What is the difference between .py and .pyc files?

.py— this is source code in Python, written by a human and readable by the interpreter..pyc— this is bytecode compiled by the interpreter from a.py` file to speed up subsequent runs.

The Python interpreter automatically compiles .py files into .pyc upon the first import of a module and saves them to avoid recompiling on each subsequent run. .pyc files are stored in the __pycache__ directory or alongside the corresponding .py file in older versions.

# Example of a `.py` file content
def greet(name):
  print(f"Hello, {name}!")

# Bytecode from a `.pyc` file (not directly readable)
# c\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00@\x00\x00\x00s\x0e\x00\x00\x00d\x00\x00GHd\x01\x00k\x02\x00\x83\x01\x00\x00d\x02\x00R\x00\xfa\x0bgreet.py\x05\x00\x00\x00s\x00\x00\x00\x00\x01\x00\x00\x00\x01\x00\x00\x00C\x00\x00\x00s\x05\x00\x00\x00\x83\x00\x00\x00\x00Sq\x00\x00\x00s\x0c\x00\x00\x00\\x01\x00\x00\x00\x00\x01\x00\x00\x00r\x02\x00\x00\x00r\x02\x00\x00\x00\xfa\x0b<frozen greet>\x01\x00\x00\x00s\x08\x00\x00\x00\xe9\x00\x00\x00\x00r\x03\x00\x00\x00r\x04\x00\x00\x00r\x0b\x00\x00\x00r\x04\x00\x00\x00N\x86\x04\x00\x00\x00GHGHGHGHGHGHGHGHGHGHGHGHGHGHGHGHGHGHG\x0c\x12\x00\x00\x00s\x00\x00\x00\x00

In summary:

File	Description	Purpose	Readability
.py	Source code	Development, writing logic	High (by humans)
.pyc	Compiled bytecode	Speed up execution	Low (by machines)

The presence of .pyc does not affect code execution, as the interpreter uses them only for module loading optimization. If a .py file is modified, the .pyc file is automatically regenerated.