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cpython/Doc/library/dis.rst
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: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:: code_info(x)
Return a formatted multi-line string with detailed code object information
for the supplied function, method, source code string or code object.
Note that the exact contents of code info strings are highly implementation
dependent and they may change arbitrarily across Python VMs or Python
releases.
.. versionadded:: 3.2
.. function:: show_code(x)
Print detailed code object information for the supplied function, method,
source code string or code object to stdout.
This is a convenient shorthand for ``print(code_info(x))``, intended for
interactive exploration at the interpreter prompt.
.. versionadded:: 3.2
.. 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 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.
**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:: DUP_TOP
Duplicates the reference on top of the stack.
.. opcode:: DUP_TOP_TWO
Duplicates the two references on top of the stack, leaving them in the
same order.
**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:: 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:: DELETE_DEREF (i)
Empties the cell contained in slot *i* of the cell and free variable storage.
Used by the :keyword:`del` statement.
.. 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``.
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