How to avoid .pyc files?

Question

Can I run the python interpreter without generating the compiled .pyc files?

Answer 1

You could make the directories that your modules exist in read-only for the user that the Python interpreter is running as.

I don't think there's a more elegant option. PEP 304 appears to have been an attempt to introduce a simple option for this, but it appears to have been abandoned.

I imagine there's probably some other problem you're trying to solve, for which disabling .py[co] would appear to be a workaround, but it'll probably be better to attack whatever this original problem is instead.

Answer 2

From "What’s New in Python 2.6 - Interpreter Changes":

Python can now be prevented from writing .pyc or .pyo files by supplying the -B switch to the Python interpreter, or by setting the PYTHONDONTWRITEBYTECODE environment variable before running the interpreter. This setting is available to Python programs as the sys.dont_write_bytecode variable, and Python code can change the value to modify the interpreter’s behaviour.

Update 2010-11-27: Python 3.2 addresses the issue of cluttering source folders with .pyc files by introducing a special __pycache__ subfolder, see What's New in Python 3.2 - PYC Repository Directories.

Answer 3

There actually IS a way to do it in Python 2.3+, but it's a bit esoteric. I don't know if you realize this, but you can do the following:

$ unzip -l /tmp/example.zip
 Archive:  /tmp/example.zip
   Length     Date   Time    Name
 --------    ----   ----    ----
     8467  11-26-02 22:30   jwzthreading.py
 --------                   -------
     8467                   1 file
$ ./python
Python 2.3 (#1, Aug 1 2003, 19:54:32) 
>>> import sys
>>> sys.path.insert(0, '/tmp/example.zip')  # Add .zip file to front of path
>>> import jwzthreading
>>> jwzthreading.__file__
'/tmp/example.zip/jwzthreading.py'

According to the zipimport library:

Any files may be present in the ZIP archive, but only files .py and .py[co] are available for import. ZIP import of dynamic modules (.pyd, .so) is disallowed. Note that if an archive only contains .py files, Python will not attempt to modify the archive by adding the corresponding .pyc or .pyo file, meaning that if a ZIP archive doesn't contain .pyc files, importing may be rather slow.

Thus, all you have to do is zip the files up, add the zipfile to your sys.path and then import them.

If you're building this for UNIX, you might also consider packaging your script using this recipe: unix zip executable, but note that you might have to tweak this if you plan on using stdin or reading anything from sys.args (it CAN be done without too much trouble).

In my experience performance doesn't suffer too much because of this, but you should think twice before importing any very large modules this way.