.PYC File Format – Code object

Fields

The fields are written in this order with no separators. An “i32” field is a little-endian 32-bit number with no type byte of its own; every other field is a marshal object.

There are no co_nlocals, co_varnames, co_cellvars or co_freevars fields in the file. Python rebuilds them from the two localsplus fields when it loads the file.

co_flags

Each bit is named by Python’s inspect module with the prefix CO_, or for a future feature in the __future__ module.

co_localspluskinds

Each byte describes the name at the same position in co_localsplusnames. Bits combine.

For example, this function compiled under Python 3.14 has seven names. Their kinds are the bytes 22 26 24 2A 2C 20 40:

def outer(a, /, b, *c, d=1, **e):
    x = a
    def inner():
        return x
    return inner

The nested function inner has one name, x, whose kind byte is 80, a free variable.

Worked example

Here is marshal.dumps(f) of a code object f, compiled under Python 3.14 from this function in t.py, starting at line 1.

def f(a):
    try:
        return a + 1
    except KeyError:
        return 0

The same code’s location table and exception table are decoded on their own pages.