cpython/Doc/library/dis.rst

:mod:`dis` --- Disassembler for Python bytecode
===============================================

.. module:: dis
   :synopsis: Disassembler for Python bytecode.


The :mod:`dis` module supports the analysis of CPython :term:`bytecode` by
disassembling it. The CPython bytecode which this module takes as an
input is defined in the file :file:`Include/opcode.h` and used by the compiler
and the interpreter.

.. impl-detail::

   Bytecode is an implementation detail of the CPython interpreter!  No
   guarantees are made that bytecode will not be added, removed, or changed
   between versions of Python.  Use of this module should not be considered to
   work across Python VMs or Python releases.


Example: Given the function :func:`myfunc`::

   def myfunc(alist):
       return len(alist)

the following command can be used to get the disassembly of :func:`myfunc`::

   >>> dis.dis(myfunc)
     2           0 LOAD_GLOBAL              0 (len)
                 3 LOAD_FAST                0 (alist)
                 6 CALL_FUNCTION            1
                 9 RETURN_VALUE

(The "2" is a line number).

The :mod:`dis` module defines the following functions and constants:


.. function:: dis(x=None)

   Disassemble the *x* object.  *x* can denote either a module, a
   class, a method, a function, a code object, a string of source code or a
   byte sequence of raw bytecode.  For a module, it disassembles all
   functions.  For a class, it disassembles all methods.  For a code object
   or sequence of raw bytecode, it prints one line per bytecode instruction.
   Strings are first compiled to code objects with the :func:`compile`
   built-in function before being disassembled.  If no object is provided,
   this function disassembles the last traceback.


.. function:: distb(tb=None)

   Disassemble the top-of-stack function of a traceback, using the last
   traceback if none was passed.  The instruction causing the exception is
   indicated.


.. function:: disassemble(code, lasti=-1)
              disco(code, lasti=-1)

   Disassemble a code object, indicating the last instruction if *lasti* was
   provided.  The output is 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.

   The parameter interpretation recognizes local and global variable names,
   constant values, branch targets, and compare operators.


.. function:: findlinestarts(code)

   This generator function uses the ``co_firstlineno`` and ``co_lnotab``
   attributes of the code object *code* to find the offsets which are starts of
   lines in the source code.  They are generated as ``(offset, lineno)`` pairs.


.. function:: findlabels(code)

   Detect all offsets in the code object *code* which are jump targets, and
   return a list of these offsets.


.. data:: opname

   Sequence of operation names, indexable using the bytecode.


.. data:: opmap

   Dictionary mapping bytecodes to operation names.


.. data:: cmp_op

   Sequence of all compare operation names.


.. data:: hasconst

   Sequence of bytecodes that have a constant parameter.


.. data:: hasfree

   Sequence of bytecodes that access a free variable.


.. data:: hasname

   Sequence of bytecodes that access an attribute by name.


.. data:: hasjrel

   Sequence of bytecodes that have a relative jump target.


.. data:: hasjabs

   Sequence of bytecodes that have an absolute jump target.


.. data:: haslocal

   Sequence of bytecodes that access a local variable.


.. data:: hascompare

   Sequence of bytecodes of Boolean operations.


.. _bytecodes:

Python Bytecode Instructions
----------------------------

The Python compiler currently generates the following bytecode instructions.


**General instructions**

.. opcode:: STOP_CODE

   Indicates end-of-code to the compiler, not used by the interpreter.


.. opcode:: NOP

   Do nothing code.  Used as a placeholder by the bytecode optimizer.


.. opcode:: POP_TOP

   Removes the top-of-stack (TOS) item.


.. opcode:: ROT_TWO

   Swaps the two top-most stack items.


.. opcode:: ROT_THREE

   Lifts second and third stack item one position up, moves top down to position
   three.


.. opcode:: ROT_FOUR

   Lifts second, third and forth stack item one position up, moves top down to
   position four.


.. opcode:: DUP_TOP

   Duplicates the reference on top of the stack.


**Unary operations**

Unary operations take the top of the stack, apply the operation, and push the
result back on the stack.

.. opcode:: UNARY_POSITIVE

   Implements ``TOS = +TOS``.


.. opcode:: UNARY_NEGATIVE

   Implements ``TOS = -TOS``.


.. opcode:: UNARY_NOT

   Implements ``TOS = not TOS``.


.. opcode:: UNARY_INVERT

   Implements ``TOS = ~TOS``.


.. opcode:: GET_ITER

   Implements ``TOS = iter(TOS)``.


**Binary operations**

Binary operations remove the top of the stack (TOS) and the second top-most
stack item (TOS1) from the stack.  They perform the operation, and put the
result back on the stack.

.. opcode:: BINARY_POWER

   Implements ``TOS = TOS1 ** TOS``.


.. opcode:: BINARY_MULTIPLY

   Implements ``TOS = TOS1 * TOS``.


.. opcode:: BINARY_FLOOR_DIVIDE

   Implements ``TOS = TOS1 // TOS``.


.. opcode:: BINARY_TRUE_DIVIDE

   Implements ``TOS = TOS1 / TOS``.


.. opcode:: BINARY_MODULO

   Implements ``TOS = TOS1 % TOS``.


.. opcode:: BINARY_ADD

   Implements ``TOS = TOS1 + TOS``.


.. opcode:: BINARY_SUBTRACT

   Implements ``TOS = TOS1 - TOS``.


.. opcode:: BINARY_SUBSCR

   Implements ``TOS = TOS1[TOS]``.


.. opcode:: BINARY_LSHIFT

   Implements ``TOS = TOS1 << TOS``.


.. opcode:: BINARY_RSHIFT

   Implements ``TOS = TOS1 >> TOS``.


.. opcode:: BINARY_AND

   Implements ``TOS = TOS1 & TOS``.


.. opcode:: BINARY_XOR

   Implements ``TOS = TOS1 ^ TOS``.


.. opcode:: BINARY_OR

   Implements ``TOS = TOS1 | TOS``.


**In-place operations**

In-place operations are like binary operations, in that they remove TOS and
TOS1, and push the result back on the stack, but the operation is done in-place
when TOS1 supports it, and the resulting TOS may be (but does not have to be)
the original TOS1.

.. opcode:: INPLACE_POWER

   Implements in-place ``TOS = TOS1 ** TOS``.


.. opcode:: INPLACE_MULTIPLY

   Implements in-place ``TOS = TOS1 * TOS``.


.. opcode:: INPLACE_FLOOR_DIVIDE

   Implements in-place ``TOS = TOS1 // TOS``.


.. opcode:: INPLACE_TRUE_DIVIDE

   Implements in-place ``TOS = TOS1 / TOS``.


.. opcode:: INPLACE_MODULO

   Implements in-place ``TOS = TOS1 % TOS``.


.. opcode:: INPLACE_ADD

   Implements in-place ``TOS = TOS1 + TOS``.


.. opcode:: INPLACE_SUBTRACT

   Implements in-place ``TOS = TOS1 - TOS``.


.. opcode:: INPLACE_LSHIFT

   Implements in-place ``TOS = TOS1 << TOS``.


.. opcode:: INPLACE_RSHIFT

   Implements in-place ``TOS = TOS1 >> TOS``.


.. opcode:: INPLACE_AND

   Implements in-place ``TOS = TOS1 & TOS``.


.. opcode:: INPLACE_XOR

   Implements in-place ``TOS = TOS1 ^ TOS``.


.. opcode:: INPLACE_OR

   Implements in-place ``TOS = TOS1 | TOS``.


.. opcode:: STORE_SUBSCR

   Implements ``TOS1[TOS] = TOS2``.


.. opcode:: DELETE_SUBSCR

   Implements ``del TOS1[TOS]``.


**Miscellaneous opcodes**

.. opcode:: PRINT_EXPR

   Implements the expression statement for the interactive mode.  TOS is removed
   from the stack and printed.  In non-interactive mode, an expression statement is
   terminated with ``POP_STACK``.


.. opcode:: BREAK_LOOP

   Terminates a loop due to a :keyword:`break` statement.


.. opcode:: CONTINUE_LOOP (target)

   Continues a loop due to a :keyword:`continue` statement.  *target* is the
   address to jump to (which should be a ``FOR_ITER`` instruction).


.. opcode:: SET_ADD (i)

   Calls ``set.add(TOS1[-i], TOS)``.  Used to implement set comprehensions.


.. opcode:: LIST_APPEND (i)

   Calls ``list.append(TOS[-i], TOS)``.  Used to implement list comprehensions.


.. opcode:: MAP_ADD (i)

   Calls ``dict.setitem(TOS1[-i], TOS, TOS1)``.  Used to implement dict
   comprehensions.

For all of the SET_ADD, LIST_APPEND and MAP_ADD instructions, while the
added value or key/value pair is popped off, the container object remains on
the stack so that it is available for further iterations of the loop.


.. opcode:: RETURN_VALUE

   Returns with TOS to the caller of the function.


.. opcode:: YIELD_VALUE

   Pops ``TOS`` and yields it from a :term:`generator`.


.. opcode:: IMPORT_STAR

   Loads all symbols not starting with ``'_'`` directly from the module TOS to the
   local namespace. The module is popped after loading all names. This opcode
   implements ``from module import *``.


.. opcode:: POP_BLOCK

   Removes one block from the block stack.  Per frame, there is a  stack of blocks,
   denoting nested loops, try statements, and such.


.. opcode:: POP_EXCEPT

   Removes one block from the block stack. The popped block must be an exception
   handler block, as implicitly created when entering an except handler.
   In addition to popping extraneous values from the frame stack, the
   last three popped values are used to restore the exception state.


.. opcode:: END_FINALLY

   Terminates a :keyword:`finally` clause.  The interpreter recalls whether the
   exception has to be re-raised, or whether the function returns, and continues
   with the outer-next block.


.. opcode:: LOAD_BUILD_CLASS

   Pushes :func:`builtins.__build_class__` onto the stack.  It is later called
   by ``CALL_FUNCTION`` to construct a class.


.. opcode:: SETUP_WITH (delta)

   This opcode performs several operations before a with block starts.  First,
   it loads :meth:`~object.__exit__` from the context manager and pushes it onto
   the stack for later use by :opcode:`WITH_CLEANUP`.  Then,
   :meth:`~object.__enter__` is called, and a finally block pointing to *delta*
   is pushed.  Finally, the result of calling the enter method is pushed onto
   the stack.  The next opcode will either ignore it (:opcode:`POP_TOP`), or
   store it in (a) variable(s) (:opcode:`STORE_FAST`, :opcode:`STORE_NAME`, or
   :opcode:`UNPACK_SEQUENCE`).


.. opcode:: WITH_CLEANUP

   Cleans up the stack when a :keyword:`with` statement block exits.  TOS is
   the context manager's :meth:`__exit__` bound method. Below TOS are 1--3
   values indicating how/why the finally clause was entered:

   * SECOND = ``None``
   * (SECOND, THIRD) = (``WHY_{RETURN,CONTINUE}``), retval
   * SECOND = ``WHY_*``; no retval below it
   * (SECOND, THIRD, FOURTH) = exc_info()

   In the last case, ``TOS(SECOND, THIRD, FOURTH)`` is called, otherwise
   ``TOS(None, None, None)``.  In addition, TOS is removed from the stack.

   If the stack represents an exception, *and* the function call returns
   a 'true' value, this information is "zapped" and replaced with a single
   ``WHY_SILENCED`` to prevent ``END_FINALLY`` from re-raising the exception.
   (But non-local gotos will still be resumed.)

   .. XXX explain the WHY stuff!


.. opcode:: STORE_LOCALS

   Pops TOS from the stack and stores it as the current frame's ``f_locals``.
   This is used in class construction.


All of the following opcodes expect arguments.  An argument is two bytes, with
the more significant byte last.

.. opcode:: STORE_NAME (namei)

   Implements ``name = TOS``. *namei* is the index of *name* in the attribute
   :attr:`co_names` of the code object. The compiler tries to use ``STORE_FAST``
   or ``STORE_GLOBAL`` if possible.


.. opcode:: DELETE_NAME (namei)

   Implements ``del name``, where *namei* is the index into :attr:`co_names`
   attribute of the code object.


.. opcode:: UNPACK_SEQUENCE (count)

   Unpacks TOS into *count* individual values, which are put onto the stack
   right-to-left.


.. opcode:: UNPACK_EX (counts)

   Implements assignment with a starred target: Unpacks an iterable in TOS into
   individual values, where the total number of values can be smaller than the
   number of items in the iterable: one the new values will be a list of all
   leftover items.

   The low byte of *counts* is the number of values before the list value, the
   high byte of *counts* the number of values after it.  The resulting values
   are put onto the stack right-to-left.


.. opcode:: DUP_TOPX (count)

   Duplicate *count* items, keeping them in the same order. Due to implementation
   limits, *count* should be between 1 and 5 inclusive.


.. opcode:: STORE_ATTR (namei)

   Implements ``TOS.name = TOS1``, where *namei* is the index of name in
   :attr:`co_names`.


.. opcode:: DELETE_ATTR (namei)

   Implements ``del TOS.name``, using *namei* as index into :attr:`co_names`.


.. opcode:: STORE_GLOBAL (namei)

   Works as ``STORE_NAME``, but stores the name as a global.


.. opcode:: DELETE_GLOBAL (namei)

   Works as ``DELETE_NAME``, but deletes a global name.


.. opcode:: LOAD_CONST (consti)

   Pushes ``co_consts[consti]`` onto the stack.


.. opcode:: LOAD_NAME (namei)

   Pushes the value associated with ``co_names[namei]`` onto the stack.


.. opcode:: BUILD_TUPLE (count)

   Creates a tuple consuming *count* items from the stack, and pushes the resulting
   tuple onto the stack.


.. opcode:: BUILD_LIST (count)

   Works as ``BUILD_TUPLE``, but creates a list.


.. opcode:: BUILD_SET (count)

   Works as ``BUILD_TUPLE``, but creates a set.


.. opcode:: BUILD_MAP (count)

   Pushes a new dictionary object onto the stack.  The dictionary is pre-sized
   to hold *count* entries.


.. opcode:: LOAD_ATTR (namei)

   Replaces TOS with ``getattr(TOS, co_names[namei])``.


.. opcode:: COMPARE_OP (opname)

   Performs a Boolean operation.  The operation name can be found in
   ``cmp_op[opname]``.


.. opcode:: IMPORT_NAME (namei)

   Imports the module ``co_names[namei]``.  TOS and TOS1 are popped and provide
   the *fromlist* and *level* arguments of :func:`__import__`.  The module
   object is pushed onto the stack.  The current namespace is not affected:
   for a proper import statement, a subsequent ``STORE_FAST`` instruction
   modifies the namespace.


.. opcode:: IMPORT_FROM (namei)

   Loads the attribute ``co_names[namei]`` from the module found in TOS. The
   resulting object is pushed onto the stack, to be subsequently stored by a
   ``STORE_FAST`` instruction.


.. opcode:: JUMP_FORWARD (delta)

   Increments bytecode counter by *delta*.


.. opcode:: POP_JUMP_IF_TRUE (target)

   If TOS is true, sets the bytecode counter to *target*.  TOS is popped.


.. opcode:: POP_JUMP_IF_FALSE (target)

   If TOS is false, sets the bytecode counter to *target*.  TOS is popped.


.. opcode:: JUMP_IF_TRUE_OR_POP (target)

   If TOS is true, sets the bytecode counter to *target* and leaves TOS
   on the stack.  Otherwise (TOS is false), TOS is popped.


.. opcode:: JUMP_IF_FALSE_OR_POP (target)

   If TOS is false, sets the bytecode counter to *target* and leaves
   TOS on the stack.  Otherwise (TOS is true), TOS is popped.


.. opcode:: JUMP_ABSOLUTE (target)

   Set bytecode counter to *target*.


.. opcode:: FOR_ITER (delta)

   ``TOS`` is an :term:`iterator`.  Call its :meth:`__next__` method.  If this
   yields a new value, push it on the stack (leaving the iterator below it).  If
   the iterator indicates it is exhausted ``TOS`` is popped, and the byte code
   counter is incremented by *delta*.


.. opcode:: LOAD_GLOBAL (namei)

   Loads the global named ``co_names[namei]`` onto the stack.


.. opcode:: SETUP_LOOP (delta)

   Pushes a block for a loop onto the block stack.  The block spans from the
   current instruction with a size of *delta* bytes.


.. opcode:: SETUP_EXCEPT (delta)

   Pushes a try block from a try-except clause onto the block stack. *delta* points
   to the first except block.


.. opcode:: SETUP_FINALLY (delta)

   Pushes a try block from a try-except clause onto the block stack. *delta* points
   to the finally block.

.. opcode:: STORE_MAP

   Store a key and value pair in a dictionary.  Pops the key and value while leaving
   the dictionary on the stack.

.. opcode:: LOAD_FAST (var_num)

   Pushes a reference to the local ``co_varnames[var_num]`` onto the stack.


.. opcode:: STORE_FAST (var_num)

   Stores TOS into the local ``co_varnames[var_num]``.


.. opcode:: DELETE_FAST (var_num)

   Deletes local ``co_varnames[var_num]``.


.. opcode:: LOAD_CLOSURE (i)

   Pushes a reference to the cell contained in slot *i* of the cell and free
   variable storage.  The name of the variable is  ``co_cellvars[i]`` if *i* is
   less than the length of *co_cellvars*.  Otherwise it is  ``co_freevars[i -
   len(co_cellvars)]``.


.. opcode:: LOAD_DEREF (i)

   Loads the cell contained in slot *i* of the cell and free variable storage.
   Pushes a reference to the object the cell contains on the stack.


.. opcode:: STORE_DEREF (i)

   Stores TOS into the cell contained in slot *i* of the cell and free variable
   storage.


.. opcode:: SET_LINENO (lineno)

   This opcode is obsolete.


.. opcode:: RAISE_VARARGS (argc)

   Raises an exception. *argc* indicates the number of parameters to the raise
   statement, ranging from 0 to 3.  The handler will find the traceback as TOS2,
   the parameter as TOS1, and the exception as TOS.


.. opcode:: CALL_FUNCTION (argc)

   Calls a function.  The low byte of *argc* indicates the number of positional
   parameters, the high byte the number of keyword parameters. On the stack, the
   opcode finds the keyword parameters first.  For each keyword argument, the value
   is on top of the key.  Below the keyword parameters, the positional parameters
   are on the stack, with the right-most parameter on top.  Below the parameters,
   the function object to call is on the stack.  Pops all function arguments, and
   the function itself off the stack, and pushes the return value.


.. opcode:: MAKE_FUNCTION (argc)

   Pushes a new function object on the stack.  TOS is the code associated with the
   function.  The function object is defined to have *argc* default parameters,
   which are found below TOS.


.. opcode:: MAKE_CLOSURE (argc)

   Creates a new function object, sets its *__closure__* slot, and pushes it on
   the stack.  TOS is the code associated with the function, TOS1 the tuple
   containing cells for the closure's free variables.  The function also has
   *argc* default parameters, which are found below the cells.


.. opcode:: BUILD_SLICE (argc)

   .. index:: builtin: slice

   Pushes a slice object on the stack.  *argc* must be 2 or 3.  If it is 2,
   ``slice(TOS1, TOS)`` is pushed; if it is 3, ``slice(TOS2, TOS1, TOS)`` is
   pushed. See the :func:`slice` built-in function for more information.


.. opcode:: EXTENDED_ARG (ext)

   Prefixes any opcode which has an argument too big to fit into the default two
   bytes.  *ext* holds two additional bytes which, taken together with the
   subsequent opcode's argument, comprise a four-byte argument, *ext* being the two
   most-significant bytes.


.. opcode:: CALL_FUNCTION_VAR (argc)

   Calls a function. *argc* is interpreted as in ``CALL_FUNCTION``. The top element
   on the stack contains the variable argument list, followed by keyword and
   positional arguments.


.. opcode:: CALL_FUNCTION_KW (argc)

   Calls a function. *argc* is interpreted as in ``CALL_FUNCTION``. The top element
   on the stack contains the keyword arguments dictionary,  followed by explicit
   keyword and positional arguments.


.. opcode:: CALL_FUNCTION_VAR_KW (argc)

   Calls a function. *argc* is interpreted as in ``CALL_FUNCTION``.  The top
   element on the stack contains the keyword arguments dictionary, followed by the
   variable-arguments tuple, followed by explicit keyword and positional arguments.


.. opcode:: HAVE_ARGUMENT

   This is not really an opcode.  It identifies the dividing line between opcodes
   which don't take arguments ``< HAVE_ARGUMENT`` and those which do ``>=
   HAVE_ARGUMENT``.