Python Decompiler Online

What is a PYC File?

PYC stands for Python Compiled. When Python imports a module, the interpreter compiles the source code (.py file) into bytecode and caches the result as a .pyc file. This cached bytecode allows subsequent imports to skip the compilation step, improving startup time.

In Python 3, .pyc files are stored in __pycache__ directories with filenames that include the Python version (e.g., module.cpython-39.pyc). In Python 2, .pyc files were placed alongside the .py source files.

PYC File Structure

A PYC file contains:

• Magic number — identifies the Python version that generated the bytecode (each Python version uses a different magic number)
• Timestamp / hash — used for cache invalidation (timestamp-based in Python < 3.7, hash-based option in 3.7+)
• Source size — the size of the original .py file (Python 3.2+)
• Code object — marshalled Python code object containing bytecode, constants, variable names, and metadata

PYC vs PYO Files

PYO (Python Optimized) files were created when running Python with the -O flag, which removed assert statements and __debug__ code. In Python 3.5 (PEP 488), PYO files were deprecated and replaced by PYC files with optimization level suffixes like .cpython-39.opt-1.pyc.

Both PYC and PYO files contain the same type of bytecode and can be decompiled using this tool. The optimization level does not affect decompilation — only the removed assertions and debug code will be missing from the output.

How Python Decompilation Works

Python bytecode retains significant information about the original source code — variable names, function signatures, docstrings, line numbers, and code structure are all preserved. This makes Python decompilation much more effective than decompiling native code.

The decompiler reads the marshalled code object from the PYC file, analyzes the bytecode instructions, reconstructs control flow (if/else, loops, try/except), and generates Python source code that produces identical bytecode when recompiled.

Common Uses for Python Decompilation

• Recovering lost source code — when you have .pyc files but the original .py source was deleted
• Understanding packaged applications — many Python apps (PyInstaller, cx_Freeze) distribute only compiled bytecode
• Security analysis — examining Python-based malware or suspicious scripts
• Learning — studying how Python bytecode relates to source code

Frequently Asked Questions