Difference between yield in Python and yield in C#

Question

What is the difference between yield keyword in Python and yield keyword in C#?

Answer 1

The most important difference is that python yield gives you an iterator, once it is fully iterated that's over.

But C# yield return gives you an iterator "factory", which you can pass it around and uses it in multiple places of your code without concerning whether it has been "looped" once before.

Take this example in python:

In [235]: def func1():
   .....:     for i in xrange(3):
   .....:         yield i
   .....:

In [236]: x1 = func1()

In [237]: for k in x1:
   .....:     print k
   .....:
0
1
2

In [238]: for k in x1:
   .....:     print k
   .....:

In [239]:

And in C#:

class Program
{
    static IEnumerable<int> Func1()
    {
        for (int i = 0; i < 3; i++)
            yield return i;
    }

    static void Main(string[] args)
    {
        var x1 = Func1();
        foreach (int k in x1) 
            Console.WriteLine(k);

        foreach (int k in x1)
            Console.WriteLine(k);
    }
}

That gives you:

0
1
2
0
1
2

Answer 2

An important distinction to note, in addition to other answers, is that yield in C# can't be used as an expression, only as a statement.

An example of yield expression usage in Python (example pasted from here):

def echo(value=None):
  print "Execution starts when 'next()' is called for the first time."
  try:
    while True:
       try:
         value = (yield value)
       except GeneratorExit:
         # never catch GeneratorExit
         raise
       except Exception, e:
         value = e
     finally:
       print "Don't forget to clean up when 'close()' is called."

generator = echo(1)
print generator.next()
# Execution starts when 'next()' is called for the first time.
# prints 1

print generator.next()
# prints None

print generator.send(2)
# prints 2

generator.throw(TypeError, "spam")
# throws TypeError('spam',)

generator.close()
# prints "Don't forget to clean up when 'close()' is called."

Answer 3

C#'s yield return is equivalent to Python's yield , and yield break is just return in Python.

Other than those minor differences, they have basically the same purpose.

Answer 4

This may help you understand it : Iterators in C#, Python, and Ruby