When are .pyc files refreshed?

python python-internals pyc
Aaron Schif picture Aaron Schif · Apr 5, 2013 · Viewed 51.1k times · Source

I understand that ".pyc" files are compiled versions of the plain-text ".py" files, created at runtime to make programs run faster. However I have observed a few things:

  1. Upon modification of "py" files, program behavior changes. This indicates that the "py" files are compiled or at least go though some sort of hashing process or compare time stamps in order to tell whether or not they should be re-compiled.
  2. Upon deleting all ".pyc" files (rm *.pyc) sometimes program behavior will change. Which would indicate that they are not being compiled on update of ".py"s.

Questions:

  • How do they decide when to be compiled?
  • Is there a way to ensure that they have stricter checking during development?

Answer

DaveTheScientist picture DaveTheScientist · Apr 5, 2013

The .pyc files are created (and possibly overwritten) only when that python file is imported by some other script. If the import is called, Python checks to see if the .pyc file's internal timestamp is not older than the corresponding .py file. If it is, it loads the .pyc; if it isn't or if the .pyc does not yet exist, Python compiles the .py file into a .pyc and loads it.

What do you mean by "stricter checking"?

Related questions

How does the @property decorator work in Python?

I would like to understand how the built-in function property works. What confuses me is that property can also be used as a decorator, but it only takes arguments when used as a built-in function and not when used as …

Read More
When is del useful in python?

I can't really think of any reason why python needs the del keyword (and most languages seem to not have a similar keyword). For instance, rather than deleting a variable, one could just assign None to it. And when deleting …

Read More
time.sleep -- sleeps thread or process?

In Python for *nix, does time.sleep() block the thread or the process?

Read More