30.12. dis — Disassembler for Python bytecode
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(x=None)
- Disassemble the x object. x 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=None)
- 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=-1)
-
dis.disco(code, lasti=-1)
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.
-
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 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.
30.12.1. 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_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_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 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.
-
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.
-
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.
-
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.
-
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 __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.