Python function attributes - uses and abuses

Question

Not many are aware of this feature  but Python s functions  and methods  can have attributes  Behold    gt  gt  gt  def foo x           pass           gt  gt  gt  foo score   10  gt  gt  gt  dir foo      call        class        delattr        dict        doc        get        getattribute        hash        init        module        name        new        reduce        reduce ex        repr        setattr        str      func closure    func code    func defaults    func dict    func doc    func globals    func name    score    gt  gt  gt  foo score 10  gt  gt  gt  foo score    1  gt  gt  gt  foo score 11   What are the possible uses and abuses of this feature in Python   One good use I m aware of is PLY s usage of the docstring to associate a syntax rule with a method  But what about custom attributes   Are there good reasons to use them

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Sometimes I use an attribute of a function for caching already computed values  You can also have a generic decorator that generalizes this approach  Be aware of concurrency issues and side effects of such functions

User · Answer

I typically use function attributes as storage for annotations  Suppose I want to write  in the style of C   indicating that a certain method should be part of the web service interface   class Foo WebService        webmethod     def bar self  arg1  arg2                  then I can define  def webmethod func       func is webmethod   True     return func   Then  when a webservice call arrives  I look up the method  check whether the underlying function has the is webmethod attribute  the actual value is irrelevant   and refuse the service if the method is absent or not meant to be called over the web

User · Answer

I typically use function attributes as storage for annotations  Suppose I want to write  in the style of C   indicating that a certain method should be part of the web service interface   class Foo WebService        webmethod     def bar self  arg1  arg2                  then I can define  def webmethod func       func is webmethod   True     return func   Then  when a webservice call arrives  I look up the method  check whether the underlying function has the is webmethod attribute  the actual value is irrelevant   and refuse the service if the method is absent or not meant to be called over the web

User · Answer

Sometimes I use an attribute of a function for caching already computed values  You can also have a generic decorator that generalizes this approach  Be aware of concurrency issues and side effects of such functions

User · Answer

You can do objects the JavaScript way    It makes no sense but it works      gt  gt  gt  def FakeObject          def test            print  foo        FakeObject test   test       return FakeObject  gt  gt  gt  x   FakeObject    gt  gt  gt  x test   foo

User · Answer

I was always of the assumption that the only reason this was possible was so there was a logical place to put a doc-string or other such stuff  I know if I used it for any production code it d confuse most who read it

User · Answer

I ve used them as static variables for a function  For example  given the following C code   int fn int i        static f   1      f    i      return f      I can implement the function similarly in Python   def fn i       fn f    i     return fn f fn f   1   This would definitely fall into the  abuses  end of the spectrum

User · Answer

I use them sparingly  but they can be pretty convenient   def log msg      log logfile write msg    Now I can use log throughout my module  and redirect output simply by setting log logfile       There are lots and lots of other ways to accomplish that  but this one s lightweight and dirt simple   And while it smelled funny the first time I did it  I ve come to believe that it smells better than having a global logfile variable

User · Answer

Sometimes I use an attribute of a function for caching already computed values  You can also have a generic decorator that generalizes this approach  Be aware of concurrency issues and side effects of such functions

User · Answer

You can do objects the JavaScript way    It makes no sense but it works      gt  gt  gt  def FakeObject          def test            print  foo        FakeObject test   test       return FakeObject  gt  gt  gt  x   FakeObject    gt  gt  gt  x test   foo

User · Answer

You can do objects the JavaScript way    It makes no sense but it works      gt  gt  gt  def FakeObject          def test            print  foo        FakeObject test   test       return FakeObject  gt  gt  gt  x   FakeObject    gt  gt  gt  x test   foo

User · Answer

Function attributes can be used to write light-weight closures that wrap code and associated data together      usr bin env python  SW DELTA   0 SW MARK    1 SW BASE    2  def stopwatch       import time     def  sw  action   SW DELTA           if action    SW DELTA           return time time   -  sw  time        elif action    SW MARK            sw  time   time time            return  sw  time        elif action    SW BASE           return  sw  time        else           raise NotImplementedError      sw  time   time time     time of creation     return  sw    test code sw stopwatch   sw2 stopwatch   import os os system  sleep 1   print sw     defaults to  SW DELTA  sw  SW MARK   os system  sleep 2   print sw   print sw2     1 00934004784  2 00644397736  3 01593494415

User · Answer

I was always of the assumption that the only reason this was possible was so there was a logical place to put a doc-string or other such stuff  I know if I used it for any production code it d confuse most who read it

User · Answer

I ve created this helper decorator to easily set function attributes   def with attrs   func attrs          Set attributes in the decorated function  at definition time      Only accepts keyword arguments      E g            with attrs counter 0  something  boing           def count it                count it counter    1         print count it counter         print count it something           Out             gt  gt  gt  0            gt  gt  gt   boing              def attr decorator fn            wraps fn          def wrapper  args    kwargs               return fn  args    kwargs           for attr  value in func attrs iteritems                setattr wrapper  attr  value           return wrapper      return attr decorator   A use case is to create a collection of factories and query the data type they can create at a function meta level  For example  very dumb one     with attrs datatype list  def factory1        return  1  2  3    with attrs datatype SomeClass  def factory2        return SomeClass    factories    factory1  factory2   def create datatype       for f in factories          if f datatype    datatype              return f       return None

User · Answer

I ve used them as static variables for a function  For example  given the following C code   int fn int i        static f   1      f    i      return f      I can implement the function similarly in Python   def fn i       fn f    i     return fn f fn f   1   This would definitely fall into the  abuses  end of the spectrum

User · Answer

I ve used them as static variables for a function  For example  given the following C code   int fn int i        static f   1      f    i      return f      I can implement the function similarly in Python   def fn i       fn f    i     return fn f fn f   1   This would definitely fall into the  abuses  end of the spectrum

User · Answer

I ve used them as static variables for a function  For example  given the following C code   int fn int i        static f   1      f    i      return f      I can implement the function similarly in Python   def fn i       fn f    i     return fn f fn f   1   This would definitely fall into the  abuses  end of the spectrum

User · Answer

I was always of the assumption that the only reason this was possible was so there was a logical place to put a doc-string or other such stuff  I know if I used it for any production code it d confuse most who read it

User · Answer

I typically use function attributes as storage for annotations  Suppose I want to write  in the style of C   indicating that a certain method should be part of the web service interface   class Foo WebService        webmethod     def bar self  arg1  arg2                  then I can define  def webmethod func       func is webmethod   True     return func   Then  when a webservice call arrives  I look up the method  check whether the underlying function has the is webmethod attribute  the actual value is irrelevant   and refuse the service if the method is absent or not meant to be called over the web

User · Answer

Function attributes can be used to write light-weight closures that wrap code and associated data together      usr bin env python  SW DELTA   0 SW MARK    1 SW BASE    2  def stopwatch       import time     def  sw  action   SW DELTA           if action    SW DELTA           return time time   -  sw  time        elif action    SW MARK            sw  time   time time            return  sw  time        elif action    SW BASE           return  sw  time        else           raise NotImplementedError      sw  time   time time     time of creation     return  sw    test code sw stopwatch   sw2 stopwatch   import os os system  sleep 1   print sw     defaults to  SW DELTA  sw  SW MARK   os system  sleep 2   print sw   print sw2     1 00934004784  2 00644397736  3 01593494415

User · Answer

Function attributes can be used to write light-weight closures that wrap code and associated data together      usr bin env python  SW DELTA   0 SW MARK    1 SW BASE    2  def stopwatch       import time     def  sw  action   SW DELTA           if action    SW DELTA           return time time   -  sw  time        elif action    SW MARK            sw  time   time time            return  sw  time        elif action    SW BASE           return  sw  time        else           raise NotImplementedError      sw  time   time time     time of creation     return  sw    test code sw stopwatch   sw2 stopwatch   import os os system  sleep 1   print sw     defaults to  SW DELTA  sw  SW MARK   os system  sleep 2   print sw   print sw2     1 00934004784  2 00644397736  3 01593494415

User · Answer

I ve created this helper decorator to easily set function attributes   def with attrs   func attrs          Set attributes in the decorated function  at definition time      Only accepts keyword arguments      E g            with attrs counter 0  something  boing           def count it                count it counter    1         print count it counter         print count it something           Out             gt  gt  gt  0            gt  gt  gt   boing              def attr decorator fn            wraps fn          def wrapper  args    kwargs               return fn  args    kwargs           for attr  value in func attrs iteritems                setattr wrapper  attr  value           return wrapper      return attr decorator   A use case is to create a collection of factories and query the data type they can create at a function meta level  For example  very dumb one     with attrs datatype list  def factory1        return  1  2  3    with attrs datatype SomeClass  def factory2        return SomeClass    factories    factory1  factory2   def create datatype       for f in factories          if f datatype    datatype              return f       return None

User · Answer

Function attributes can be used to write light-weight closures that wrap code and associated data together      usr bin env python  SW DELTA   0 SW MARK    1 SW BASE    2  def stopwatch       import time     def  sw  action   SW DELTA           if action    SW DELTA           return time time   -  sw  time        elif action    SW MARK            sw  time   time time            return  sw  time        elif action    SW BASE           return  sw  time        else           raise NotImplementedError      sw  time   time time     time of creation     return  sw    test code sw stopwatch   sw2 stopwatch   import os os system  sleep 1   print sw     defaults to  SW DELTA  sw  SW MARK   os system  sleep 2   print sw   print sw2     1 00934004784  2 00644397736  3 01593494415

User · Answer

Sometimes I use an attribute of a function for caching already computed values  You can also have a generic decorator that generalizes this approach  Be aware of concurrency issues and side effects of such functions

User · Answer

I use them sparingly  but they can be pretty convenient   def log msg      log logfile write msg    Now I can use log throughout my module  and redirect output simply by setting log logfile       There are lots and lots of other ways to accomplish that  but this one s lightweight and dirt simple   And while it smelled funny the first time I did it  I ve come to believe that it smells better than having a global logfile variable

User · Answer

I was always of the assumption that the only reason this was possible was so there was a logical place to put a doc-string or other such stuff  I know if I used it for any production code it d confuse most who read it

User · Answer

You can do objects the JavaScript way    It makes no sense but it works      gt  gt  gt  def FakeObject          def test            print  foo        FakeObject test   test       return FakeObject  gt  gt  gt  x   FakeObject    gt  gt  gt  x test   foo

User · Answer

I typically use function attributes as storage for annotations  Suppose I want to write  in the style of C   indicating that a certain method should be part of the web service interface   class Foo WebService        webmethod     def bar self  arg1  arg2                  then I can define  def webmethod func       func is webmethod   True     return func   Then  when a webservice call arrives  I look up the method  check whether the underlying function has the is webmethod attribute  the actual value is irrelevant   and refuse the service if the method is absent or not meant to be called over the web

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