opname='STORE_NAME', opcode=90, optype='name', inst_size=2, arg=0, argval='f', argrepr='f', has_arg=True, offset=6, starts_line=None, is_jump_target=False), Instruction(opname='LOAD_CONST', opcode=100, optype='const', inst_size=2, arg=2, argval=None, argrepr='None', has_arg=True, offset=8, starts_line=None, is_jump_target=False), Instruction(opname='RETURN_VALUE', opcode=83, optype=None, inst_size=2, arg=None, argval=None, argrepr='', has_arg=False, offset=10, starts_line=None, is_jump_target=False)]
>>> code_obj = ops[0]
>>> print(list(dis.Bytecode(code_obj.argval)))
[Instruction(opname='LOAD_CONST', opcode=100, optype='const', inst_size=2, arg=1, argval=1, argrepr='1', has_arg=True, offset=0, starts_line=2, is_jump_target=False), Instruction(opname='RETURN_VALUE', opcode=83, optype=None, inst_size=2, arg=None, argval=None, argrepr='', has_arg=False, offset=2, starts_line=None, is_jump_target=False)]

xdis isn't going to give you much useful functionality here. It's just giving you some more object'd output that you could theoretically muck with a bit easier than the regular dis module. But, the stock module tells us all we need to know:

>>> from dis import dis
>>> def f(a, b):
... return 1
...
>>> dis(f)
 2 0 LOAD_CONST 1 (1)
 2 RETURN_VALUE

Note how the disassembly includes only two opcodes. LOAD_CONST pushes a 1 onto the stack (the cpython runtime is stack based) and RETURN_VALUE returns from the function with the value on the top of the stack. There is no mention of a nor b here. And this makes sense. They aren't used! Byte the byte code doesn't concern itself with function arguments. It will emit the necessary ops to put them on the stack (where needed):

>>> def f(a, b):
... return a + b
...
>>> dis(f)
 2 0 LOAD_FAST 0 (a)
 2 LOAD_FAST 1 (b)
 4 BINARY_ADD
 6 RETURN_VALUE

Note here that LOAD_FAST gets a and b and pushes them onto the stack for BINARY_ADD (which adds the top two values on the stack and pushes the result).

You can get at what you want by using __code__, specifically:

params_and_locals = f.__code__.co_varnames
num_args = f.__code__.co_argcount + f.__code__.co_kwonlyargcount
params = params_and_locals[:num_args]

If you have a code object

def f(a, b, *, c=True):
 e = a + b
 if c:
 return a * e

code_obj = f.__code__

then the positional parameters are

code_obj.co_varnames[:code_obj.co_argcount]

# --> ('a', 'b')

and the keyword only parameters are

code_obj.co_varnames[code_obj.co_argcount : code_obj.co_argcount + code_obj.co_kwonlyargcount]

# --> ('c',)