32.12. "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.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.

dis.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.

dis.disassemble(code[, lasti])

   Disassembles 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.disco(code[, lasti])

   A synonym for "disassemble()".  It is more convenient to type, and
   kept for compatibility with earlier Python releases.

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 access a constant.

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.


32.12.1. Python Bytecode Instructions
=====================================

The Python compiler currently generates the following bytecode
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.

ROT_FOUR()

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

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.

UNARY_POSITIVE()

   Implements "TOS = +TOS".

UNARY_NEGATIVE()

   Implements "TOS = -TOS".

UNARY_NOT()

   Implements "TOS = not TOS".

UNARY_CONVERT()

   Implements "TOS = `TOS`".

UNARY_INVERT()

   Implements "TOS = ~TOS".

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.

BINARY_POWER()

   Implements "TOS = TOS1 ** TOS".

BINARY_MULTIPLY()

   Implements "TOS = TOS1 * TOS".

BINARY_DIVIDE()

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

BINARY_FLOOR_DIVIDE()

   Implements "TOS = TOS1 // TOS".

BINARY_TRUE_DIVIDE()

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

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 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_DIVIDE()

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

INPLACE_FLOOR_DIVIDE()

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

INPLACE_TRUE_DIVIDE()

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

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".

The slice opcodes take up to three parameters.

SLICE+0()

   Implements "TOS = TOS[:]".

SLICE+1()

   Implements "TOS = TOS1[TOS:]".

SLICE+2()

   Implements "TOS = TOS1[:TOS]".

SLICE+3()

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

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

STORE_SLICE+0()

   Implements "TOS[:] = TOS1".

STORE_SLICE+1()

   Implements "TOS1[TOS:] = TOS2".

STORE_SLICE+2()

   Implements "TOS1[:TOS] = TOS2".

STORE_SLICE+3()

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

DELETE_SLICE+0()

   Implements "del TOS[:]".

DELETE_SLICE+1()

   Implements "del TOS1[TOS:]".

DELETE_SLICE+2()

   Implements "del TOS1[:TOS]".

DELETE_SLICE+3()

   Implements "del TOS2[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_TOP".

PRINT_ITEM()

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

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.

PRINT_NEWLINE()

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

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.

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).

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.

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.

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 *".

EXEC_STMT()

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

POP_BLOCK()

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

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.

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.

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.  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 "__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.)

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.

DUP_TOPX(count)

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

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.

   New in version 2.7.

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
   bytecode 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.

SET_LINENO(lineno)

   This opcode is obsolete.

RAISE_VARARGS(argc)

   Raises an exception. *argc* indicates the number of arguments 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 callable object.  The low byte of *argc* indicates the
   number of positional arguments, the high byte the number of keyword
   arguments. The stack contains keyword arguments on top (if any),
   then the positional arguments below that (if any), then the
   callable object to call below that. Each keyword argument is
   represented with two values on the stack: the argument’s name, and
   its value, with the argument’s value above the name on the stack.
   The positional arguments are pushed in the order that they are
   passed in to the callable object, with the right-most positional
   argument on top. "CALL_FUNCTION" pops all arguments and the
   callable object off the stack, calls the callable object with those
   arguments, and pushes the return value returned by the callable
   object.

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 *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.

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 callable object, similarly to "CALL_FUNCTION". *argc*
   represents the number of keyword and positional arguments,
   identically to "CALL_FUNCTION". The top of the stack contains an
   iterable object containing additional positional arguments. Below
   that are keyword arguments (if any), positional arguments (if any)
   and a callable object, identically to "CALL_FUNCTION". Before the
   callable object is called, the iterable object is “unpacked” and
   its contents are appended to the positional arguments passed in.
   The iterable object is ignored when computing the value of "argc".

CALL_FUNCTION_KW(argc)

   Calls a callable object, similarly to "CALL_FUNCTION". *argc*
   represents the number of keyword and positional arguments,
   identically to "CALL_FUNCTION". The top of the stack contains a
   mapping object containing additional keyword arguments. Below that
   are keyword arguments (if any), positional arguments (if any) and a
   callable object, identically to "CALL_FUNCTION". Before the
   callable is called, the mapping object at the top of the stack is
   “unpacked” and its contents are appended to the keyword arguments
   passed in. The mapping object at the top of the stack is ignored
   when computing the value of "argc".

CALL_FUNCTION_VAR_KW(argc)

   Calls a callable object, similarly to "CALL_FUNCTION_VAR" and
   "CALL_FUNCTION_KW". *argc* represents the number of keyword and
   positional arguments, identically to "CALL_FUNCTION". The top of
   the stack contains a mapping object, as per "CALL_FUNCTION_KW".
   Below that is an iterable object, as per "CALL_FUNCTION_VAR". Below
   that are keyword arguments (if any), positional arguments (if any)
   and a callable object, identically to "CALL_FUNCTION". Before the
   callable is called, the mapping object and iterable object are each
   “unpacked” and their contents passed in as keyword and positional
   arguments respectively, identically to "CALL_FUNCTION_VAR" and
   "CALL_FUNCTION_KW". The mapping object and iterable object are both
   ignored when computing the value of "argc".

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".
