Today I learned how to read the bytecode from a file of compiled Python bytecode (.pyc).

How to read bytecode from a .pyc file

If you have a .pyc file, you can use the modules dis and marshal from the standard library to get the corresponding bytecode:

import dis
import marshal

with open(path_to_pyc_file, "rb") as f:
    _ = f.read(16)  # Header is 16 bytes in 3.6+.
    # _ = f.read(8)  # Header is 8 bytes in <3.6.
    loaded = marshal.load(f)

dis.dis(loaded)

Example

Suppose that you have a file fibonacci.py with the following code:

def fibonacci(num):
    """Computes terms of the Fibonacci sequence."""
    if num <= 1:
        return 1
    return fibonacci(num - 1) + fibonacci(num - 2)

If you import your function fibonacci into the REPL or from another file, Python will compile the bytecode and write it to a .pyc file.

The quickest way to force Python to compile and dump the bytecode in a file is with this command:

❯ python -c "import fibonacci"

This will create a folder __pycache__ (if it doesn't exist yet) and write the bytecode to a file. Because I'm running Python 3.11 at the time of writing, the file that I got was fibonacci.cpython-311.pyc.

Now, to get the bytecode back from that file, I run the code from above, but I specify the path to the file fibonacci.cpython-311.pyc:

import dis
import marshal

with open("__pycache__/fibonacci.cpython-311.pyc", "rb") as f:
    _ = f.read(16)  # Header is 16 bytes in 3.6+.
    loaded = marshal.load(f)

dis.dis(loaded)

If you run the code above, you get the bytecode associated with the .pyc file you opened:

  0           0 RESUME                   0

  1           2 LOAD_CONST               0 (<code object fibonacci at 0x100f997d0, file "/Users/rodrigogs/Documents/tmp/fibonacci.py", line 1>)
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (fibonacci)
              8 LOAD_CONST               1 (None)
             10 RETURN_VALUE

Disassembly of <code object fibonacci at 0x100f997d0, file "/Users/rodrigogs/Documents/tmp/fibonacci.py", line 1>:
  1           0 RESUME                   0

  3           2 LOAD_FAST                0 (num)
              4 LOAD_CONST               1 (1)
              6 COMPARE_OP               1 (<=)
             12 POP_JUMP_FORWARD_IF_FALSE     2 (to 18)

  4          14 LOAD_CONST               1 (1)
             16 RETURN_VALUE

  5     >>   18 LOAD_GLOBAL              1 (NULL + fibonacci)
             30 LOAD_FAST                0 (num)
             32 LOAD_CONST               1 (1)
             34 BINARY_OP               10 (-)
             38 PRECALL                  1
             42 CALL                     1
             52 LOAD_GLOBAL              1 (NULL + fibonacci)
             64 LOAD_FAST                0 (num)
             66 LOAD_CONST               2 (2)
             68 BINARY_OP               10 (-)
             72 PRECALL                  1
             76 CALL                     1
             86 BINARY_OP                0 (+)
             90 RETURN_VALUE

That's it for now! Stay tuned and I'll see you around!

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References

• pycDcode, https://github.com/BarakAharoni/pycDcode [last accessed 25-07-2023];