Question

In this blog article they use the construct:

  @measured   def some_func():     #...   # Presumably outputs something like 'some_func() is finished in 121.333 s' somewhere 

This @measured directive doesn’t seem to work with raw python. What is it?

UPDATE: I see from Triptych that @something is valid, but is where can I find @measured, is it in a library somewhere, or is the author of this blog using something from his own private code base?

Answer 1

@measured decorates the some_func() function, using a function or class named measured. The @ is the decorator syntax, measured is the decorator function name.

Decorators can be a bit hard to understand, but they are basically used to either wrap code around a function, or inject code into one.

For example the measured function (used as a decorator) is probably implemented like this…

import time  def measured(orig_function):     # When you decorate a function, the decorator func is called     # with the original function as the first argument.     # You return a new, modified function. This returned function     # is what the to-be-decorated function becomes.      print 'INFO: This from the decorator function'     print 'INFO: I am about to decorate %s' % (orig_function)      # This is what some_func will become:     def newfunc(*args, **kwargs):         print 'INFO: This is the decorated function being called'          start = time.time()          # Execute the old function, passing arguments         orig_func_return = orig_function(*args, **kwargs)         end = time.time()          print 'Function took %s seconds to execute' % (end - start)         return orig_func_return # return the output of the original function      # Return the modified function, which..     return newfunc  @measured def some_func(arg1):     print 'This is my original function! Argument was %s' % arg1  # We call the now decorated function.. some_func(123)  #.. and we should get (minus the INFO messages): This is my original function! Argument was 123 # Function took 7.86781311035e-06 to execute 

The decorator syntax is just a shorter and neater way of doing the following:

def some_func():     print 'This is my original function!'  some_func = measured(some_func) 

There are some decorators included with Python, for example staticmethod – but measured is not one of them:

>>> type(measured) Traceback (most recent call last):   File '<stdin>', line 1, in <module> NameError: name 'measured' is not defined 

Check the projects import statements to see where the function or class is coming from. If it uses from blah import * you’ll need to check all of those files (which is why import * is discouraged), or you could just do something like grep -R def measured *