[Tutor] Stupid bug

[Steven D'Aprano]

Wayne Werner wrote:

> Including the sister bug - continually importing something and you still get
> the old function because you forgot to delete the .pyc file. Whoops!

Er, whoops is right... I think you may have misinterpreted what you were 
seeing.

When you import a module for the first time, Python checks the date 
stamps on the .pyc file (if it exists) and ONLY uses it if it is newer 
than the .py file. If you update the source code, and then import the 
module, the obsolete .pyc file will be ignored and over-ridden. You 
don't have to delete the .pyc file in order for Python to use a newer 
source file.

However, if the date stamps are screwed up, of course anything could 
happen. Or if you move the .pyc files into an earlier part of the 
PYTHONPATH, so they are seen before the .py files.

Note also that what I described only occurs the first time you import a 
module. The second and subsequent time you import a module, it is not 
retrieved from either the .py or the .pyc file, it is retrieved from the 
cache in sys.modules.

Python is not really designed for reloading modules on the fly. There is 
the reload() function (built-in in Python 2.x, moved into a library in 
Python 3.x) but it is a fairly feeble thing, only good for the most 
simple interactive use. Unless you're using a library that offers proper 
reload functionality, the safest way to ensure you're using the latest 
version of the module you've just edited is to exist the interpreter and 
start it up again.


-- 
Steven