
``dis`` --- Disassembler for Python bytecode
********************************************

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

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

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

**CPython implementation 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 ``myfunc()``:

   def myfunc(alist):
       return len(alist)

the following command can be used to get the disassembly of
``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 ``dis`` module defines the following functions and constants:

dis.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.

   New in version 3.2.

dis.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.

   New in version 3.2.

dis.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 ``compile()`` built-in function before being disassembled.
   If no object is provided, this function disassembles the last
   traceback.

dis.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.

dis.disassemble(code, lasti=-1)
dis.disco(code, lasti=-1)

   Disassemble a code object, indicating the last instruction if
   *lasti* was provided.  The output is divided in the following
   columns:

   1. the line number, for the first instruction of each line

   2. the current instruction, indicated as ``-->``,

   3. a labelled instruction, indicated with ``>>``,

   4. the address of the instruction,

   5. the operation code name,

   6. operation parameters, and

   7. interpretation of the parameters in parentheses.

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

dis.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.

dis.findlabels(code)

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

dis.opname

   Sequence of operation names, indexable using the bytecode.

dis.opmap

   Dictionary mapping operation names to bytecodes.

dis.cmp_op

   Sequence of all compare operation names.

dis.hasconst

   Sequence of bytecodes that have a constant parameter.

dis.hasfree

   Sequence of bytecodes that access a free variable.

dis.hasname

   Sequence of bytecodes that access an attribute by name.

dis.hasjrel

   Sequence of bytecodes that have a relative jump target.

dis.hasjabs

   Sequence of bytecodes that have an absolute jump target.

dis.haslocal

   Sequence of bytecodes that access a local variable.

dis.hascompare

   Sequence of bytecodes of Boolean operations.


Python Bytecode Instructions
============================

The Python compiler currently generates the following bytecode
instructions.

**General instructions**

STOP_CODE

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

NOP

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

POP_TOP

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

ROT_TWO

   Swaps the two top-most stack items.

ROT_THREE

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

DUP_TOP

   Duplicates the reference on top of the stack.

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.

UNARY_POSITIVE

   Implements ``TOS = +TOS``.

UNARY_NEGATIVE

   Implements ``TOS = -TOS``.

UNARY_NOT

   Implements ``TOS = not TOS``.

UNARY_INVERT

   Implements ``TOS = ~TOS``.

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.

BINARY_POWER

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

BINARY_MULTIPLY

   Implements ``TOS = TOS1 * TOS``.

BINARY_FLOOR_DIVIDE

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

BINARY_TRUE_DIVIDE

   Implements ``TOS = TOS1 / TOS``.

BINARY_MODULO

   Implements ``TOS = TOS1 % TOS``.

BINARY_ADD

   Implements ``TOS = TOS1 + TOS``.

BINARY_SUBTRACT

   Implements ``TOS = TOS1 - TOS``.

BINARY_SUBSCR

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

BINARY_LSHIFT

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

BINARY_RSHIFT

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

BINARY_AND

   Implements ``TOS = TOS1 & TOS``.

BINARY_XOR

   Implements ``TOS = TOS1 ^ TOS``.

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.

INPLACE_POWER

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

INPLACE_MULTIPLY

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

INPLACE_FLOOR_DIVIDE

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

INPLACE_TRUE_DIVIDE

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

INPLACE_MODULO

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

INPLACE_ADD

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

INPLACE_SUBTRACT

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

INPLACE_LSHIFT

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

INPLACE_RSHIFT

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

INPLACE_AND

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

INPLACE_XOR

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

INPLACE_OR

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

STORE_SUBSCR

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

DELETE_SUBSCR

   Implements ``del TOS1[TOS]``.

**Miscellaneous opcodes**

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``.

BREAK_LOOP

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

CONTINUE_LOOP(target)

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

SET_ADD(i)

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

LIST_APPEND(i)

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

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.

RETURN_VALUE

   Returns with TOS to the caller of the function.

YIELD_VALUE

   Pops ``TOS`` and yields it from a *generator*.

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 *``.

POP_BLOCK

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

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.

END_FINALLY

   Terminates a ``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.

LOAD_BUILD_CLASS

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

SETUP_WITH(delta)

   This opcode performs several operations before a with block starts.
   First, it loads ``__exit__()`` from the context manager and pushes
   it onto the stack for later use by ``WITH_CLEANUP``.  Then,
   ``__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
   (``POP_TOP``), or store it in (a) variable(s) (``STORE_FAST``,
   ``STORE_NAME``, or ``UNPACK_SEQUENCE``).

WITH_CLEANUP

   Cleans up the stack when a ``with`` statement block exits.  TOS is
   the context manager's ``__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.)

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.

STORE_NAME(namei)

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

DELETE_NAME(namei)

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

UNPACK_SEQUENCE(count)

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

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.

STORE_ATTR(namei)

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

DELETE_ATTR(namei)

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

STORE_GLOBAL(namei)

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

DELETE_GLOBAL(namei)

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

LOAD_CONST(consti)

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

LOAD_NAME(namei)

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

BUILD_TUPLE(count)

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

BUILD_LIST(count)

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

BUILD_SET(count)

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

BUILD_MAP(count)

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

LOAD_ATTR(namei)

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

COMPARE_OP(opname)

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

IMPORT_NAME(namei)

   Imports the module ``co_names[namei]``.  TOS and TOS1 are popped
   and provide the *fromlist* and *level* arguments of
   ``__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.

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.

JUMP_FORWARD(delta)

   Increments bytecode counter by *delta*.

POP_JUMP_IF_TRUE(target)

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

POP_JUMP_IF_FALSE(target)

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

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.

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.

JUMP_ABSOLUTE(target)

   Set bytecode counter to *target*.

FOR_ITER(delta)

   ``TOS`` is an *iterator*.  Call its ``__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*.

LOAD_GLOBAL(namei)

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

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.

SETUP_EXCEPT(delta)

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

SETUP_FINALLY(delta)

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

STORE_MAP

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

LOAD_FAST(var_num)

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

STORE_FAST(var_num)

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

DELETE_FAST(var_num)

   Deletes local ``co_varnames[var_num]``.

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)]``.

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.

STORE_DEREF(i)

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

DELETE_DEREF(i)

   Empties the cell contained in slot *i* of the cell and free
   variable storage. Used by the ``del`` statement.

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.

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.

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.

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.

BUILD_SLICE(argc)

   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 ``slice()`` built-in function for
   more information.

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.

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.

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.

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.

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``.
