Utility Classes :: YieldSeq

Overview

YieldSeq() was dreamed up as a solution to the problem of wanting your generator to return objects of different types on sequential yields. Without YieldSeq(), nasty, imprecise workarounds are required:

@yields(IsOneOf(int, float))
def foo():
	yield 5
	yield 5.0
	yield 6
	yield 6.0

Where IsOneOf() is the IsOneOf utility class.

With the YieldSeq() utility class in hand, however, you can express this in a much more type-safe way:

@yields(YieldSeq(int, float, int, float))
def foo():
	yield 5
	yield 5.0
	yield 6
	yield 6.0

This approach is better because it retains a higher degree of precision as opposed to the IsOneOf()-based approach.

Details

• The YieldSeq() constructor requires at least one argument; an invocation like

  YieldSeq()

  will raise a TypeError exception.

• YieldSeq instances will compare equal (==) only if they have the same sequence of types. All YieldSeq instances that compare equal will hash to the same value.

• If the typechecked generator yields more values than the YieldSeq instance is prepared for, a _TC_YieldCountError exception will be raised with the number of expected yields.

  It is expected that this exception will be caught and converted to a TypeCheckError before it reaches the user.

• As can be deduced from the examples given above, the YieldSeq instance will check sequential conditions against sequential yields. This sequence will not be interrupted if a particular yielded value fails to typecheck; the following code, for example, will examine all values yielded by the generator:

  @yields(YieldSeq(int, float, int, float))
  def gen_foo():
  	yield 5
  	yield 5.0
  	yield "6"
  	yield 6.0
  	
  gen = gen_foo()
  while True:
  	try:
  		print gen.next()
  	except StopIteration:
  		break
  	except TypeCheckError:
  		pass

  Even though "6" clearly fails its typecheck and will raise a TypeCheckError, the values 5, 5.0 and 6.0 will still be printed.