17.10.1 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.

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.

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 & 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.

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[: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 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.

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.

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.

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_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 zero: Pushes a new 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 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 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_LINENO 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 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.

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 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.

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