Book Excerpt: The Python Standard Library by Example

Acquiring Function Properties

The partial object does not have __name__ or __doc__ attributes by default, and without those attributes, decorated functions are more difficult to debug. Using update_wrapper() copies or adds attributes from the original function to the partial object.

import functools 

def myfunc(a, b=2): 
    """Docstring for myfunc()."""
    print ’ called myfunc with:’, (a, b)
    return 

def show_details(name, f): 
   """Show details of a callable object."""
   print %s:’ % name
   print ’ object:’,f
   print ’ __name__:’,
   try: 
      print f.__name__
   except AttributeError: 
      print ’(no __name__)’
   print ’ __doc__’, repr(f.__doc__)
   print
   return 

show_details(’myfunc’, myfunc) 

p1 = functools.partial(myfunc, b=4) 
show_details(’raw wrapper’, p1) 

print Updating wrapper:
print assign:’, functools.WRAPPER_ASSIGNMENTS
print update:’, functools.WRAPPER_UPDATES
print 

functools.update_wrapper(p1, myfunc) 
show_details(’updated wrapper’, p1) 

The attributes added to the wrapper are defined in WRAPPER_ASSIGNMENTS, while WRAPPER_UPDATES lists values to be modified.

$ python functools_update_wrapper.py 

myfunc:
 object: <function myfunc at 0x100da2050>
 __name__: myfunc
 __doc__ ’Docstring for myfunc().’ 

raw wrapper:
 object: <functools.partial object at 0x100d993c0>
 __name__: (no __name__)
 __doc__ ’partial(func, *args, **keywords) -new function with parti 
al application\n of the given arguments and keywords.\n’ 

Updating wrapper:
 assign: (’__module__’, ’__name__’, ’__doc__’)
 update: (’__dict__’,) 

updated wrapper:
 object: <functools.partial object at 0x100d993c0>
 __name__: myfunc
 __doc__ ’Docstring for myfunc().’ 

Other Callables

Partials work with any callable object, not just with stand-alone functions.

import functools 

class MyClass(object): 
   """Demonstration class for functools""" 

   def method1(self, a, b=2): 
      """Docstring for method1().""" 
      print ’ called method1 with:’, (self, a, b)
      return 

   def method2(self, c, d=5): 
      """Docstring for method2""" 
       print ’ called method2 with:’, (self, c, d)
       return 
       wrapped_method2 = functools.partial(method2, ’wrapped c’)
       functools.update_wrapper(wrapped_method2, method2) 
  
     def __call__(self, e, f=6): 
       """Docstring for MyClass.__call__"""
        print ’ called object with:’, (self, e, f)
        return 

def show_details(name, f): 
    """Show details of a callable object."""
     print %s:’ % name
     print ’ object:’,f
     print ’ __name__:’,
     try: 
        print f.__name__
     except AttributeError: 
        print ’(no __name__)’
     print ’ __doc__’, repr(f.__doc__)
     return 

o = MyClass() 

show_details(’method1 straight’, o.method1)
o.method1(’no default for a’, b=3) 
print 

p1 = functools.partial(o.method1, b=4) 
functools.update_wrapper(p1, o.method1) 
show_details(’method1 wrapper’, p1) 
p1(a goes here’) 
print 

show_details(’method2’, o.method2)
o.method2(’no default for c’, d=6) 
print 

show_details(’wrapped method2’, o.wrapped_method2) 
o.wrapped_method2(’no default for c’, d=6) 
print 

show_details(’instance’, o) 
o(’no default for e’) 
print 
p2 = functools.partial(o, f=7) 
show_details(’instance wrapper’, p2) 
p2(e goes here’) 

This example creates partials from an instance and methods of an instance.

$ python functools_method.py 

method1 straight: 
  object: <bound method MyClass.method1 of <__main__.MyClass object
at 0x100da3550>> 
  __name__: method1 
  __doc__ ’Docstring for method1().’ 
  called method1 with: (<__main__.MyClass object at 0x100da3550>, ’n 
o default for a’, 3) 

method1 wrapper: 
  object: <functools.partial object at 0x100d99470> 
  __name__: method1 
  __doc__ ’Docstring for method1().’ 
  called method1 with: (<__main__.MyClass object at 0x100da3550>, ’a 
 goes here’, 4) 

method2: 
  object: <bound method MyClass.method2 of <__main__.MyClass object
at 0x100da3550>> 
  __name__: method2 
  __doc__ ’Docstring for method2’ 
  called method2 with: (<__main__.MyClass object at 0x100da3550>, ’n 
o default for c’, 6) 

wrapped method2: 
  object: <functools.partial object at 0x100d993c0> 
  __name__: method2 
  __doc__ ’Docstring for method2’ 
  called method2 with: (’wrapped c’, ’no default for c’, 6) 

instance: 
  object: <__main__.MyClass object at 0x100da3550> 
  __name__: (no __name__) 
  __doc__ ’Demonstration class for functools’ 
  called object with: (<__main__.MyClass object at 0x100da3550>, ’no 
    default for e’, 6) 
  instance wrapper:
    object: <functools.partial object at 0x100d994c8>
    __name__: (no __name__) 
    __doc__ ’partial(func, *args, **keywords) -new function with part 
  ial application\n of the given arguments and keywords.\n’
    called object with: (<__main__.MyClass object at 0x100da3550>, ’e
  goes here’, 7) 
______________________

Webinar
One Click, Universal Protection: Implementing Centralized Security Policies on Linux Systems

As Linux continues to play an ever increasing role in corporate data centers and institutions, ensuring the integrity and protection of these systems must be a priority. With 60% of the world's websites and an increasing share of organization's mission-critical workloads running on Linux, failing to stop malware and other advanced threats on Linux can increasingly impact an organization's reputation and bottom line.

Learn More

Sponsored by Bit9

Webinar
Linux Backup and Recovery Webinar

Most companies incorporate backup procedures for critical data, which can be restored quickly if a loss occurs. However, fewer companies are prepared for catastrophic system failures, in which they lose all data, the entire operating system, applications, settings, patches and more, reducing their system(s) to “bare metal.” After all, before data can be restored to a system, there must be a system to restore it to.

In this one hour webinar, learn how to enhance your existing backup strategies for better disaster recovery preparedness using Storix System Backup Administrator (SBAdmin), a highly flexible bare-metal recovery solution for UNIX and Linux systems.

Learn More

Sponsored by Storix