Is there a Python equivalent of the Haskell 'let'

Question

Is there a Python equivalent of the Haskell \'let\' expression that would allow me to write something like:

list2 = [let (name,size)=lookup(productId) in (ba         


        

Answer 1

Although you can simply write this as:

list2 = [(barcode(pid), metric(lookup(pid)[1]))
         for pid in list]

You could define LET yourself to get:

list2 = [LET(('size', lookup(pid)[1]),
             lambda o: (barcode(pid), metric(o.size)))
         for pid in list]

or even:

list2 = map(lambda pid: LET(('name_size', lookup(pid),
                             'size', lambda o: o.name_size[1]),
                            lambda o: (barcode(pid), metric(o.size))),
            list)

as follows:

import types

def _obj():
  return lambda: None

def LET(bindings, body, env=None):
  '''Introduce local bindings.
  ex: LET(('a', 1,
           'b', 2),
          lambda o: [o.a, o.b])
  gives: [1, 2]

  Bindings down the chain can depend on
  the ones above them through a lambda.
  ex: LET(('a', 1,
           'b', lambda o: o.a + 1),
          lambda o: o.b)
  gives: 2
  '''
  if len(bindings) == 0:
    return body(env)

  env = env or _obj()
  k, v = bindings[:2]
  if isinstance(v, types.FunctionType):
    v = v(env)

  setattr(env, k, v)
  return LET(bindings[2:], body, env)