Hi,
Could you please give me some advice on the piece of code I am
writing?
My system has several possible outputs, some of them are not always
needed. I started to get confused with the code flow conditions needed
to avoid doing unnecessary work. So I am trying to restructure it
using lazy evaluation.
In the new mechanism I am coding I have a repository with two types of
objects: infos and routines. In the begining I have a list of
routines. Each routine tells which infos it can compute. The execution
is triggered when the value of an info is requested. In the example
below I have 3 routines
Routine "ReadData" computes info "gender" and info "birth_year"
Routine "YearToAge" computes info "age" (using info "birth_year")
Routine "ComputeMHF" computes info "max_heart_frequency" (using info
"gender" and info "age")
/--> gender ----------------------------\
ReadData --| | --> ComputeMHF --
> max_heart_frequency
\--> birth_year --> YearToAge --> age --/
So for instance if all I need is info "age", only the routines
"ReadData" and "YearToAge" are computed.
The code below implements the example. There are 3 files:
- test.py: the test case for the example
- routines.py: the routines (classes) of the example
- repository.py: the lazy evaluation mechanism (independent of the
example)
My questions are:
- Is there a more standard (pythonic) way to do what I am trying to
do? Are there libraries, design patterns, functional programming
structures to use to achieve what I am looking for (i.e. am I trying
to reinvent the wheel)?
- Is the coding style good?
- Can I avoid the eval command in Repository.add_routine? What I want
there is to be able to have a generic code for the repository which
does not depend on the files containing the routines I want it to
hold.
Note: The routines do not need to declare the info they depend on.
They request the info in the computation phase.
test.py
===
import unittest
from repository import Repository
ROUTINE_LIST = """
ReadData routines
YearToAge routines
ComputeMHF routines
"""
class Test(unittest.TestCase):
def test_age(self):
repo = Repository(ROUTINE_LIST)
self.assertEqual(repo['age'], 30)
def test_max_heart_frequency(self):
repo = Repository(ROUTINE_LIST)
self.assertEqual(repo['max_heart_frequency'], 181)
===
routines.py
===
from repository import AbstractRoutine
class ReadData(AbstractRoutine):
def __init__(self):
super(ReadData, self).__init__(self.__class__.__name__,
['birth_year', 'gender'])
def compute(self, repo):
repo['birth_year'] = 1979
repo['gender'] = 'F'
class YearToAge(AbstractRoutine):
def __init__(self):
super(YearToAge, self).__init__(self.__class__.__name__,
['age'])
def compute(self, repo):
repo['age'] = 2009 - repo['birth_year']
class ComputeMHF(AbstractRoutine):
def __init__(self):
super(ComputeMHF, self).__init__(self.__class__.__name__,
['max_heart_frequency'])
def compute(self, repo):
gender = repo['gender']
age = repo['age']
mhf = 211 - age if gender == 'F' else 205 - age
repo['max_heart_frequency'] = mhf
===
repostory.py
===
from StringIO import StringIO
class AbstractRoutine(object):
def __init__(self, name, infos_provided):
self.name = name
self.infos_provided = infos_provided
self.computed = False
def compute(self):
raise NotImplementedError
class Info(object):
def __init__(self, name, routine):
self.name = name
self.routine = routine
self.computed = False
self.value = None
class Repository(object):
def __init__(self, routine_definition_lines):
self._infos = {}
self.add_routines(routine_definition_lines)
def add_routines(self, definition_lines):
for line in StringIO(definition_lines):
line = line.strip()
if line == '':
continue
name, file_name = line.split()
self.add_routine(name, file_name)
def add_routine(self, class_name, file_name):
routine = None # only to cheat pylint
cmd = "from %s import %s\nroutine = %s()" % (file_name,
class_name,
class_name)
exec(cmd) # XXX: ugly
if not isinstance(routine, AbstractRoutine):
raise ValueError('Class %s is not AbstractRoutine'
% class_name)
for info_name in routine.infos_provided:
info = Info(info_name, routine)
self._infos[info_name] = info
def __setitem__(self, key, value):
if key not in self._infos:
raise ValueError('info %s not defined in repository' %
key)
info = self._infos[key]
if info.computed:
raise ValueError('info %s has already been computed' %
key)
info.value = value
info.computed = True
def __getitem__(self, key):
if key not in self._infos:
raise ValueError('info %s not defined in repository' %
key)
info = self._infos[key]
if not info.computed:
print('Calling routine %s to compute info %s'
% (info.routine.name, info.name))
info.routine.compute(self)
if not info.computed:
raise ValueError('routine %s did not compute info %s'
%
(info.routine.name, key))
return info.value
===
Thanks a lot!
Marko
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