31.12. dis — Disassembler for Python bytecode

The dis module supports the analysis of Python 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 Include/opcode.h and used by the compiler and the interpreter.

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.opname

Sequence of operation names, indexable using the bytecode.

dis.opmap

Dictionary mapping bytecodes to operation names.

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.

31.12.1. 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_STACK.

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()

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

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.

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_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.

JUMP_IF_TRUE(delta)

If TOS is true, increment the bytecode counter by delta. TOS is left on the stack.

JUMP_IF_FALSE(delta)

If TOS is false, increment the bytecode counter by delta. TOS is not changed.

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 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 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 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.