Skip to main content

Python code object transformers

Project description

Bytecode transformers for CPython inspired by the ast module’s NodeTransformer.

CodeTransformer API


Just like the NodeTransformer, we write visit_* methods that define how we act on an instruction.

For example (taken from my lazy library):

def visit_UNARY_NOT(self, instr):
    Replace the `not` operator to act on the values that the thunks
    This makes `not` lazy.
    yield self.LOAD_CONST(_lazy_not).steal(instr)
    # TOS = _lazy_not
    # TOS1 = arg

    yield Instruction(ops.ROT_TWO)
    # TOS = arg
    # TOS1 = _lazy_not

    yield Instruction(ops.CALL_FUNCTION, 1)
    # TOS = _lazy_not(arg)

This visitor is applied to a unary not instruction (not a) and replaces it with code that is like: _lazy_not(a)

These methods will act on any opcode.

These methods are passed an Instruction object as the argument.


Code objects also have some data other than their bytecode. We can act on these things as well.

These methods are passed the type that occupied the given field.

  1. visit_name: A transformer for the co_names field.

  2. visit_varname: A transformer for the co_varnames field.

  3. visit_freevar: A transformer for the co_freevars field.

  4. visit_cellvar: A transformer for the co_cellvars field.

  5. visit_default: A transformer for the co_defaults field.

  6. visit_const: A transformer for the co_consts field.

A note about visit_const: One should be sure to call super().visit_const(const) inside of their definiton to recursivly apply your transformer to nested code objects.


One of the best uses of a bytecode transform is to make something available at runtime without putting a name in the namespace. We can do this by putting a new entry in the co_consts.

The const_index function accepts the value you want to put into the consts and returns the index as an int. This will create a new entry if needed.

The LOAD_CONST method of a CodeTransformer is a shortcut that returns a LOAD_CONST instruction object with the argument as the index of the object passed.


steal is a method of the Instruction object that steals the jump target of another instruction. For example, if an instruction a is jumping to instruction b and instruction c steals b, then a will jump to b. This is useful when you are replacing an instruction with a transformer but want to preserve jumps.

Project details

Download files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

Source Distribution

codetransformer-0.2.0.tar.gz (4.6 kB view hashes)

Uploaded Source

Supported by

AWS AWS Cloud computing and Security Sponsor Datadog Datadog Monitoring Fastly Fastly CDN Google Google Download Analytics Microsoft Microsoft PSF Sponsor Pingdom Pingdom Monitoring Sentry Sentry Error logging StatusPage StatusPage Status page