Python Byte Code Instructions

The Python compiler currently generates the following byte code instructions.

STOP_CODE

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

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.

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_NEG

Implements TOS = -TOS.

UNARY_NOT

Implements TOS = not TOS.

UNARY_CONVERT

Implements TOS = `TOS`.

UNARY_INVERT

Implements TOS = 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.

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 and TOS.

BINARY_XOR

Implements TOS = TOS1 ^ TOS.

BINARY_OR

Implements TOS = TOS1 or 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[:TOS1].

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].

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. There is one such instruction for each item in the 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.

BREAK_LOOP

Terminates a loop due to a break statement.

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.

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-block. 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.

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_LOCAL 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_TUPLE    count

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

UNPACK_LIST    count

Unpacks TOS into count individual values.

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    zero

Pushes an empty dictionary object onto the stack. The argument is ignored and set to zero by the compiler.

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]. The module object is pushed onto the stack. The current name space is not affected: for a proper import statement, a subsequent STORE_FAST instruction modifies the name space.

IMPORT_FROM    namei

Imports the attribute co_names[namei]. The module to import from is found in TOS and left there.

JUMP_FORWARD    delta

Increments byte code counter by delta.

JUMP_IF_TRUE    delta

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

JUMP_IF_FALSE    delta

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

JUMP_ABSOLUTE    target

Set byte code counter to target.

FOR_LOOP    delta

Iterate over a sequence. TOS is the current index, TOS1 the sequence. First, the next element is computed. If the sequence is exhausted, increment byte code counter by delta. Otherwise, push the sequence, the incremented counter, and the current item onto the stack.

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.

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].

SET_LINE_NO    lineno

Sets the current line number to lineno.

RAISE_VARARGS    argc

Raises an exception. argc indicates the number of parameters to the raise statement, ranging from 1 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.

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.

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.