Doc/library/dis.rst

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

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

**Source code:** :source:`Lib/dis.py`

--------------

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([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.


.. function:: distb([tb])

   Disassembles 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])

   Disassembles 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:: disco(code[, lasti])

   A synonym for :func:`disassemble`.  It is more convenient to type, and kept
   for compatibility with earlier Python releases.


.. 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 operation names to bytecodes.


.. 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.


.. 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 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_CONVERT ()

   Implements ``TOS = `TOS```.


.. opcode:: UNARY_INVERT ()

   Implements ``TOS = ~TOS``.


.. opcode:: GET_ITER ()

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

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_DIVIDE ()

   Implements ``TOS = TOS1 / TOS`` when ``from __future__ import division`` is
   not in effect.


.. opcode:: BINARY_FLOOR_DIVIDE ()

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


.. opcode:: BINARY_TRUE_DIVIDE ()

   Implements ``TOS = TOS1 / TOS`` when ``from __future__ import division`` is
   in effect.


.. 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 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_DIVIDE ()

   Implements in-place ``TOS = TOS1 / TOS`` when ``from __future__ import
   division`` is not in effect.


.. opcode:: INPLACE_FLOOR_DIVIDE ()

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


.. opcode:: INPLACE_TRUE_DIVIDE ()

   Implements in-place ``TOS = TOS1 / TOS`` when ``from __future__ import
   division`` is in effect.


.. 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``.

The slice opcodes take up to three parameters.


.. opcode:: SLICE+0 ()

   Implements ``TOS = TOS[:]``.


.. opcode:: SLICE+1 ()

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


.. opcode:: SLICE+2 ()

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


.. opcode:: SLICE+3 ()

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

Slice assignment needs even an additional parameter.  As any statement, they put
nothing on the stack.


.. opcode:: STORE_SLICE+0 ()

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


.. opcode:: STORE_SLICE+1 ()

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


.. opcode:: STORE_SLICE+2 ()

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


.. opcode:: STORE_SLICE+3 ()

   Implements ``TOS2[TOS1:TOS] = TOS3``.


.. opcode:: DELETE_SLICE+0 ()

   Implements ``del TOS[:]``.


.. opcode:: DELETE_SLICE+1 ()

   Implements ``del TOS1[TOS:]``.


.. opcode:: DELETE_SLICE+2 ()

   Implements ``del TOS1[:TOS]``.


.. opcode:: DELETE_SLICE+3 ()

   Implements ``del TOS2[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 :opcode:`POP_TOP`.


.. opcode:: PRINT_ITEM ()

   Prints TOS to the file-like object bound to ``sys.stdout``.  There is one
   such instruction for each item in the :keyword:`print` statement.


.. opcode:: PRINT_ITEM_TO ()

   Like ``PRINT_ITEM``, but prints the item second from TOS to the file-like
   object at TOS.  This is used by the extended print statement.


.. opcode:: PRINT_NEWLINE ()

   Prints a new line on ``sys.stdout``.  This is generated as the last operation
   of a :keyword:`print` statement, unless the statement ends with a comma.


.. opcode:: PRINT_NEWLINE_TO ()

   Like ``PRINT_NEWLINE``, but prints the new line on the file-like object on
   the TOS.  This is used by the extended print statement.


.. 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 :opcode:`FOR_ITER` instruction).


.. opcode:: LIST_APPEND (i)

   Calls ``list.append(TOS[-i], TOS)``.  Used to implement list comprehensions.
   While the appended value is popped off, the list object remains on the stack
   so that it is available for further iterations of the loop.


.. opcode:: LOAD_LOCALS ()

   Pushes a reference to the locals of the current scope on the stack. This is
   used in the code for a class definition: After the class body is evaluated,
   the locals are passed to the class definition.


.. 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:: EXEC_STMT ()

   Implements ``exec TOS2,TOS1,TOS``.  The compiler fills missing optional
   parameters with ``None``.


.. 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:: 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:: BUILD_CLASS ()

   Creates a new class object.  TOS is the methods dictionary, TOS1 the tuple of
   the names of the base classes, and TOS2 the class name.


.. 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.  On top of
   the stack are 1--3 values indicating how/why the finally clause was entered:

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

   Under them is EXIT, the context manager's :meth:`__exit__` bound method.

   In the last case, ``EXIT(TOP, SECOND, THIRD)`` is called, otherwise
   ``EXIT(None, None, None)``.

   EXIT is removed from the stack, leaving the values above it in the same
   order. In addition, if the stack represents an exception, *and* the function
   call returns a 'true' value, this information is "zapped", to prevent
   ``END_FINALLY`` from re-raising the exception.  (But non-local gotos should
   still be resumed.)

   .. XXX explain the WHY stuff!


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:: 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_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 bytecode
   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 *func_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 :opcode:`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 :opcode:`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 :opcode:`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``.