Given a python .pyc file, is there a tool that let me view the bytecode?

Question

A Python module is automatically compiled into a .pyc file by CPython interpreter. The .pyc file, which contains the bytecode, is in binary format (marshaled code?). Is there a GUI (or command line) tool that let me view the bytecode?

Answer 1

Every *.pyc file is a binary file containing next things:

• a four-byte magic number - it's simply bytes that change with each change to the marshalling code;
• a four-byte modification timestamp - is the Unix modification timestamp of the source file that generated the .pyc, so that it can be recompiled if the source changes;
• since version Python3.3+ next four bytes is field that encodes the size of the source file as a long;
• a marshalled code object.

Why not just use the CPython`s built-in features for this task?

---

A file view_pyc_file.py

import platform
import time
import sys
import binascii
import marshal
import dis
import struct


def view_pyc_file(path):
    """Read and display a content of the Python`s bytecode in a pyc-file."""

    file = open(path, 'rb')

    magic = file.read(4)
    timestamp = file.read(4)
    size = None

    if sys.version_info.major == 3 and sys.version_info.minor >= 3:
        size = file.read(4)
        size = struct.unpack('I', size)[0]

    code = marshal.load(file)

    magic = binascii.hexlify(magic).decode('utf-8')
    timestamp = time.asctime(time.localtime(struct.unpack('I', timestamp)[0]))

    dis.disassemble(code)

    print('-' * 80)
    print(
        'Python version: {}\nMagic code: {}\nTimestamp: {}\nSize: {}'
        .format(platform.python_version(), magic, timestamp, size)
    )

    file.close()


if __name__ == '__main__':
    view_pyc_file(sys.argv[1])

Tested with next CPython`s versions:

• 2.7.9
• 3.4.2
• 3.5.2

---

Demonstration

Content of a file main.py

$ cat main.py
print("Never give up")

Create and to read pyc-file by the python2.7

setivolkylany$~/Downloads/temp/temp$ python2.7 -m py_compile main.py 
setivolkylany$~/Downloads/temp/temp$ python2.7 view_pyc_file.py ./main.pyc
  1           0 LOAD_CONST               0 ('Never give up')
              3 PRINT_ITEM          
              4 PRINT_NEWLINE       
              5 LOAD_CONST               1 (None)
              8 RETURN_VALUE        
--------------------------------------------------------------------------------
Python version: 2.7.9
Magic code: 03f30d0a
Timestamp: Fri Mar 10 15:08:20 2017
Size: None

Create and to read pyc-file by the python3.4

setivolkylany$~/Downloads/temp/temp$ python3.4 -m py_compile main.py 
setivolkylany$~/Downloads/temp/temp$ python3.4 view_pyc_file.py __pycache__/main.cpython-34.pyc 
  1           0 LOAD_NAME                0 (print)
              3 LOAD_CONST               0 ('Never give up')
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
              9 POP_TOP
             10 LOAD_CONST               1 (None)
             13 RETURN_VALUE
--------------------------------------------------------------------------------
Python version: 3.4.2
Magic code: ee0c0d0a
Timestamp: Fri Mar 10 15:08:20 2017
Size: 23

Create and to read pyc-file by the python3.5

setivolkylany$~/Downloads/temp/temp$ python3.5 -m py_compile main.py 
setivolkylany$~/Downloads/temp/temp$ python3.5 view_pyc_file.py __pycache__/main.cpython-35.pyc 
  1           0 LOAD_NAME                0 (print)
              3 LOAD_CONST               0 ('Never give up')
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
              9 POP_TOP
             10 LOAD_CONST               1 (None)
             13 RETURN_VALUE
--------------------------------------------------------------------------------
Python version: 3.5.2
Magic code: 160d0d0a
Timestamp: Fri Mar 10 15:08:20 2017
Size: 23

---

Based on:

• How can I understand a .pyc file content
• https://gist.github.com/anonymous/35c08092a6eb70cdd723
• https://nedbatchelder.com/blog/200804/the_structure_of_pyc_files.html

Answer 2

Extending the code form @PADYMKO based on @Apteryx 's note about the PEP:

def view_pyc_file(path):
    """Read and display a content of the Python`s bytecode in a pyc-file."""

    with open(path, 'rb') as file:

        magic = file.read(4)
        bit_field = None
        timestamp = None
        hashstr = None
        size = None

        if sys.version_info.major == 3 and sys.version_info.minor >= 7:
            bit_field = int.from_bytes(file.read(4), byteorder=sys.byteorder)
            if 1 & bit_field == 1:
                hashstr = file.read(8)
            else:
                timestamp = file.read(4)
                size = file.read(4)
                size = struct.unpack('I', size)[0]
        elif sys.version_info.major == 3 and sys.version_info.minor >= 3:
            timestamp = file.read(4)
            size = file.read(4)
            size = struct.unpack('I', size)[0]
        else:
            timestamp = file.read(4)

        code = marshal.load(file)

    magic = binascii.hexlify(magic).decode('utf-8')
    timestamp = time.asctime(time.localtime(struct.unpack('I', timestamp)[0]))

    dis.disassemble(code)

    print('-' * 80)
    print(
        'Python version: {}\nMagic code: {}\nTimestamp: {}\nSize: {}\nHash: {}\nBitfield: {}'
        .format(platform.python_version(), magic, timestamp, size, hashstr, bit_field)
    )

Answer 3

There's a visual python disassembler called PyChrisanthemum.

To do it the command-line way you can use module dis (python 2.7.3, python 3.2.3), as OP already found out.