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How do I write a decorator that restores the current working directory to what it was before the decorated function was called? In other words, if I use the decorator on a function that does an os.chdir(), the cwd will not be changed after the function is called.
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When you apply a decorator to a class or a class member, you are actually calling a function that is going to receive details of what is being decorated, and the decorator implementation will then be able to transform the code dynamically, adding extra functionality, and reducing boilerplate code.
The parameter (s) passed to the decorator will depend on where the decorator will be used. The first parameter is commonly called target. The sealed decorator will be used only on class declarations, so your function will receive a single parameter, the target, which will be of type Function.
When to Use a Decorator in Python You'll use a decorator when you need to change the behavior of a function without modifying the function itself. A few good examples are when you want to add logging, test performance, perform caching, verify permissions, and so on. You can also use one when you need to run the same code on multiple functions.
You'll use a decorator when you need to change the behavior of a function without modifying the function itself. A few good examples are when you want to add logging, test performance, perform caching, verify permissions, and so on.
The answer for a decorator has been given; it works at the function definition stage as requested.
With Python 2.5+, you also have an option to do that at the function call stage using a context manager:
from __future__ import with_statement # needed for 2.5 ≤ Python < 2.6 import contextlib, os @contextlib.contextmanager def remember_cwd(): curdir= os.getcwd() try: yield finally: os.chdir(curdir) which can be used if needed at the function call time as:
print "getcwd before:", os.getcwd() with remember_cwd(): walk_around_the_filesystem() print "getcwd after:", os.getcwd() It's a nice option to have.
EDIT: I added error handling as suggested by codeape. Since my answer has been voted up, it's fair to offer a complete answer, all other issues aside.
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