path: root/Lib/dis.py

"""Disassembler of Python byte code into mnemonics."""

import sys
import types
import collections
import io

from opcode import *
from opcode import (
    __all__ as _opcodes_all,
    _cache_format,
    _inline_cache_entries,
    _nb_ops,
    _specializations,
    _specialized_instructions,
)

__all__ = ["code_info", "dis", "disassemble", "distb", "disco",
           "findlinestarts", "findlabels", "show_code",
           "get_instructions", "Instruction", "Bytecode"] + _opcodes_all
del _opcodes_all

_have_code = (types.MethodType, types.FunctionType, types.CodeType,
              classmethod, staticmethod, type)

FORMAT_VALUE = opmap['FORMAT_VALUE']
FORMAT_VALUE_CONVERTERS = (
    (None, ''),
    (str, 'str'),
    (repr, 'repr'),
    (ascii, 'ascii'),
)
MAKE_FUNCTION = opmap['MAKE_FUNCTION']
MAKE_FUNCTION_FLAGS = ('defaults', 'kwdefaults', 'annotations', 'closure')

LOAD_CONST = opmap['LOAD_CONST']
LOAD_GLOBAL = opmap['LOAD_GLOBAL']
BINARY_OP = opmap['BINARY_OP']
JUMP_BACKWARD = opmap['JUMP_BACKWARD']
FOR_ITER = opmap['FOR_ITER']
LOAD_ATTR = opmap['LOAD_ATTR']

CACHE = opmap["CACHE"]

_all_opname = list(opname)
_all_opmap = dict(opmap)
_empty_slot = [slot for slot, name in enumerate(_all_opname) if name.startswith("<")]
for spec_op, specialized in zip(_empty_slot, _specialized_instructions):
    # fill opname and opmap
    _all_opname[spec_op] = specialized
    _all_opmap[specialized] = spec_op

deoptmap = {
    specialized: base for base, family in _specializations.items() for specialized in family
}

def _try_compile(source, name):
    """Attempts to compile the given source, first as an expression and
       then as a statement if the first approach fails.

       Utility function to accept strings in functions that otherwise
       expect code objects
    """
    try:
        c = compile(source, name, 'eval')
    except SyntaxError:
        c = compile(source, name, 'exec')
    return c

def dis(x=None, *, file=None, depth=None, show_caches=False, adaptive=False):
    """Disassemble classes, methods, functions, and other compiled objects.

    With no argument, disassemble the last traceback.

    Compiled objects currently include generator objects, async generator
    objects, and coroutine objects, all of which store their code object
    in a special attribute.
    """
    if x is None:
        distb(file=file, show_caches=show_caches, adaptive=adaptive)
        return
    # Extract functions from methods.
    if hasattr(x, '__func__'):
        x = x.__func__
    # Extract compiled code objects from...
    if hasattr(x, '__code__'):  # ...a function, or
        x = x.__code__
    elif hasattr(x, 'gi_code'):  #...a generator object, or
        x = x.gi_code
    elif hasattr(x, 'ag_code'):  #...an asynchronous generator object, or
        x = x.ag_code
    elif hasattr(x, 'cr_code'):  #...a coroutine.
        x = x.cr_code
    # Perform the disassembly.
    if hasattr(x, '__dict__'):  # Class or module
        items = sorted(x.__dict__.items())
        for name, x1 in items:
            if isinstance(x1, _have_code):
                print("Disassembly of %s:" % name, file=file)
                try:
                    dis(x1, file=file, depth=depth, show_caches=show_caches, adaptive=adaptive)
                except TypeError as msg:
                    print("Sorry:", msg, file=file)
                print(file=file)
    elif hasattr(x, 'co_code'): # Code object
        _disassemble_recursive(x, file=file, depth=depth, show_caches=show_caches, adaptive=adaptive)
    elif isinstance(x, (bytes, bytearray)): # Raw bytecode
        _disassemble_bytes(x, file=file, show_caches=show_caches)
    elif isinstance(x, str):    # Source code
        _disassemble_str(x, file=file, depth=depth, show_caches=show_caches, adaptive=adaptive)
    else:
        raise TypeError("don't know how to disassemble %s objects" %
                        type(x).__name__)

def distb(tb=None, *, file=None, show_caches=False, adaptive=False):
    """Disassemble a traceback (default: last traceback)."""
    if tb is None:
        try:
            tb = sys.last_traceback
        except AttributeError:
            raise RuntimeError("no last traceback to disassemble") from None
        while tb.tb_next: tb = tb.tb_next
    disassemble(tb.tb_frame.f_code, tb.tb_lasti, file=file, show_caches=show_caches, adaptive=adaptive)

# The inspect module interrogates this dictionary to build its
# list of CO_* constants. It is also used by pretty_flags to
# turn the co_flags field into a human readable list.
COMPILER_FLAG_NAMES = {
     1: "OPTIMIZED",
     2: "NEWLOCALS",
     4: "VARARGS",
     8: "VARKEYWORDS",
    16: "NESTED",
    32: "GENERATOR",
    64: "NOFREE",
   128: "COROUTINE",
   256: "ITERABLE_COROUTINE",
   512: "ASYNC_GENERATOR",
}

def pretty_flags(flags):
    """Return pretty representation of code flags."""
    names = []
    for i in range(32):
        flag = 1<<i
        if flags & flag:
            names.append(COMPILER_FLAG_NAMES.get(flag, hex(flag)))
            flags ^= flag
            if not flags:
                break
    else:
        names.append(hex(flags))
    return ", ".join(names)

class _Unknown:
    def __repr__(self):
        return "<unknown>"

# Sentinel to represent values that cannot be calculated
UNKNOWN = _Unknown()

def _get_code_object(x):
    """Helper to handle methods, compiled or raw code objects, and strings."""
    # Extract functions from methods.
    if hasattr(x, '__func__'):
        x = x.__func__
    # Extract compiled code objects from...
    if hasattr(x, '__code__'):  # ...a function, or
        x = x.__code__
    elif hasattr(x, 'gi_code'):  #...a generator object, or
        x = x.gi_code
    elif hasattr(x, 'ag_code'):  #...an asynchronous generator object, or
        x = x.ag_code
    elif hasattr(x, 'cr_code'):  #...a coroutine.
        x = x.cr_code
    # Handle source code.
    if isinstance(x, str):
        x = _try_compile(x, "<disassembly>")
    # By now, if we don't have a code object, we can't disassemble x.
    if hasattr(x, 'co_code'):
        return x
    raise TypeError("don't know how to disassemble %s objects" %
                    type(x).__name__)

def _deoptop(op):
    name = _all_opname[op]
    return _all_opmap[deoptmap[name]] if name in deoptmap else op

def _get_code_array(co, adaptive):
    return co._co_code_adaptive if adaptive else co.co_code

def code_info(x):
    """Formatted details of methods, functions, or code."""
    return _format_code_info(_get_code_object(x))

def _format_code_info(co):
    lines = []
    lines.append("Name:              %s" % co.co_name)
    lines.append("Filename:          %s" % co.co_filename)
    lines.append("Argument count:    %s" % co.co_argcount)
    lines.append("Positional-only arguments: %s" % co.co_posonlyargcount)
    lines.append("Kw-only arguments: %s" % co.co_kwonlyargcount)
    lines.append("Number of locals:  %s" % co.co_nlocals)
    lines.append("Stack size:        %s" % co.co_stacksize)
    lines.append("Flags:             %s" % pretty_flags(co.co_flags))
    if co.co_consts:
        lines.append("Constants:")
        for i_c in enumerate(co.co_consts):
            lines.append("%4d: %r" % i_c)
    if co.co_names:
        lines.append("Names:")
        for i_n in enumerate(co.co_names):
            lines.append("%4d: %s" % i_n)
    if co.co_varnames:
        lines.append("Variable names:")
        for i_n in enumerate(co.co_varnames):
            lines.append("%4d: %s" % i_n)
    if co.co_freevars:
        lines.append("Free variables:")
        for i_n in enumerate(co.co_freevars):
            lines.append("%4d: %s" % i_n)
    if co.co_cellvars:
        lines.append("Cell variables:")
        for i_n in enumerate(co.co_cellvars):
            lines.append("%4d: %s" % i_n)
    return "\n".join(lines)

def show_code(co, *, file=None):
    """Print details of methods, functions, or code to *file*.

    If *file* is not provided, the output is printed on stdout.
    """
    print(code_info(co), file=file)

Positions = collections.namedtuple(
    'Positions',
    [
        'lineno',
        'end_lineno',
        'col_offset',
        'end_col_offset',
    ],
    defaults=[None] * 4
)

_Instruction = collections.namedtuple(
    "_Instruction",
    [
        'opname',
        'opcode',
        'arg',
        'argval',
        'argrepr',
        'offset',
        'starts_line',
        'is_jump_target',
        'positions'
    ],
    defaults=[None]
)

_Instruction.opname.__doc__ = "Human readable name for operation"
_Instruction.opcode.__doc__ = "Numeric code for operation"
_Instruction.arg.__doc__ = "Numeric argument to operation (if any), otherwise None"
_Instruction.argval.__doc__ = "Resolved arg value (if known), otherwise same as arg"
_Instruction.argrepr.__doc__ = "Human readable description of operation argument"
_Instruction.offset.__doc__ = "Start index of operation within bytecode sequence"
_Instruction.starts_line.__doc__ = "Line started by this opcode (if any), otherwise None"
_Instruction.is_jump_target.__doc__ = "True if other code jumps to here, otherwise False"
_Instruction.positions.__doc__ = "dis.Positions object holding the span of source code covered by this instruction"

_ExceptionTableEntry = collections.namedtuple("_ExceptionTableEntry",
    "start end target depth lasti")

_OPNAME_WIDTH = 20
_OPARG_WIDTH = 5

class Instruction(_Instruction):
    """Details for a bytecode operation

       Defined fields:
         opname - human readable name for operation
         opcode - numeric code 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
         positions - Optional dis.Positions object holding the span of source code
                     covered by this instruction
    """

    def _disassemble(self, lineno_width=3, mark_as_current=False, offset_width=4):
        """Format instruction details for inclusion in disassembly output

        *lineno_width* sets the width of the line number field (0 omits it)
        *mark_as_current* inserts a '-->' marker arrow as part of the line
        *offset_width* sets the width of the instruction offset field
        """
        fields = []
        # Column: Source code line number
        if lineno_width:
            if self.starts_line is not None:
                lineno_fmt = "%%%dd" % lineno_width
                fields.append(lineno_fmt % self.starts_line)
            else:
                fields.append(' ' * lineno_width)
        # Column: Current instruction indicator
        if mark_as_current:
            fields.append('-->')
        else:
            fields.append('   ')
        # Column: Jump target marker
        if self.is_jump_target:
            fields.append('>>')
        else:
            fields.append('  ')
        # Column: Instruction offset from start of code sequence
        fields.append(repr(self.offset).rjust(offset_width))
        # Column: Opcode name
        fields.append(self.opname.ljust(_OPNAME_WIDTH))
        # Column: Opcode argument
        if self.arg is not None:
            fields.append(repr(self.arg).rjust(_OPARG_WIDTH))
            # Column: Opcode argument details
            if self.argrepr:
                fields.append('(' + self.argrepr + ')')
        return ' '.join(fields).rstrip()


def get_instructions(x, *, first_line=None, show_caches=False, adaptive=False):
    """Iterator for the opcodes in methods, functions or code

    Generates a series of Instruction named tuples giving the details of
    each operations in the supplied code.

    If *first_line* is not None, it indicates the line number that should
    be reported for the first source line in the disassembled code.
    Otherwise, the source line information (if any) is taken directly from
    the disassembled code object.
    """
    co = _get_code_object(x)
    linestarts = dict(findlinestarts(co))
    if first_line is not None:
        line_offset = first_line - co.co_firstlineno
    else:
        line_offset = 0
    return _get_instructions_bytes(_get_code_array(co, adaptive),
                                   co._varname_from_oparg,
                                   co.co_names, co.co_consts,
                                   linestarts, line_offset,
                                   co_positions=co.co_positions(),
                                   show_caches=show_caches)

def _get_const_value(op, arg, co_consts):
    """Helper to get the value of the const in a hasconst op.

       Returns the dereferenced constant if this is possible.
       Otherwise (if it is a LOAD_CONST and co_consts is not
       provided) returns the dis.UNKNOWN sentinel.
    """
    assert op in hasconst

    argval = UNKNOWN
    if op == LOAD_CONST:
        if co_consts is not None:
            argval = co_consts[arg]
    return argval

def _get_const_info(op, arg, co_consts):
    """Helper to get optional details about const references

       Returns the dereferenced constant and its repr if the value
       can be calculated.
       Otherwise returns the sentinel value dis.UNKNOWN for the value
       and an empty string for its repr.
    """
    argval = _get_const_value(op, arg, co_consts)
    argrepr = repr(argval) if argval is not UNKNOWN else ''
    return argval, argrepr

def _get_name_info(name_index, get_name, **extrainfo):
    """Helper to get optional details about named references

       Returns the dereferenced name as both value and repr if the name
       list is defined.
       Otherwise returns the sentinel value dis.UNKNOWN for the value
       and an empty string for its repr.
    """
    if get_name is not None:
        argval = get_name(name_index, **extrainfo)
        return argval, argval
    else:
        return UNKNOWN, ''

def _parse_varint(iterator):
    b = next(iterator)
    val = b & 63
    while b&64:
        val <<= 6
        b = next(iterator)
        val |= b&63
    return val

def _parse_exception_table(code):
    iterator = iter(code.co_exceptiontable)
    entries = []
    try:
        while True:
            start = _parse_varint(iterator)*2
            length = _parse_varint(iterator)*2
            end = start + length
            target = _parse_varint(iterator)*2
            dl = _parse_varint(iterator)
            depth = dl >> 1
            lasti = bool(dl&1)
            entries.append(_ExceptionTableEntry(start, end, target, depth, lasti))
    except StopIteration:
        return entries

def _is_backward_jump(op):
    return 'JUMP_BACKWARD' in opname[op]

def _get_instructions_bytes(code, varname_from_oparg=None,
                            names=None, co_consts=None,
                            linestarts=None, line_offset=0,
                            exception_entries=(), co_positions=None,
                            show_caches=False):
    """Iterate over the instructions in a bytecode string.

    Generates a sequence of Instruction namedtuples giving the details of each
    opcode.  Additional information about the code's runtime environment
    (e.g. variable names, co_consts) can be specified using optional
    arguments.

    """
    co_positions = co_positions or iter(())
    get_name = None if names is None else names.__getitem__
    labels = set(findlabels(code))
    for start, end, target, _, _ in exception_entries:
        for i in range(start, end):
            labels.add(target)
    starts_line = None
    for offset, op, arg in _unpack_opargs(code):
        if linestarts is not None:
            starts_line = linestarts.get(offset, None)
            if starts_line is not None:
                starts_line += line_offset
        is_jump_target = offset in labels
        argval = None
        argrepr = ''
        positions = Positions(*next(co_positions, ()))
        deop = _deoptop(op)
        if arg is not None:
            #  Set argval to the dereferenced value of the argument when
            #  available, and argrepr to the string representation of argval.
            #    _disassemble_bytes needs the string repr of the
            #    raw name index for LOAD_GLOBAL, LOAD_CONST, etc.
            argval = arg
            if deop in hasconst:
                argval, argrepr = _get_const_info(deop, arg, co_consts)
            elif deop in hasname:
                if deop == LOAD_GLOBAL:
                    argval, argrepr = _get_name_info(arg//2, get_name)
                    if (arg & 1) and argrepr:
                        argrepr = "NULL + " + argrepr
                elif deop == LOAD_ATTR:
                    argval, argrepr = _get_name_info(arg//2, get_name)
                    if (arg & 1) and argrepr:
                        argrepr = "NULL|self + " + argrepr
                else:
                    argval, argrepr = _get_name_info(arg, get_name)
            elif deop in hasjabs:
                argval = arg*2
                argrepr = "to " + repr(argval)
            elif deop in hasjrel:
                signed_arg = -arg if _is_backward_jump(deop) else arg
                argval = offset + 2 + signed_arg*2
                if deop == FOR_ITER:
                    argval += 2
                argrepr = "to " + repr(argval)
            elif deop in haslocal or deop in hasfree:
                argval, argrepr = _get_name_info(arg, varname_from_oparg)
            elif deop in hascompare:
                argval = cmp_op[arg>>4]
                argrepr = argval
            elif deop == FORMAT_VALUE:
                argval, argrepr = FORMAT_VALUE_CONVERTERS[arg & 0x3]
                argval = (argval, bool(arg & 0x4))
                if argval[1]:
                    if argrepr:
                        argrepr += ', '
                    argrepr += 'with format'
            elif deop == MAKE_FUNCTION:
                argrepr = ', '.join(s for i, s in enumerate(MAKE_FUNCTION_FLAGS)
                                    if arg & (1<<i))
            elif deop == BINARY_OP:
                _, argrepr = _nb_ops[arg]
        yield Instruction(_all_opname[op], op,
                          arg, argval, argrepr,
                          offset, starts_line, is_jump_target, positions)
        caches = _inline_cache_entries[deop]
        if not caches:
            continue
        if not show_caches:
            # We still need to advance the co_positions iterator:
            for _ in range(caches):
                next(co_positions, ())
            continue
        for name, size in _cache_format[opname[deop]].items():
            for i in range(size):
                offset += 2
                # Only show the fancy argrepr for a CACHE instruction when it's
                # the first entry for a particular cache value:
                if i == 0:
                    data = code[offset: offset + 2 * size]
                    argrepr = f"{name}: {int.from_bytes(data, sys.byteorder)}"
                else:
                    argrepr = ""
                yield Instruction(
                    "CACHE", CACHE, 0, None, argrepr, offset, None, False,
                    Positions(*next(co_positions, ()))
                )

def disassemble(co, lasti=-1, *, file=None, show_caches=False, adaptive=False):
    """Disassemble a code object."""
    linestarts = dict(findlinestarts(co))
    exception_entries = _parse_exception_table(co)
    _disassemble_bytes(_get_code_array(co, adaptive),
                       lasti, co._varname_from_oparg,
                       co.co_names, co.co_consts, linestarts, file=file,
                       exception_entries=exception_entries,
                       co_positions=co.co_positions(), show_caches=show_caches)

def _disassemble_recursive(co, *, file=None, depth=None, show_caches=False, adaptive=False):
    disassemble(co, file=file, show_caches=show_caches, adaptive=adaptive)
    if depth is None or depth > 0:
        if depth is not None:
            depth = depth - 1
        for x in co.co_consts:
            if hasattr(x, 'co_code'):
                print(file=file)
                print("Disassembly of %r:" % (x,), file=file)
                _disassemble_recursive(
                    x, file=file, depth=depth, show_caches=show_caches, adaptive=adaptive
                )

def _disassemble_bytes(code, lasti=-1, varname_from_oparg=None,
                       names=None, co_consts=None, linestarts=None,
                       *, file=None, line_offset=0, exception_entries=(),
                       co_positions=None, show_caches=False):
    # Omit the line number column entirely if we have no line number info
    show_lineno = bool(linestarts)
    if show_lineno:
        maxlineno = max(linestarts.values()) + line_offset
        if maxlineno >= 1000:
            lineno_width = len(str(maxlineno))
        else:
            lineno_width = 3
    else:
        lineno_width = 0
    maxoffset = len(code) - 2
    if maxoffset >= 10000:
        offset_width = len(str(maxoffset))
    else:
        offset_width = 4
    for instr in _get_instructions_bytes(code, varname_from_oparg, names,
                                         co_consts, linestarts,
                                         line_offset=line_offset,
                                         exception_entries=exception_entries,
                                         co_positions=co_positions,
                                         show_caches=show_caches):
        new_source_line = (show_lineno and
                           instr.starts_line is not None and
                           instr.offset > 0)
        if new_source_line:
            print(file=file)
        is_current_instr = instr.offset == lasti
        print(instr._disassemble(lineno_width, is_current_instr, offset_width),
              file=file)
    if exception_entries:
        print("ExceptionTable:", file=file)
        for entry in exception_entries:
            lasti = " lasti" if entry.lasti else ""
            end = entry.end-2
            print(f"  {entry.start} to {end} -> {entry.target} [{entry.depth}]{lasti}", file=file)

def _disassemble_str(source, **kwargs):
    """Compile the source string, then disassemble the code object."""
    _disassemble_recursive(_try_compile(source, '<dis>'), **kwargs)

disco = disassemble                     # XXX For backwards compatibility


# Rely on C `int` being 32 bits for oparg
_INT_BITS = 32
# Value for c int when it overflows
_INT_OVERFLOW = 2 ** (_INT_BITS - 1)

def _unpack_opargs(code):
    extended_arg = 0
    caches = 0
    for i in range(0, len(code), 2):
        # Skip inline CACHE entries:
        if caches:
            caches -= 1
            continue
        op = code[i]
        deop = _deoptop(op)
        caches = _inline_cache_entries[deop]
        if deop in hasarg:
            arg = code[i+1] | extended_arg
            extended_arg = (arg << 8) if deop == EXTENDED_ARG else 0
            # The oparg is stored as a signed integer
            # If the value exceeds its upper limit, it will overflow and wrap
            # to a negative integer
            if extended_arg >= _INT_OVERFLOW:
                extended_arg -= 2 * _INT_OVERFLOW
        else:
            arg = None
            extended_arg = 0
        yield (i, op, arg)

def findlabels(code):
    """Detect all offsets in a byte code which are jump targets.

    Return the list of offsets.

    """
    labels = []
    for offset, op, arg in _unpack_opargs(code):
        if arg is not None:
            deop = _deoptop(op)
            if deop in hasjrel:
                if _is_backward_jump(deop):
                    arg = -arg
                label = offset + 2 + arg*2
                if deop == FOR_ITER:
                    label += 2
            elif deop in hasjabs:
                label = arg*2
            else:
                continue
            if label not in labels:
                labels.append(label)
    return labels

def findlinestarts(code):
    """Find the offsets in a byte code which are start of lines in the source.

    Generate pairs (offset, lineno)
    """
    lastline = None
    for start, end, line in code.co_lines():
        if line is not None and line != lastline:
            lastline = line
            yield start, line
    return

def _find_imports(co):
    """Find import statements in the code

    Generate triplets (name, level, fromlist) where
    name is the imported module and level, fromlist are
    the corresponding args to __import__.
    """
    IMPORT_NAME = opmap['IMPORT_NAME']

    consts = co.co_consts
    names = co.co_names
    opargs = [(op, arg) for _, op, arg in _unpack_opargs(co.co_code)
                  if op != EXTENDED_ARG]
    for i, (op, oparg) in enumerate(opargs):
        if op == IMPORT_NAME and i >= 2:
            from_op = opargs[i-1]
            level_op = opargs[i-2]
            if (from_op[0] in hasconst and level_op[0] in hasconst):
                level = _get_const_value(level_op[0], level_op[1], consts)
                fromlist = _get_const_value(from_op[0], from_op[1], consts)
                yield (names[oparg], level, fromlist)

def _find_store_names(co):
    """Find names of variables which are written in the code

    Generate sequence of strings
    """
    STORE_OPS = {
        opmap['STORE_NAME'],
        opmap['STORE_GLOBAL']
    }

    names = co.co_names
    for _, op, arg in _unpack_opargs(co.co_code):
        if op in STORE_OPS:
            yield names[arg]


class Bytecode:
    """The bytecode operations of a piece of code

    Instantiate this with a function, method, other compiled object, string of
    code, or a code object (as returned by compile()).

    Iterating over this yields the bytecode operations as Instruction instances.
    """
    def __init__(self, x, *, first_line=None, current_offset=None, show_caches=False, adaptive=False):
        self.codeobj = co = _get_code_object(x)
        if first_line is None:
            self.first_line = co.co_firstlineno
            self._line_offset = 0
        else:
            self.first_line = first_line
            self._line_offset = first_line - co.co_firstlineno
        self._linestarts = dict(findlinestarts(co))
        self._original_object = x
        self.current_offset = current_offset
        self.exception_entries = _parse_exception_table(co)
        self.show_caches = show_caches
        self.adaptive = adaptive

    def __iter__(self):
        co = self.codeobj
        return _get_instructions_bytes(_get_code_array(co, self.adaptive),
                                       co._varname_from_oparg,
                                       co.co_names, co.co_consts,
                                       self._linestarts,
                                       line_offset=self._line_offset,
                                       exception_entries=self.exception_entries,
                                       co_positions=co.co_positions(),
                                       show_caches=self.show_caches)

    def __repr__(self):
        return "{}({!r})".format(self.__class__.__name__,
                                 self._original_object)

    @classmethod
    def from_traceback(cls, tb, *, show_caches=False, adaptive=False):
        """ Construct a Bytecode from the given traceback """
        while tb.tb_next:
            tb = tb.tb_next
        return cls(
            tb.tb_frame.f_code, current_offset=tb.tb_lasti, show_caches=show_caches, adaptive=adaptive
        )

    def info(self):
        """Return formatted information about the code object."""
        return _format_code_info(self.codeobj)

    def dis(self):
        """Return a formatted view of the bytecode operations."""
        co = self.codeobj
        if self.current_offset is not None:
            offset = self.current_offset
        else:
            offset = -1
        with io.StringIO() as output:
            _disassemble_bytes(_get_code_array(co, self.adaptive),
                               varname_from_oparg=co._varname_from_oparg,
                               names=co.co_names, co_consts=co.co_consts,
                               linestarts=self._linestarts,
                               line_offset=self._line_offset,
                               file=output,
                               lasti=offset,
                               exception_entries=self.exception_entries,
                               co_positions=co.co_positions(),
                               show_caches=self.show_caches)
            return output.getvalue()


def _test():
    """Simple test program to disassemble a file."""
    import argparse

    parser = argparse.ArgumentParser()
    parser.add_argument('infile', type=argparse.FileType('rb'), nargs='?', default='-')
    args = parser.parse_args()
    with args.infile as infile:
        source = infile.read()
    code = compile(source, args.infile.name, "exec")
    dis(code)

if __name__ == "__main__":
    _test()