Bytecode API

Constants

__version__

Module version string (ex: '0.1').

UNSET

Singleton used to mark the lack of value. It is different than None.

Functions

dump_bytecode(bytecode, \*, lineno=False)

Dump a bytecode to the standard output. ConcreteBytecode, Bytecode and ControlFlowGraph are accepted for bytecode.

If lineno is true, show also line numbers and instruction index/offset.

This function is written for debug purpose.

Instruction classes

Instr

class Instr(name: str, arg=UNSET, \*, lineno: int=None)

Abstract instruction.

The type of the arg parameter (and the arg attribute) depends on the operation:

  • If the operation has a jump argument (has_jump(), ex: JUMP_ABSOLUTE): arg must be a Label (if the instruction is used in Bytecode) or a BasicBlock (used in ControlFlowGraph).

  • If the operation has a cell or free argument (ex: LOAD_DEREF): arg must be a CellVar or FreeVar instance.

  • If the operation has a local variable (ex: LOAD_FAST): arg must be a variable name, type str.

  • If the operation has a constant argument (LOAD_CONST): arg must not be a Label or BasicBlock instance.

  • If the operation has a compare argument (COMPARE_OP): arg must be a Compare enum.

  • If the operation has no argument (ex: DUP_TOP), arg must not be set.

  • Otherwise (the operation has an argument, ex: CALL_FUNCTION), arg must be an integer (int) in the range 0..2,147,483,647.

To replace the operation name and the argument, the set() method must be used instead of modifying the name attribute and then the arg attribute. Otherwise, an exception is be raised if the previous operation requires an argument and the new operation has no argument (or the opposite).

Attributes:

arg

Argument value.

It can be UNSET if the instruction has no argument.

lineno

Line number (int >= 1), or None.

name

Operation name (str). Setting the name updates the opcode attibute.

opcode

Operation code (int). Setting the operation code updates the name attribute.

Changed in version 0.3: The op attribute was renamed to opcode.

Methods:

copy()

Create a copy of the instruction.

is_final() bool

Is the operation a final operation?

Final operations:

  • RETURN_VALUE

  • RAISE_VARARGS

  • BREAK_LOOP

  • CONTINUE_LOOP

  • unconditional jumps: is_uncond_jump()

has_jump() bool

Does the operation have a jump argument?

More general than is_cond_jump() and is_uncond_jump(), it includes other operations. Examples:

  • FOR_ITER

  • SETUP_EXCEPT

  • CONTINUE_LOOP

is_cond_jump() bool

Is the operation an conditional jump?

Conditional jumps:

  • JUMP_IF_FALSE_OR_POP

  • JUMP_IF_TRUE_OR_POP

  • POP_JUMP_IF_FALSE

  • POP_JUMP_IF_TRUE

is_uncond_jump() bool

Is the operation an unconditional jump?

Unconditional jumps:

  • JUMP_FORWARD

  • JUMP_ABSOLUTE

set(name: str, arg=UNSET)

Modify the instruction in-place: replace name and arg attributes, and update the opcode attribute.

Changed in version 0.3: The lineno parameter has been removed.

stack_effect(jump: bool = None) int

Operation effect on the stack size as computed by dis.stack_effect().

The jump argument takes one of three values. None (the default) requests the largest stack effect. This works fine with most instructions. True returns the stack effect for taken branches. False returns the stack effect for non-taken branches.

Changed in version 0.8: stack_method was changed from a property to a method in order to add the keyword argument jump.

pre_and_post_stack_effect(jump: Optional[bool] = None) Tuple[int, int]

Effect of the instruction on the stack before and after its execution.

The impact on the stack before the instruction reflects how many values from the stacks are used/popped. The impact on the stack after the instruction execution reflects how many values are pushed back on the stack. Those are deduced from dis.stack_effect() and manual analysis.

The jump argument has the same meaning as in Instr.stack_effect().

New in version 0.12.

ConcreteInstr

class ConcreteInstr(name: str, arg=UNSET, \*, lineno: int=None)

Concrete instruction Inherit from Instr.

If the operation requires an argument, arg must be an integer (int) in the range 0..2,147,483,647. Otherwise, arg must not by set.

Concrete instructions should only be used in ConcreteBytecode.

Attributes:

arg

Argument value: an integer (int) in the range 0..2,147,483,647, or UNSET. Setting the argument value can change the instruction size (size).

size

Read-only size of the instruction in bytes (int): between 1 byte (no agument) and 6 bytes (extended argument).

Static method:

static disassemble(code: bytes, offset: int) ConcreteInstr

Create a concrete instruction from a bytecode string.

Methods:

get_jump_target(instr_offset: int) int or None

Get the absolute target offset of a jump. Return None if the instruction is not a jump.

The instr_offset parameter is the offset of the instruction. It is required by relative jumps.

Note

Starting with Python 3.10, this quantity is expressed in term of instruction offset rather than byte offset, and is hence twice smaller than in 3.9 for identical code.

assemble() bytes

Assemble the instruction to a bytecode string.

Compare

class Compare

Enum for the argument of the COMPARE_OP instruction.

Equality test:

  • Compare.EQ (2): x == y

  • Compare.NE (3): x != y

  • Compare.IS (8): x is y

  • Compare.IS_NOT (9): x is not y

Inequality test:

  • Compare.LT (0): x < y

  • Compare.LE (1): x <= y

  • Compare.GT (4): x > y

  • Compare.GE (5): x >= y

Other tests:

  • Compare.IN (6): x in y

  • Compare.NOT_IN (7): x not in y

  • Compare.EXC_MATCH (10): used to compare exceptions in except: blocks

Label

class Label

Pseudo-instruction used as targets of jump instructions.

Label targets are “resolved” by Bytecode.to_concrete_bytecode.

Labels must only be used in Bytecode.

SetLineno

class SetLineno(lineno: int)

Pseudo-instruction to set the line number of following instructions.

lineno must be greater or equal than 1.

lineno

Line number (int), read-only attribute.

Bytecode classes

BaseBytecode

class BaseBytecode

Base class of bytecode classes.

Attributes:

argcount

Argument count (int), default: 0.

cellvars

Names of the cell variables (list of str), default: empty list.

docstring

Documentation string aka “docstring” (str), None, or UNSET. Default: UNSET.

If set, it is used by ConcreteBytecode.to_code() as the first constant of the created Python code object.

filename

Code filename (str), default: '<string>'.

first_lineno

First line number (int), default: 1.

flags

Flags (int).

freevars

List of free variable names (list of str), default: empty list.

posonlyargcount

Positional-only argument count (int), default: 0.

New in Python 3.8

kwonlyargcount

Keyword-only argument count (int), default: 0.

name

Code name (str), default: '<module>'.

Changed in version 0.3: Attribute kw_only_argcount renamed to kwonlyargcount.

Bytecode

class Bytecode

Abstract bytecode: list of abstract instructions (Instr). Inherit from BaseBytecode and list.

A bytecode must only contain objects of the 4 following types:

It is possible to use concrete instructions (ConcreteInstr), but abstract instructions are preferred.

Attributes:

argnames

List of the argument names (list of str), default: empty list.

Static methods:

static from_code(code) Bytecode

Create an abstract bytecode from a Python code object.

Methods:

legalize()

Check the validity of all the instruction and remove the SetLineno instances after updating the instructions.

to_concrete_bytecode(compute_jumps_passes: int = None) ConcreteBytecode

Convert to concrete bytecode with concrete instructions.

Resolve jump targets: replace abstract labels (Label) with concrete instruction offsets (relative or absolute, depending on the jump operation). It will also add EXTENDED_ARG prefixes to jump instructions to ensure that the target instructions can be reached.

If compute_jumps_passes is not None, it sets the upper limit for the number of passes that can be made to generate EXTENDED_ARG prefixes for jump instructions. If None then an internal default is used. The number of passes is, in theory, limited only by the number of input instructions, however a much smaller default is used because the algorithm converges quickly on most code. For example, running CPython 3.6.5 unittests on OS X 11.13 results in 264996 compiled methods, only one of which requires 5 passes, and none requiring more.

to_code(compute_jumps_passes: int = None, stacksize: int = None, *, check_pre_and_post: bool = True) types.CodeType

Convert to a Python code object.

It is based on to_concrete_bytecode() and so resolve jump targets.

compute_jumps_passes: see to_concrete_bytecode()

stacksize: see ConcreteBytecode.to_code()

check_pre_and_post: see ConcreteBytecode.to_code()

compute_stacksize(*, check_pre_and_post: bool = True) int

Compute the stacksize needed to execute the code. Will raise an exception if the bytecode is invalid.

This computation requires to build the control flow graph associated with the code.

check_pre_and_post Allows caller to disable checking for stack underflow

update_flags(is_async: bool = None)

Update the object flags by calling :py:func:infer_flags on itself.

ConcreteBytecode

class ConcreteBytecode

List of concrete instructions (ConcreteInstr). Inherit from BaseBytecode.

A concrete bytecode must only contain objects of the 2 following types:

Label and Instr must not be used in concrete bytecode.

Attributes:

consts

List of constants (list), default: empty list.

names

List of names (list of str), default: empty list.

varnames

List of variable names (list of str), default: empty list.

Static methods:

static from_code(code, \*, extended_arg=false) ConcreteBytecode

Create a concrete bytecode from a Python code object.

If extended_arg is true, create EXTENDED_ARG instructions. Otherwise, concrete instruction use extended argument (size of 6 bytes rather than 3 bytes).

Methods:

legalize()

Check the validity of all the instruction and remove the SetLineno instances after updating the instructions.

to_code(stacksize: int = None, *, check_pre_and_post: bool = True) types.CodeType

Convert to a Python code object.

On Python older than 3.6, raise an exception on negative line number delta.

stacksize Allows caller to explicitly specify a stacksize. If not specified a ControlFlowGraph is created internally in order to call ControlFlowGraph.compute_stacksize(). It’s cheaper to pass a value if the value is known.

check_pre_and_post Allows caller to disable checking for stack underflow

to_bytecode() Bytecode

Convert to abstract bytecode with abstract instructions.

compute_stacksize(*, check_pre_and_post: bool = True) int

Compute the stacksize needed to execute the code. Will raise an exception if the bytecode is invalid.

This computation requires to build the control flow graph associated with the code.

check_pre_and_post Allows caller to disable checking for stack underflow

update_flags(is_async: bool = None)

Update the object flags by calling :py:func:infer_flags on itself.

BasicBlock

class BasicBlock

Basic block. Inherit from list.

A basic block is a straight-line code sequence of abstract instructions (Instr) with no branches in except to the entry and no branches out except at the exit.

A block must only contain objects of the 3 following types:

It is possible to use concrete instructions (ConcreteInstr) in blocks, but abstract instructions (Instr) are preferred.

Only the last instruction can have a jump argument, and the jump argument must be a basic block (BasicBlock).

Labels (Label) must not be used in blocks.

Attributes:

next_block

Next basic block (BasicBlock), or None.

Methods:

legalize(first_lineno: int)

Check the validity of all the instruction and remove the SetLineno instances after updating the instructions. first_lineno specifies the line number to use for instruction without a set line number encountered before the first SetLineno instance.

get_jump()

Get the target block (BasicBlock) of the jump if the basic block ends with an instruction with a jump argument. Otherwise, return None.

ControlFlowGraph

class ControlFlowGraph

Control flow graph (CFG): list of basic blocks (BasicBlock). A basic block is a straight-line code sequence of abstract instructions (Instr) with no branches in except to the entry and no branches out except at the exit. Inherit from BaseBytecode.

Labels (Label) must not be used in blocks.

This class is not designed to emit code, but to analyze and modify existing code. Use Bytecode to emit code.

Attributes:

argnames

List of the argument names (list of str), default: empty list.

Methods:

static from_bytecode(bytecode: Bytecode) ControlFlowGraph

Convert a Bytecode object to a ControlFlowGraph object: convert labels to blocks.

Splits blocks after final instructions (Instr.is_final()) and after conditional jumps (Instr.is_cond_jump()).

legalize(first_lineno: int)

Legalize all the blocks of the CFG.

add_block(instructions=None) BasicBlock

Add a new basic block. Return the newly created basic block.

get_block_index(block: BasicBlock) int

Get the index of a block in the bytecode.

Raise a ValueError if the block is not part of the bytecode.

New in version 0.3.

split_block(block: BasicBlock, index: int) BasicBlock

Split a block into two blocks at the specific instruction. Return the newly created block, or block if index equals 0.

to_bytecode() Bytecode

Convert to a bytecode object using labels.

compute_stacksize(*, check_pre_and_post: bool = True) int

Compute the stack size required by a bytecode object. Will raise an exception if the bytecode is invalid.

check_pre_and_post Allows caller to disable checking for stack underflow

update_flags(is_async: bool = None)

Update the object flags by calling :py:func:infer_flags on itself.

to_code(stacksize: int = None, *, check_pre_and_post: bool = True)

Convert to a Python code object. Refer to descriptions of Bytecode.to_code() and ConcreteBytecode.to_code().

check_pre_and_post Allows caller to disable checking for stack underflow

Cell and Free Variables

CellVar

class CellVar(name: str)

Cell variable used for instruction argument by operations taking a cell or free variable name.

Attributes:

name

Name of the cell variable (str).

FreeVar

class FreeVar(name: str)

Free variable used for instruction argument by operations taking a cell or free variable name.

Attributes:

name

Name of the free variable (str).

Line Numbers

The line number can set directly on an instruction using the lineno parameter of the constructor. Otherwise, the line number if inherited from the previous instruction, starting at first_lineno of the bytecode.

SetLineno pseudo-instruction can be used to set the line number of following instructions.

Compiler Flags

class CompilerFlags
OPTIMIZED

Set if a code object only uses fast locals

NEWLOCALS

Set if the code execution should be done with a new local scope

VARARGS

Set if a code object expects variable number of positional arguments

VARKEYWORDS

Set if a code object expects variable number of keyword arguments

NESTED

Set if a code object correspond to function defined in another function

GENERATOR

Set if a code object is a generator (contains yield instructions)

NOFREE

Set if a code object does not use free variables

COROUTINE

Set if a code object is a coroutine. New in Python 3.5

ITERABLE_COROUTINE

Set if a code object is an iterable coroutine. New in Python 3.5

ASYNC_GENERATOR

Set if a code object is an asynchronous generator. New in Python 3.6

FUTURE_GENERATOR_STOP

Set if a code object is defined in a context in which generator_stop has been imported from __future__

infer_flags(bytecode, async: bool = None) CompilerFlags

Infer the correct values for the compiler flags for a given bytecode based on the instructions. The flags that can be inferred are :

  • OPTIMIZED

  • GENRATOR

  • NOFREE

  • COROUTINE

  • ASYNC_GENERATOR

Force the code to be marked as asynchronous if True, prevent it from being marked as asynchronous if False and simply infer the best solution based on the opcode and the existing flag if None.