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

===============================================

.. module:: dis

   :synopsis: Disassembler for Python bytecode.

The :mod:`dis` module supports the analysis of Python :term:`bytecode` by disassembling

it.  Since there is no Python assembler, this module defines the Python assembly

language.  The Python 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.

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 disassemble.  It is more convenient to type, and kept for

   compatibility with earlier Python releases.

.. 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.

.. 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 ``POP_STACK``.

.. 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 ``FOR_ITER`` instruction).

.. opcode:: LIST_APPEND ()

   Calls ``list.append(TOS1, TOS)``.  Used to implement list comprehensions.

.. 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:: 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()