Way to have compiled python files in a separate folder?

Update:

In Python 3.8 -X pycache_prefix=PATH command-line option enables writing .pyc files to a parallel tree rooted at the given directory instead of to the code tree. See $PYTHONPYCACHEPREFIX envvar^{credits: @RobertT' answer}

The location of the cache is reported in sys.pycache_prefix (None indicates the default location in __pycache__ [since Python 3.2] subdirectories).

To turn off caching the compiled Python bytecode, -B may be set, then Python won’t try to write .pyc files on the import of source modules. See $PYTHONDONTWRITEBYTECODE envvar^{credits: @Maleev's answer}

^{Old [Python 2] answer:}

There is PEP 304: Controlling Generation of Bytecode Files. Its status is Withdrawn and corresponding patch rejected. Therefore there might be no direct way to do it.

If you don't need source code then you may just delete *.py files. *.pyc files can be used as is or packed in an egg.

In the dark and ancient days of 2003, PEP 304 came forth to challenge this problem. Its patch was found wanting. Environment variable platform dependencies and version skews ripped it to shreds and left its bits scattered across the wastelands.

After years of suffering, a new challenger rose in the last days of 2009. Barry Warsaw summoned PEP 3147 and sent it to do battle, wielding a simple weapon with skill. The PEP crushed the cluttering PYC files, silenced the waring Unladen Swallow and CPython interpreter each trying to argue its PYC file should be triumphant, and allowed Python to rest easy with its dead ghosts occasionally running in the dead of night. PEP 3147 was found worthy by the dictator and was knighted into the official roles in the days of 3.2.

As of 3.2, Python stores a module's PYC files in __pycache__ under the module's directory. Each PYC file contains the name and version of the interpreter, e.g., __pycache__/foo.cpython-33.pyc. You might also have a __pycache__/foo.cpython-32.pyc compiled by an earlier version of Python. The right magic happens: the correct one is used and recompiled if out of sync with the source code. At runtime, look at the module's mymodule.__cached__ for the pyc filename and parse it with imp.get_tag(). See the What's New section for more information.

TL;DR - Just works in Python 3.2 and above. Poor hacks substitute for versions before that.

And only almost ten years later, Python 3.8 finally provides support for keeping bytecode in separate parallel filesystem tree by setting environment variable PYTHONPYCACHEPREFIX or using -X pycache_prefix=PATH argument (official doc here).