3.18.1 Python Byte Code Instructions

The Python compiler currently generates the following byte code instructions.

STOP_CODE (byte code instruction): Indicates end-of-code to the compiler, not used by the interpreter.

POP_TOP (byte code instruction): Removes the top-of-stack (TOS) item.

ROT_TWO (byte code instruction): Swaps the two top-most stack items.

ROT_THREE (byte code instruction): Lifts second and third stack item one position up, moves top down to position three.

DUP_TOP (byte code instruction): 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 (byte code instruction): Implements TOS = +TOS.

UNARY_NEG (byte code instruction): Implements TOS = -TOS.

UNARY_NOT (byte code instruction): Implements TOS = not TOS.

UNARY_CONVERT (byte code instruction): Implements TOS = `TOS`.

UNARY_INVERT (byte code instruction): 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 (byte code instruction): Implements TOS = TOS1 ** TOS.

BINARY_MULTIPLY (byte code instruction): Implements TOS = TOS1 * TOS.

BINARY_DIVIDE (byte code instruction): Implements TOS = TOS1 / TOS.

BINARY_MODULO (byte code instruction): Implements TOS = TOS1 % TOS.

BINARY_ADD (byte code instruction): Implements TOS = TOS1 + TOS.

BINARY_SUBTRACT (byte code instruction): Implements TOS = TOS1 - TOS.

BINARY_SUBSCR (byte code instruction): Implements TOS = TOS1[TOS].

BINARY_LSHIFT (byte code instruction): Implements TOS = TOS1 << TOS.

BINARY_RSHIFT (byte code instruction): Implements TOS = TOS1 >> TOS.

BINARY_AND (byte code instruction): Implements TOS = TOS1 and TOS.

BINARY_XOR (byte code instruction): Implements TOS = TOS1 ^ TOS.

BINARY_OR (byte code instruction): Implements TOS = TOS1 or TOS.

The slice opcodes take up to three parameters.

SLICE+0 (byte code instruction): Implements TOS = TOS[:].

SLICE+1 (byte code instruction): Implements TOS = TOS1[TOS:].

SLICE+2 (byte code instruction): Implements TOS = TOS1[:TOS1].

SLICE+3 (byte code instruction): Implements TOS = TOS2[TOS1:TOS].

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

STORE_SLICE+0 (byte code instruction): Implements TOS[:] = TOS1.

STORE_SLICE+1 (byte code instruction): Implements TOS1[TOS:] = TOS2.

STORE_SLICE+2 (byte code instruction): Implements TOS1[:TOS] = TOS2.

STORE_SLICE+3 (byte code instruction): Implements TOS2[TOS1:TOS] = TOS3.

DELETE_SLICE+0 (byte code instruction): Implements del TOS[:].

DELETE_SLICE+1 (byte code instruction): Implements del TOS1[TOS:].

DELETE_SLICE+2 (byte code instruction): Implements del TOS1[:TOS].

DELETE_SLICE+3 (byte code instruction): Implements del TOS2[TOS1:TOS].

STORE_SUBSCR (byte code instruction): Implements TOS1[TOS] = TOS2.

DELETE_SUBSCR (byte code instruction): Implements del TOS1[TOS].

PRINT_EXPR (byte code instruction): 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 (byte code instruction): Prints TOS. There is one such instruction for each item in the print statement.

PRINT_NEWLINE (byte code instruction): 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 (byte code instruction): Terminates a loop due to a break statement.

LOAD_LOCALS (byte code instruction): 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 (byte code instruction): Returns with TOS to the caller of the function.

EXEC_STMT (byte code instruction): Implements exec TOS2,TOS1,TOS. The compiler fills missing optional parameters with None.

POP_BLOCK (byte code instruction): Removes one block from the block stack. Per frame, there is a stack of blocks, denoting nested loops, try statements, and such.

END_FINALLY (byte code instruction): 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 (byte code instruction): 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 (byte code instruction) 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 (byte code instruction) namei: Implements del name, where namei is the index into co_names attribute of the code object.

UNPACK_TUPLE (byte code instruction) count: Unpacks TOS into count individual values, which are put onto the stack right-to-left.

UNPACK_LIST (byte code instruction) count: Unpacks TOS into count individual values.

STORE_ATTR (byte code instruction) namei: Implements TOS.name = TOS1, where namei is the index of name in co_names.

DELETE_ATTR (byte code instruction) namei: Implements del TOS.name, using namei as index into co_names.

STORE_GLOBAL (byte code instruction) namei: Works as STORE_NAME, but stores the name as a global.

DELETE_GLOBAL (byte code instruction) namei: Works as DELETE_NAME, but deletes a global name.

LOAD_CONST (byte code instruction) consti: Pushes co_consts[consti] onto the stack.

LOAD_NAME (byte code instruction) namei: Pushes the value associated with co_names[namei] onto the stack.

BUILD_TUPLE (byte code instruction) count: Creates a tuple consuming count items from the stack, and pushes the resulting tuple onto the stack.

BUILD_LIST (byte code instruction) count: Works as BUILD_TUPLE, but creates a list.

BUILD_MAP (byte code instruction) zero: Pushes an empty dictionary object onto the stack. The argument is ignored and set to zero by the compiler.

LOAD_ATTR (byte code instruction) namei: Replaces TOS with getattr(TOS,co_names[namei].

COMPARE_OP (byte code instruction) opname: Performs a boolean operation. The operation name can be found in cmp_op[opname].

IMPORT_NAME (byte code instruction) namei: Imports the module co_names[namei]. The module object is pushed onto the stack. The current name space is not affect: for a proper import statement, a subsequent STORE_FAST instruction modifies the name space.

IMPORT_FROM (byte code instruction) namei: Imports the attribute co_names[namei]. The module to import from is found in TOS and left there.

JUMP_FORWARD (byte code instruction) delta: Increments byte code counter by delta.

JUMP_IF_TRUE (byte code instruction) delta: If TOS is true, increment the byte code counter by delta. TOS is left on the stack.

JUMP_IF_FALSE (byte code instruction) delta: If TOS is false, increment the byte code counter by delta. TOS is not changed.

JUMP_ABSOLUTE (byte code instruction) target: Set byte code counter to target.

FOR_LOOP (byte code instruction) 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 (byte code instruction) namei: Loads the global named co_names[namei] onto the stack.

SETUP_LOOP (byte code instruction) 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 (byte code instruction) delta: Pushes a try block from a try-except clause onto the block stack. delta points to the first except block.

SETUP_FINALLY (byte code instruction) delta: Pushes a try block from a try-except clause onto the block stack. delta points to the finally block.

LOAD_FAST (byte code instruction) var_num: Pushes a reference to the local co_varnames[var_num] onto the stack.

STORE_FAST (byte code instruction) var_num: Stores TOS into the local co_varnames[var_num].

DELETE_FAST (byte code instruction) var_num: Deletes local co_varnames[var_num].

SET_LINE_NO (byte code instruction) lineno: Sets the current line number to lineno.

RAISE_VARARGS (byte code instruction) 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 (byte code instruction) 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 (byte code instruction) 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 (byte code instruction) argc: Pushes a slice object on the stack. If argc is three, creates TOS3[TOS2:TOS1:TOS]. Otherwise, expects three arguments.

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