Signature and parameters

## New feature of python 3 in inspect module.

>>> _fields = ['name','shares','price']
>>> from inspect import Parameter, Signature
>>> parms = [ Parameter(fname, Parameter.POSITIONAL_OR_KEYWORD) for fname in _fields]
>>> parms
[<Parameter "name">, <Parameter "shares">, <Parameter "price">]
>>> sig = Signature(parms)
>>> print(sig)
(name, shares, price)

Binding the signature to arguments. 

>>> def foo(*args,**kwargs):
...     bound = sig.bind(*args,**kwargs)
...     for name,val in bound.arguments.items():
...         print(name,val)
...         
...     
... 
>>> foo(1,2,3)
name 1
shares 2
price 3

## Also keyword arguments are working.

>>> foo(1,2,price=3)
name 1
shares 2
price 3


Ex 1 : Taking fields from class and values at instance creation time. 

class Structure:
  _fields = []
  ''' fields will take a list from class attributes '''
  def __init__(self,*args):
    ''' args will take parameters during instance initialization '''
    if len(args) != self._fields:
      raise TypeError('Wrong args')
    for name,value in zip(self._fields, args):
      ''' setting attribute by taking fields and instance initialization values '''
      setattr(self,name,value)

class Students(Structure):
  ''' Inherited class '''
  _fields = ['name','rollnumber','class']

class Teacher(Structure):
  _fields = ['name','salary']
  
## Problem with above approach, it doesn't care about the number of arguments and we can't pass key word arguments. 
## Solution is to use signature in parent class to get rid of these issues.

from inspect import Parameter, Signature

def make_signature(names):
  return Signature(Parameter(name, Parameter.POSITIONAL_OR_KEYWORD)  for name in names)

class Structure:
  __signature__ = make_signature([])
  def __init__(self, *args, **kwargs):
    bound = self.__signature__.bind(*args,**kwargs)
    for name,val in bound.arguments.items():
      setattr(self,name,val)

class Students(Structure):
  ''' Inherited class '''
  __signature__ = make_signature(['name','rollnumber','class'])
  
class Teacher(Structure):
  __signature__ = make_signature(['name','salary'])